Compression Testing for Solid Food Texture: A Comprehensive Guide for Research and Product Development

Abstract

This article provides a comprehensive overview of compression testing methodologies for solid food texture analysis, tailored for researchers, scientists, and product development professionals. It covers fundamental principles, from defining key texture attributes like hardness, cohesiveness, and springiness, to detailed methodological protocols including Texture Profile Analysis (TPA) and back-extrusion tests. The content addresses common challenges in test standardization, data interpretation, and optimization for diverse materials, from traditional foods to novel products like plant-based analogues and cultured meat. By correlating instrumental data with sensory evaluation and exploring validation frameworks, this guide serves as an essential resource for ensuring data accuracy, product consistency, and successful innovation in food science and related biomedical applications.

The Fundamentals of Food Texture and Compression Mechanics

In the field of solid food texture research, the mechanical properties of materials serve as fundamental indicators of quality, functionality, and consumer acceptance. Texture analysis, primarily concerned with evaluating mechanical characteristics where a material is subjected to a controlled force, generates deformation curves that reveal critical information about material behavior [1]. Within this domain, hardness and firmness represent two essential yet frequently conflated parameters in the characterization of solid foods. The accurate distinction between these attributes is critical for researchers and product developers seeking to quantify material performance under mechanical stress.

Hardness is formally defined as the stress or force required to break a food, typically measured through large deformation or destructive compression tests [2]. In contrast, firmness represents a moderate level of hardness, generally associated with non-destructive compression at low strain levels, typically around 0.1 [2]. This distinction is not merely semantic but reflects fundamental differences in testing methodologies, deformation behaviors, and ultimate application of results. The ambiguity in terminology presents significant challenges in comparative research, quality assurance protocols, and standardized reporting across laboratories.

Compression testing serves as the principal methodological foundation for quantifying these attributes, providing researchers with precise, reproducible data that correlates with sensory perceptions. As the scientific community moves toward greater standardization in texture analysis, clarity in defining and measuring these fundamental properties becomes increasingly important for advancing research in food science, pharmaceuticals, and related fields where material texture influences product performance and safety.

Theoretical Foundations of Key Texture Attributes

Defining Hardness and Firmness

The theoretical distinction between hardness and firmness extends beyond mere terminology to encompass fundamental differences in material behavior and measurement approaches. From a materials science perspective, hardness characterizes a material's resistance to permanent deformation or fracture, representing the point at which the internal structure undergoes catastrophic failure [2]. This failure typically develops through stress concentration at structural imperfections, with crack propagation continuing across the sample, resulting in structural breakdown. Hardness measurements are therefore appropriately applied to foods that undergo rupture or fracture when compressed, such as brittle snacks, crisp fruits, or structured gels.

Conversely, firmness describes a material's resistance to elastic deformation under small compressive strains, typically in the range of 0.1, where the material behavior remains primarily within the elastic region [2]. This parameter is particularly relevant for quality assessment of agricultural products like fruits, where destructive testing is undesirable, and for soft solid foods that deform without fracturing. The physiological basis for this distinction lies in human perception: when consumers gently squeeze fruit to assess ripeness, they are evaluating firmness, whereas when they bite through a hard candy, they perceive hardness [2].

From an instrumental perspective, the key differentiators between these attributes include:

• Degree of Deformation: Hardness measurements employ large deformation tests (often to strains of 0.75 or until fracture), while firmness assessments use small deformation tests (typically strains of ~0.1) [2].
• Nature of Measurement: Hardness captures the force or stress at structural failure, while firmness measures the force-deformation relationship within the elastic region.
• Material Response: Hardness reflects irreversible structural changes, while firmness characterizes reversible, elastic deformation.

Measurement Principles and Units

The quantification of hardness and firmness presents complexities in both methodology and reporting units. Instrumentally, hardness can be reported either as force (Newtons, N) or stress (Pascals, Pa), with significant implications for data interpretation and cross-study comparisons [2]. Force measurements dominate empirical quality control applications, while stress calculations are preferred in fundamental research where material properties independent of geometry are desired.

The challenge in reporting stress values lies in accurately determining the contact area during compression, which is well-defined in die loading but ill-defined in plate loading configurations [2]. As Muller (1973) astutely observed, "if I sit on a chair—all is well, if I sit on a pin—all is not well"; in both cases the force (body weight) is identical, but the stress differs dramatically due to contact area [2]. This analogy perfectly illustrates why contact geometry must be carefully considered in test design and data reporting.

Firmness measurements typically report force (N) or occasionally the slope of the force-distance curve (N/mm), as the small deformations involved often preclude accurate stress calculations due to uncertain and changing contact areas during compression [2]. The speed of testing further influences results, as materials subjected to compression at speeds exceeding their relaxation capacity demonstrate built-up stresses that may inflate measured values [2]. For modulus testing, sufficiently slow compression speeds are essential to allow stress relaxation, though this principle is often overlooked in practical hardness testing where higher speeds may better simulate actual consumption conditions.

Table 1: Comparative Analysis of Hardness and Firmness Attributes

Attribute	Definition	Typical Strain	Material Behavior	Common Units	Typical Applications
Hardness	Stress or force required to break a food	Large (0.75 or until fracture)	Destructive, fracture	N or Pa	Brittle foods, crisp fruits, snacks
Firmness	Resistance to elastic deformation	Small (~0.1)	Non-destructive, elastic recovery	N or N/mm	Fruit ripeness, soft solids, quality grading

Quantitative Framework for Texture Analysis

Compression Testing Modalities

Texture analysis employs diverse compression testing modalities, each designed to extract specific mechanical properties from food materials. The fundamental approach involves applying controlled deformation to a sample while precisely measuring the force response, generating force-distance or force-time curves that reveal critical texture attributes [3]. These tests can be configured in fundamental, empirical, or imitative ways, depending on the research objectives and required data specificity.

The most common compression test variants include:

• Measure the Force to go to a chosen Distance: This approach applies a predetermined deformation and records the resistance force, typically used to establish whether a specific force causes failure or irreversible deformation [3].
• Measure the Distance to go to a chosen Force: This method applies increasing compression until a target force is reached, suitable for measuring compactability in materials like granules or powders [3].
• Stress Relaxation: This test holds a constant deformation for a specified time while monitoring force decay, quantifying material relaxation behavior and useful for recoverable materials like bread or foams [3].
• Creep Recovery: This modality applies a constant load for a defined period then monitors deformation recovery, separating instantaneous (elastic) from retarded (viscoelastic) recovery [3].
• Texture Profile Analysis (TPA): This specialized double-compression test simulates two bites, generating multiple texture parameters including hardness, cohesiveness, springiness, and chewiness [3] [4].

The selection of appropriate test modality depends fundamentally on the material characteristics and the specific texture attributes of interest. For instance, stress relaxation tests provide exceptional insight into the viscoelastic properties of baked goods, while single compression to failure better characterizes brittle materials.

Experimental Parameters and Data Interpretation

The accurate quantification of texture attributes requires careful control of experimental parameters that significantly influence results. Test speed represents a critical factor, as materials compressed at velocities exceeding their relaxation capacity demonstrate artificially elevated hardness values due to built-up stresses [2]. In Texture Profile Analysis, researchers have documented a logarithmic increase in "hardness" with speed up to 2 mm/s [2].

Contact geometry equally influences measurements, with neither die nor plate loading producing isotropic force distributions [2]. Die loading provides defined contact area but introduces shear components at the perimeter, while plate loading lacks defined initial contact area but avoids shear artifacts [2]. A sophisticated solution employing two dies of different diameters enables separation of shear and compressive forces, though this approach remains rarely implemented in routine testing [2].

Data interpretation from compression tests yields multiple quantitative parameters:

• Hardness: Maximum force during first compression cycle (N) [3]
• Fracture Force/Yield Point: Force at which material structure fails (N) [3]
• Stiffness: Resistance to deformation, often represented as slope of force-distance curve (N/mm) [5]
• Springiness: Degree to which sample returns to original height after deformation [3]
• Cohesiveness: Extent of material deformation before rupture [3]

Table 2: Quantitative Texture Parameters from Compression Testing

Parameter	Definition	Interpretation	Typical Range in Foods	Measurement Standard
Hardness	Peak force during first compression	Resistance to deformation	10-500 N	TPA, ASTM D695
Stiffness	Slope of initial linear portion of force-deformation curve	Material rigidity	50-420 kPa	Rheological measurement
Fracturability	Force at first significant break	Brittleness, crispiness	5-100 N	TPA modification
Springiness	Height recovery between first and second compression	Elastic recovery	0.1-0.9 ratio	TPA
Cohesiveness	Ratio of work during second compression to first compression	Internal bond strength	0.1-0.8 ratio	TPA

Recent research on plant-based and animal meats demonstrates the practical application of these parameters, with reported stiffness values varying from 419 kPa for plant-based turkey to 57 kPa for tofu, while animal turkey, sausage, and hotdog consistently ranked between these extremes [5]. This quantitative approach enables direct comparison across product categories and formulation variations.

Research Reagent Solutions and Materials

The implementation of robust compression testing methodologies requires specialized equipment and accessories designed to address specific material characteristics and research questions. The core instrumentation includes texture analyzers capable of precise force measurement and displacement control, with capacities ranging from 1N to 250N depending on application requirements [1]. These systems typically incorporate calibrated load cells, temperature control capabilities, and software for parameter configuration and data acquisition [3] [6].

Table 3: Essential Research Reagents and Equipment for Texture Analysis

Item	Function	Application Examples	Technical Specifications
Texture Analyzer	Applies controlled compression/tension while measuring force response	Universal texture testing	Capacity: 1-250N; Accuracy: ±0.5% of reading [6]
Cylinder Probes	General compression of soft solids	Cakes, gels, doughs	Various diameters; Stainless steel or Delrin [3]
Compression Platens	Uniform compression of flat materials	Packaging, foams, plastics	Multiple diameters available [3]
Ottawa Cell	Bulk compression of multi-particle samples	Grains, irregular pieces	Standardized bulk compression [3]
Powder Compaction Rigs	Assess granule or powder compressibility	Pharmaceuticals, powder blends	Granule strength assessment [3]
Kramer Shear Cell	Bulk shear and extrusion measurement	Meats, fruits, cereals	5 or 10-blade configurations [6]
Volodkevitch Bite Set	Simulates incisor teeth shearing	Meat, vegetables, crispy foods	Bite force measurement [6]
Probe Adapters	Secure mounting and alignment of probes	Universal application	Magnetic or quick-twist options [3]
Heavy Duty Platform	Stable base with temperature isolation	Temperature-sensitive samples	Prevents heat transfer [3]

Specialized fixtures extend application possibilities, with over 70 available probes, grips, and jigs designed for specific test scenarios [6]. The Volodkevitch Bite Set fixture exemplifies the translation of human action to measurable quantity, featuring upper and lower "teeth" that simulate incisor biting through food products [6]. Similarly, the Kramer Type Shear Cell measures bulk shear and extrusion forces through multiple blades that compress and extrude samples through bottom openings, particularly valuable for products with irregular shapes and sizes [6].

For specialized applications, custom fixtures can be developed, such as the burger consistency test jig that provides objective measurement of uncooked hamburger patties using a cylindrical probe that opens into an inverted cone shape [6]. Similarly, pasta stickiness assessment employs a rectangular probe that compresses cooked pasta sheets, measuring the withdrawal force required to separate the probe [6]. These specialized approaches demonstrate the adaptability of compression testing to diverse research needs across food categories.

Experimental Protocols for Texture Analysis

Texture Profile Analysis (TPA) Protocol

Texture Profile Analysis represents a standardized double-compression methodology that simulates the human mastication process, generating multiple quantitative texture parameters from a single test. The protocol employs two consecutive compression cycles with a defined pause between them, typically conducted to a strain of 0.75 (75% deformation) unless sample integrity requires modification [3] [4].

Sample Preparation:

• Prepare samples with uniform geometry, typically cylindrical shapes (diameter: 20-50mm, height: 10-20mm)
• Condition samples at standardized temperature (typically 20-25°C) for minimum 2 hours before testing
• For heterogeneous materials, increase replication (8-12 replicates) to account for variability [3]

Instrument Configuration:

• Select appropriate load cell capacity (typically 50-500N for solid foods)
• Use flat-platen compression probe with diameter exceeding sample diameter
• Set test speed to 1-2 mm/s compression, 5 mm/s return [3]
• Program compression distance to achieve 75% strain (or modified strain if necessary)
• Insert 3-5 second pause between compression cycles

Test Execution:

• Position sample centrally on base platform
• Initiate test sequence: first compression → hold → return → pause → second compression → return
• Record force-time curve throughout test sequence
• Repeat for minimum 6 replicates per sample type

Data Analysis:

• Hardness: Maximum force during first compression (N) [4]
• Cohesiveness: Ratio of work during second compression to first compression (Area₂/Area₁)
• Springiness: Distance of sample recovery between first and second compression (mm)
• Gumminess: Hardness × Cohesiveness (N)
• Chewiness: Gumminess × Springiness (N×mm)

Application in dysphagia research demonstrates TPA's clinical relevance, with studies confirming that pureed meat dishes with food-shaping agents showed significantly increased hardness and adhesiveness (p < 0.001) while maintaining cohesiveness, meeting International Dysphagia Diet Standardisation Initiative (IDDSI) Level 4 criteria for safe swallowing [4].

Fundamental Compression Testing Protocol

Fundamental compression tests characterize material properties under single compression, providing data suitable for calculating engineering parameters such as modulus and fracture stress.

Sample Preparation:

• Machine samples to parallel surfaces with height-to-diameter ratio of 1:1 to 2:1
• Measure sample dimensions precisely (±0.1mm)
• Condition at 23°C and 50% relative humidity for 48 hours where appropriate [7]

Test Configuration:

• Select compression platen larger than sample diameter
• Set test speed to 1 mm/s for controlled compression
• Determine appropriate end point: specific strain, force, or sample failure
• For stress relaxation tests: program compression to set strain followed by 60-120s hold

Execution and Data Collection:

• Pre-load sample to 0.1N to ensure contact
• Compress sample at constant crosshead speed
• Record force-deformation data at minimum 100Hz sampling rate
• For fracture testing, continue until visible sample failure

Data Analysis:

• Calculate engineering stress (Force/Initial cross-sectional area)
• Calculate engineering strain (Deformation/Initial height)
• Plot stress-strain curve
• Determine elastic modulus from initial linear region slope
• Identify yield point (deviation from linearity)
• Record fracture stress and strain where applicable

This protocol successfully differentiates texture characteristics in applied research, as demonstrated in studies of osmo-air-dried apple rings where compression-relaxation tests effectively discriminated between cultivars (Golden Delicious vs. Pink Lady) and drying temperatures based on fracturability indices and relaxation coefficients [8].

Implementation Workflow for Texture Testing

The strategic implementation of compression testing for texture analysis requires systematic approach from experimental design through data interpretation. The following workflow visualization outlines the critical decision points and methodological considerations:

Diagram 1: Texture Testing Implementation Workflow

This decision framework guides researchers through critical methodological choices based on their specific research objectives, material characteristics, and desired output parameters. The pathway begins with clear definition of research goals, then proceeds through sequential decisions regarding attribute selection, test protocol, and appropriate fixture selection.

Implementation requires consideration of several practical aspects:

• Replication: For homogeneous materials (gels, processed foods), 4-6 replicates may suffice, while heterogeneous foods (natural products, multi-component systems) require 8-12 replicates for statistical confidence [3]
• Environmental Control: Temperature and humidity standardization is critical, particularly for hygroscopic materials, with conditioning at 23°C and 50% relative humidity for 48 hours recommended by international standards [7]
• Speed Selection: Test speed should reflect application context, with slower speeds (0.5-1 mm/s) appropriate for fundamental properties and higher speeds (1-2 mm/s) better simulating eating conditions
• Data Reporting: Clearly specify whether results represent force (N) or stress (Pa), including contact geometry and calculation methods for stress values

This systematic approach ensures appropriate methodology selection for specific research contexts, enabling comparable, reproducible results across studies and laboratories.

Compression testing is a fundamental mechanical test used to determine the behavior of materials when subjected to compressive loads. In the context of solid food texture research, it provides invaluable insights into key mechanical properties such as firmness, hardness, elasticity, and fracturability. This quantitative approach allows researchers to move beyond subjective sensory evaluations to obtain reproducible, objective data on food texture. The core principle involves placing a food sample between two plates and applying a compressive force while precisely measuring the resulting deformation and stress. The data generated is essential for understanding how food products will behave during processing, packaging, and consumption, and is particularly crucial for product development, quality control, and shelf-life studies [9].

For researchers and scientists in food and pharmaceutical development, mastering compression testing is key to designing products with specific textural attributes, from the crisp snap of a biscuit to the desired firmness of a pharmaceutical tablet. The methodology allows for the quantification of a food's response to crushing loads, simulating everything from the bite of a consumer to the stresses encountered during transport and storage [3] [9].

Core Principles: The Interplay of Force, Distance, and Stress

The fundamental relationship in compression testing is described by the concepts of force, distance (deformation), and the derived property, stress. Force is the compressive load applied to the sample, measured in Newtons (N). Distance or deformation is the change in the sample's height as a result of the applied force, measured in meters (m). Stress is the internal resistance of the material to deformation, calculated as the applied force per unit cross-sectional area (Pascals, Pa).

The data collected from these measurements allows for the construction of a force-deformation or stress-strain curve, from which critical textural properties are derived. The analysis can be approached in two primary ways, as outlined in Table 1 [3].

Table 1: Fundamental Approaches to Compression Testing

Test Objective	Control Variable	Measured Variable	Typical Application in Food Research
Measure force to a chosen distance	Distance (Deformation)	Force	Establishing the force required to cause failure or irreversible deformation in a product (e.g., fracture force of a biscuit).
Measure distance to a chosen force	Force (Load)	Distance (Deformation)	Measuring the compactability of a sample (e.g., compressibility of food powders or granules).

Beyond these basic approaches, specialized tests like Stress Relaxation (holding a distance for a chosen time) and Creep Recovery (holding a force for a chosen time) provide deeper insights into the viscoelastic nature of many food materials, characterizing how they relax or recover after the application of a load [3].

Essential Instrumentation and Research Reagents

A robust compression test requires specific instrumentation and materials to ensure accurate and repeatable data. The core of the system is a Texture Analyser or Universal Testing Machine equipped with a calibrated load cell to measure force and a means to accurately measure crosshead displacement [3] [9].

Table 2: The Scientist's Toolkit for Food Compression Testing

Tool/Component	Function & Specification	Research Application
Texture Analyser / Universal Testing Machine	Applies controlled compressive force; core instrument.	The primary apparatus for performing all compression tests [3].
Load Cell	Measures the applied force; must be selected for an appropriate force range (e.g., 0.02 N to 2,000 kN).	Ensures accurate force measurement; critical for calculating stress [9].
Cylindrical Probe / Platen	A flat, rigid plate that compresses the sample.	General compression of soft solids like cakes, gels, and doughs; provides uniform compression [3].
Heavy-Duty Platform	Provides a flat, stable base for the sample.	Ensures consistent sample placement and minimizes instrument vibration [3].
Ottawa Cell	A specialized fixture for bulk compression of multiple particles or irregular pieces.	Provides a repeatable method for non-uniform samples like grains, nuts, or berries [3].
Powder Compaction Rig	Assesses the compressibility of granules or powders.	Used in research on food powders or the development of tablet-based food products [3].
Adhesives	Used to secure the sample to the platen and prevent slipping.	Essential for testing sticky or cohesive products like cheeses or chewy candies.
Lubricants	Applied to contact surfaces to minimize friction and barrelling effects.	Used to reduce friction between the sample and platen for more uniform deformation [10].
Food-Shaping Agents	Polysaccharide-based additives (e.g., dextrin, xanthan gum).	Used in model food systems to modify and standardize texture, as seen in dysphagia food research [11].

Experimental Protocols for Food Texture Analysis

Protocol 1: Basic Uniaxial Compression Test

This protocol is designed to determine fundamental properties like firmness and compressive strength of a solid food sample.

• Sample Preparation: Prepare samples into uniform cylindrical or cubical shapes. The aspect ratio (height/diameter) should be kept low to avoid buckling, typically not much more than 1:1. Record sample dimensions (diameter and height) precisely [10].
• Mounting: Center the sample on the heavy-duty platform beneath the Texture Analyser. For some materials, a lubricant may be applied to the contact surfaces to minimize friction and barrelling [10].
• Test Setup: Select a suitable compression platen and attach it to the instrument's load cell. Program the testing method:
  • Test Type: Compression
  • Mode: Measure force to a chosen distance, OR measure distance to a chosen force (see Table 1).
  • Pre-test Speed: 1.0 mm/s
  • Test Speed: 1.0 mm/s
  • Post-test Speed: 10.0 mm/s
  • Strain/Target Distance: Set based on the sample (e.g., 25%, 50%, or 80% deformation).
  • Trigger Force: 0.1 N (to define the point of contact).
• Execution: Initiate the test. The instrument will lower the platen, compress the sample, and retract.
• Data Analysis: From the resulting force-deformation curve, calculate:
  • Firmness/Hardness: The peak force (N) achieved during compression.
  • Compressibility: The area under the compression curve (J), representing the work done to deform the sample.
  • Fracture Force/Yield Point: The force (N) at which the sample structure fails (evident by a sudden drop in force).

Protocol 2: Texture Profile Analysis (TPA)

TPA is a two-cycle compression test that simulates the mastication process, providing insights into how a food behaves when chewed. It is a cornerstone of objective texture measurement in food science [11].

• Sample Preparation: Prepare samples as in Protocol 1. Consistency in sample size and shape is critical for reproducible TPA results.
• Mounting: Identical to Protocol 1.
• Test Setup: Program a two-cycle compression test.
  • Test Type: TPA (Two-cycle compression)
  • Mode: Measure force to a chosen distance.
  • Strain: Typically 50-75% of the sample's original height (this must be standardized for a given product).
  • Between Cycle Time: A brief pause (e.g., 1-5 seconds) is set between the two compression cycles.
• Execution: Initiate the test. The instrument will compress the sample, retract to the starting point, pause, and then perform a second identical compression.
• Data Analysis: Key parameters are extracted from the TPA curve generated by the two cycles [11]:
  • Hardness: Force at the first compression peak (N).
  • Springiness: The height the sample recovers to during the pause between cycles (mm or as a ratio).
  • Cohesiveness: The ratio of the area under the second compression curve to the area under the first compression curve (dimensionless).
  • Adhesiveness: The negative area of the first cycle, representing the work necessary to pull the compressing plunger away from the sample (J).

The following workflow diagram illustrates the key stages of a TPA experiment:

Diagram 1: Texture Profile Analysis (TPA) Experimental Workflow.

Table 3: Key Parameters Derived from Texture Profile Analysis (TPA)

TPA Parameter	Definition	Interpretation in Food Texture
Hardness	The peak force during the first compression cycle.	Perceived firmness or resistance to biting.
Springiness	The degree to which a sample returns to its original height after the first compression.	Elastic recovery or rubberiness.
Cohesiveness	The ratio of the area under the second compression curve to the area under the first compression curve.	The internal strength of the food's structure.
Adhesiveness	The negative force area for the first bite.	The work needed to overcome attractive forces between the food and other surfaces (e.g., tongue, palate); stickiness.
Chewiness	The product of Hardness × Cohesiveness × Springiness.	The energy required to masticate a solid food to a state ready for swallowing.
Gumminess	The product of Hardness × Cohesiveness (for semi-solids).	The energy required to disintegrate a semi-solid food to a state ready for swallowing.

Applications in Solid Food Research: A Case Study on Pureed Meat

Compression testing is vital in developing safe and palatable texture-modified foods. A recent study on hospital pureed meat dishes provides an excellent case study of its application. The research aimed to objectively classify texture according to the International Dysphagia Diet Standardisation Initiative (IDDSI) framework and evaluate the impact of a 1% food-shaping agent on textural consistency [11].

Methods:

• Samples: 18 common pureed meat dishes (pork, chicken, fish).
• Treatments: With and without a 1% w/w polysaccharide-based food-shaping agent.
• Analysis: Subjective IDDSI Level 4 tests (fork pressure, fork drip, spoon tilt) and objective Texture Profile Analysis (TPA) using a texture analyzer. TPA hardness values were interpreted via the Universal Design Foods (UDF) framework [11].

Results:

• Subjective vs. Objective Discrepancy: Only 33% (6/18) of original purees passed all IDDSI tests, despite visually resembling purees. However, TPA confirmed that all samples, with and without the agent, met the objective UDF Stage 4 hardness standard (<5 × 10³ N/m²) for safe swallowing.
• Effect of Shaping Agent: Adding the 1% shaping agent brought all samples into IDDSI compliance, enhancing textural consistency. TPA quantified this by showing a significant increase (p < 0.001) in hardness and adhesiveness, while cohesiveness remained unchanged [11].

Conclusion: This case highlights the critical importance of compression testing. It revealed a significant discrepancy between visual/subjective assessments and objective measurements, underscoring a potential safety risk in dysphagia diets. The study successfully demonstrated a dual-modality texture grading model (IDDSI + TPA) that enhances dietary safety and reproducibility, a model that can be applied across solid food research [11].

A Texture Analyser is a fundamental instrument in solid food texture research, providing objective quantification of mechanical properties that correlate with sensory perception. This device operates by deforming a sample in a controlled manner and precisely measuring its response. For researchers in food science and drug development, it transforms subjective textural attributes into reproducible quantitative data, making it indispensable for product development, quality control, and fundamental research. The process involves a mechanical action—typically compression, tension, or shear—followed by the collection of force, distance, and time data, which is output as a curve for analysis [12].

This application note details the working principles and protocols of Texture Analysis, with a specific focus on compression testing within the context of solid food research.

Mechanical Operation of a Texture Analyser

The core mechanical operation of a Texture Analyser involves a motor-driven travelling arm that moves upward or downward to apply a controlled deformation to a sample. This arm is fitted with a load cell, a transducer that measures the force response of the sample as it is being deformed [12].

The essential mechanical actions are as follows:

• Controlled Deformation: The instrument moves a probe or attachment to compress or stretch the sample at a predefined speed and to a predefined distance or force [12] [13].
• Force Response Measurement: The load cell continuously records the resisting force exerted by the sample throughout the deformation process. This force response is a direct result of the sample's mechanical properties [12] [14].
• Data Acquisition: The system simultaneously collects triplet data of Force, Distance, and Time. This data is the primary output, typically presented as a force-time or force-distance curve on a graph [12].

The system's versatility is achieved through a wide array of interchangeable probes and attachments that can be mounted on the arm and the base. These allow the instrument to perform various test types, including compression, extrusion, cutting, bending, and stretching, to mimic real-world interactions [12] [14].

Data Output and Interpretation

The raw data collected by the Texture Analyser is presented as a curve, where force is plotted against either time or distance. The shape of this curve provides a fingerprint of the sample's textural properties [12]. Key features of the graph are analysed to extract specific quantitative parameters.

Table 1: Key Textural Parameters Derived from a Force-Time Curve

Parameter	Definition	Correlates With Sensory Attribute
Hardness	The peak force during the first compression cycle [15] [14].	Firmness, resistance to biting.
Fracturability	The first significant peak in the first compression cycle (if present) [15] [14].	Brittleness, crunchiness.
Adhesiveness	The negative force area recorded when the probe withdraws from the sample [15] [16].	Stickiness, work required to overcome attraction to the probe.
Cohesiveness	The ratio of the positive force area during the second compression to that during the first compression [15] [16].	Internal bonding strength of the product.
Springiness	The ratio of the time (or distance) during the second compression to that during the first compression [15] [16].	The degree to which a product returns to its original shape after deformation.
Gumminess	The product of Hardness and Cohesiveness [15].	The energy required to disintegrate a semi-solid food to a state ready for swallowing.
Chewiness	The product of Hardness, Cohesiveness, and Springiness [15].	The energy required to masticate a solid food to a state ready for swallowing.
Resilience	The ratio of the decompression area to the compression area in the first cycle [15].	How quickly a material recovers from deformation.

Diagram 1: Texture Profile Analysis (TPA) workflow.

Detailed Experimental Protocol: Texture Profile Analysis (TPA)

Texture Profile Analysis is a double compression test that simulates the action of the jaw biting down on a food sample twice. It is one of the most widely used methods for quantifying multiple textural parameters in a single test [15] [16].

Principle

A bite-size piece of food is compressed twice in a reciprocating motion to simulate mastication. Data is extracted from the resulting force-time curve to provide parameters that correlate with sensory evaluation [15].

Materials and Equipment

Table 2: Research Reagent Solutions and Essential Materials

Item	Function/Description
Texture Analyser	A stable micro systems or equivalent instrument with a calibrated load cell suitable for the expected force range of the sample [12].
Compression Platen	A large, flat probe (e.g., 75 mm or 100 mm diameter) with a surface area greater than the sample to ensure true uniaxial compression [3] [15].
Heavy-Duty Platform	A flat, stable base to support the sample and raise it away from the instrument base to avoid temperature effects [3].
Sample Preparation Tools	Corers, cutters, and blades to prepare samples of consistent geometry (e.g., cylinders of 20mm height and 25mm diameter) [15].

Step-by-Step Procedure

• Sample Preparation: Prepare samples with consistent geometry. For many solid foods, cylindrical shapes are used. Record the exact height and diameter of each sample. Consistency is critical for reproducible results [15].
• Instrument Setup:
  • Mount the chosen compression platen to the instrument's arm.
  • Ensure the heavy-duty platform is clean and securely positioned.
  • Calibrate the instrument for force and distance according to the manufacturer's instructions.
• Method Definition (Typical TPA Settings):
  • Test Type: Compression
  • Pre-test Speed: 1.0 mm/s (Slower speeds are better for accurate trigger detection, especially for soft products) [15].
  • Test Speed: 1.0 mm/s (Should mimic chewing speed; must be identical to post-test speed for accurate cohesiveness calculation) [15].
  • Post-test Speed: 1.0 mm/s
  • Target Mode: Strain (Percentage deformation)
  • Strain: Typically 50-80% for solid foods (Must be sufficient to cause significant deformation, often fracture, to mimic mastication) [15].
  • Trigger Force: 5 g (May need adjustment to ensure contact is detected without "overshooting") [15].
  • Time Between Compressions: 5 seconds (Allows for partial sample recovery) [15].
• Test Execution:
  • Place the sample centrally on the platform.
  • Initiate the test method. The instrument will automatically perform the two-cycle compression test.
  • Repeat for a statistically appropriate number of replicates (8-12 for heterogeneous foods) [3].
• Data Analysis:
  • Use the instrument's software (e.g., Exponent Connect) to analyse the resulting force-time curve.
  • The software will automatically place anchors and calculate parameters like Hardness, Cohesiveness, Springiness, Gumminess, Chewiness, and Adhesiveness based on the definitions in Table 1 [15] [16].

Diagram 2: TPA force-time curve with key parameters. Cohesiveness = Area 4:6 / Area 1:3.

Application in Solid Food Research: Compression Testing

Compression testing is a fundamental application for solid foods, measuring a sample's resistance to being squashed. It can be performed as a simple single compression or as the more complex TPA described above [3].

Table 3: Compression Test Types and Applications for Solid Foods

Test Type	Protocol Summary	Measured Properties	Example Food Application
Force to a Distance	Compress sample to a fixed distance or % deformation [3].	Firmness, Hardness, Fracture Force.	Firmness of biscuits, bread, or fruit [3].
Stress Relaxation	Compress to a distance and hold for a defined time [3].	Relaxation behaviour, recovery, indication of freshness.	Springiness of cake, freshness of bread [3].
Creep Recovery	Apply a constant force for a time, then release and monitor recovery [3].	Instantaneous and retarded recovery, viscoelastic properties.	Behaviour of cheese or dense gels.
Cyclic Compression	Perform multiple continuous compression cycles on one sample [3].	Fatigue, work input, and recovery over multiple cycles.	Chewing simulation for meat analogues.

Critical Considerations for Compression Testing

• Probe Selection: The probe must have a surface area equal to or larger than the sample to ensure pure compression and not penetration or shear [3] [15].
• Sample Geometry: For single-piece compression, sample dimensions must be meticulously controlled, as variations in height and cross-sectional area directly impact the force results [3] [15].
• Test Speed: The rate of compression affects the force response. A slower speed allows for greater sample relaxation. The chosen speed should aim to reproduce conditions relevant to the test (e.g., chewing speeds) [15].
• Extent of Deformation: The chosen strain level must be appropriate. To mimic the destructive process of mastication, deformation values high enough to break the sample are often required (e.g., >70% for gels) [15].

The Texture Analyser functions as a precise and objective tool for quantifying the mechanical properties of solid foods. Its operation, from mechanical action to data output, provides researchers with a reliable method to correlate instrumental measurements with sensory perception. The detailed protocol for Texture Profile Analysis offers a standardized approach to deconstruct complex textural attributes like hardness, chewiness, and cohesiveness into quantifiable metrics. By adhering to rigorous methodological considerations, compression testing becomes a powerful technique for driving research and development, ensuring product consistency, and deepening the understanding of food structure and texture.

Distinguishing Between Compression, Penetration, and Shear Testing

In the field of solid food texture research, the objective measurement of mechanical properties is essential for understanding product performance, quality control, and consumer acceptance. Compression, penetration, and shear testing represent three fundamental mechanical testing approaches that simulate different aspects of how foods behave when subjected to external forces. These methods provide complementary data on textural properties, from bulk deformation and firmness to surface rupture and cutting resistance. Within a broader thesis on compression testing for solid food texture, understanding the distinctions, applications, and appropriate protocols for each method is critical for designing valid experiments and interpreting results accurately. This guide provides researchers with a clear comparative framework and detailed methodologies for implementing these tests effectively.

Fundamental Principles and Comparative Analysis

The following table summarizes the core characteristics, measured properties, and typical applications of compression, penetration, and shear testing in food research.

Testing Method	Fundamental Principle	Primary Measured Properties	Typical Food Applications
Compression Testing [17] [18] [19]	Applying a force that pushes a sample together from two ends, typically between parallel plates, to cause bulk deformation without rupture.	Firmness, Hardness, Stiffness, Springiness, Elastic Modulus, Compressibility, Cohesiveness [18] [20] [19]	Bread (firmness, springiness), Cheese (ripening, firmness), Cake, Butter, Gels (Bloom strength), Biscuits (fracture force) [18] [19]
Penetration Testing [21] [22]	Using a probe smaller than the sample to puncture or penetrate the surface and internal structure, often mimicking a bite.	Firmness (at surface and interior), Hardness, Rupture Strength, Yield Stress [21] [22]	Fruits (e.g., apple firmness), Gels (e.g., gelatin Bloom test), Products with skin or crust, Soft solids [18] [21]
Shear Testing [17] [23]	Applying parallel, offset forces to cause a "sliding past" or cutting failure within the material.	Shear Strength, Shear Modulus, Cutting Force, Toughness [17] [23]	Meat (tenderness), Fibrous vegetables, Adhesives (bond strength), Multi-layered products [17]

Detailed Experimental Protocols

Protocol for Uniaxial Compression Testing

Uniaxial compression is a foundational method for determining the bulk textural properties of solid foods.

• Objective: To determine the firmness, stiffness, and deformability of a solid food sample under axial load.
• Equipment & Reagents:
  • Universal Testing Machine (UTM) or Texture Analyser equipped with a calibrated load cell [24] [19].
  • Two flat, parallel compression platens (e.g., 75 mm or 100 mm diameter), typically made of stainless steel or aluminium [18].
  • Heavy-duty platform to ensure stability [18].
  • Temperature control chamber or pre-equilibration incubator, if testing temperature-sensitive samples [18].
• Sample Preparation:
  • Prepare samples with a uniform geometry (e.g., cylinders or cubes) where the cross-sectional area is smaller than the platen surface area [20].
  • For baked goods like bread or cake, a common sample size is a 25 mm cube. For butter or cheese, a cylindrical core of defined dimensions (e.g., 30 mm height x 20 mm diameter) is appropriate.
  • Condition samples to a consistent, specified temperature (e.g., 20°C for butter per ISO 16305) before testing, as temperature significantly influences texture [19].
• Test Procedure:
  • Setup: Mount the compression platens on the UTM. Secure the lower platen on the base and the upper platen on the moving crosshead. Select a load cell with a capacity suitable for the expected forces.
  • Calibration: Perform instrument calibration according to manufacturer guidelines.
  • Positioning: Place the sample centrally on the lower platen.
  • Test Parameters:
    • Pre-test speed: 1.0 mm/s (to approach the sample without impact).
    • Test speed: 1.0 - 2.0 mm/s (a standard rate for many food products).
    • Strain/Deformation: Compress the sample to a predefined strain (e.g., 25%, 50%, or 75% of its original height) or until a structural failure occurs.
    • Post-test speed: 10.0 mm/s (to return the crosshead to the start position).
  • Data Acquisition: Initiate the test. The software will record force (N) versus time (s) or distance (mm), generating a force-deformation curve.
• Data Analysis:
  • Firmness/Hardness is the peak force (N) achieved during the first compression cycle.
  • Stiffness is the slope (N/mm) of the initial linear region of the force-deformation curve.
  • For Texture Profile Analysis (TPA), a two-cycle compression test is performed. From the resulting curve, calculate Springiness (degree of recovery), Cohesiveness (strength of internal bonds), and Chewiness (Hardness × Cohesiveness × Springiness) [20] [24].

Protocol for Penetration/Puncture Testing

Penetration testing assesses the force required to rupture a surface or penetrate into a material, which is critical for products with skins or gels.

• Objective: To measure the force required for a probe to puncture a sample's surface and penetrate to a specified depth, indicating surface strength and internal firmness.
• Equipment & Reagents:
  • UTM or Texture Analyser [21].
  • Penetration probe, selected based on the application: cylindrical (blunt or sharp-edged), spherical, or conical (e.g., for Bloom testing) [21] [19].
  • A rigid sample support base with a hole smaller than the sample but larger than the probe to avoid interference [21].
• Sample Preparation:
  • For fruits (e.g., apples), use whole fruits with skin intact. A flat surface may be created by cutting a small portion from opposite sides.
  • For gels (e.g., gelatin), prepare in standard containers according to relevant standards (e.g., ISO 9665 for Bloom test). Gels must be matured at a controlled temperature (e.g., 10°C) for a precise duration (e.g., 17±1 hours) prior to testing [19].
• Test Procedure:
  • Setup: Mount the selected penetration probe on the UTM crosshead. Ensure the sample support base is secure.
  • Positioning: Place the sample on the support base, ensuring the probe will contact the desired location (e.g., the center of a gel or the equatorial region of a fruit).
  • Test Parameters:
    • Test speed: Standard speeds vary; 0.5 - 1.0 mm/s is common. The Bloom test for gelatin specifies 0.5 mm/s [19].
    • Penetration depth: This is typically set to a distance that ensures the probe has fully ruptured the surface and measured the characteristic peak force. For the Bloom test, the probe penetrates 4 mm into the gel.
  • Data Acquisition: Start the test. The instrument records the force as the probe penetrates the sample.
• Data Analysis:
  • The key parameter is the Peak Force (N) or Puncture Strength encountered during penetration, which often corresponds to the point of rupture of the surface or membrane [21].
  • For gels, this peak force is reported as the Bloom Strength or Gel Strength [19].

Protocol for Shear Testing

Shear testing measures the resistance of a food material to cutting forces, which is directly related to textural attributes like tenderness and toughness.

• Objective: To determine the force required to cut or shear a food sample.
• Equipment & Reagents:
  • UTM or Texture Analyser.
  • Shear fixtures: Warner-Bratzler blade for meats (a slotted blade with a V-shaped notch), Kramer Shear Cell (a multi-blade device that combines compression and shear), or a single flat blade [17] [23].
• Sample Preparation:
  • For meat tenderness testing using a Warner-Bratzler blade, core samples of uniform diameter (e.g., 1.27 cm) are taken parallel to the muscle fibers.
  • For the Kramer Shear Press, a defined weight of the product (e.g., particulate foods like peas or corn) is placed in the cell.
• Test Procedure:
  • Setup: Mount the shear fixture on the UTM crosshead.
  • Positioning: For a Warner-Bratzler test, place the meat core perpendicularly through the slot in the blade. For a Kramer cell, distribute the sample evenly in the base compartment.
  • Test Parameters:
    • Test speed: A crosshead speed of 50 - 500 mm/min is typical, with 200 mm/min being a common standard for the Warner-Bratzler test.
  • Data Acquisition: Start the test. The blade will move down, shearing through the sample. The force is recorded throughout the travel.
• Data Analysis:
  • The primary result is the Maximum Shear Force (N), which is the peak force required to shear the sample. For meat, a lower force indicates greater tenderness [23].

Workflow and Decision Framework

The following diagram illustrates the logical decision process for selecting the appropriate mechanical test based on the research objective and sample characteristics.

Diagram 1: Decision workflow for selecting a texture testing method.

The experimental workflow for performing a texture test, from sample preparation to data interpretation, follows a consistent path. The following diagram outlines this generalized protocol.

Diagram 2: Generalized experimental workflow for texture analysis.

Essential Research Reagent Solutions

Successful and reproducible texture testing relies on the use of appropriate equipment and accessories. The following table details key components of a texture analysis toolkit.

Item	Function/Description	Example Use Cases
Universal Testing Machine (UTM) [24]	A versatile instrument that applies controlled forces and measures material response. The core system for all tests.	Used for compression, penetration, and shear tests by changing fixtures.
Compression Platens [18]	Flat, rigid plates between which a sample is compressed for bulk property measurement.	Testing firmness of cheese, bread, and butter; performing TPA.
Cylindrical Penetration Probes [21] [19]	Probes of various diameters (e.g., 1/2 inch for Bloom test) used to puncture a sample.	Measuring fruit firmness; determining gel strength (Bloom).
Shear Fixtures [17] [23]	Blades or cells designed to apply a cutting force. Includes Warner-Bratzler blades and Kramer Shear Cells.	Objective measurement of meat tenderness; testing of fibrous products.
Temperature Control System [18]	An accessory to maintain or precondition samples to a specific temperature during testing.	Essential for testing temperature-sensitive materials like fats (butter) and gels.
Texture Analysis Software [19]	Software for controlling test parameters, acquiring data, and analyzing force-deformation curves.	Automated calculation of parameters like hardness, springiness, and cohesiveness.

Compression, penetration, and shear testing are distinct yet complementary mechanical tests that provide a comprehensive picture of solid food texture. Compression characterizes bulk deformation properties, penetration focuses on surface and localized failure, and shear quantifies resistance to cutting. The choice of method must be driven by the specific research question and the physicochemical nature of the food. By adhering to standardized protocols, using appropriate equipment, and applying a logical decision framework, researchers can generate robust, reproducible data. This objective data is indispensable for correlating instrumental measurements with sensory perception, ultimately driving innovation and ensuring quality in food product development.

Compression testing stands as a fundamental methodology in food texture research, providing critical quantitative data that bridges the gap between subjective sensory perception and objective mechanical properties. This technique, which involves applying controlled force to a food sample to measure its resistance to deformation, has evolved far beyond basic firmness measurements. Within the context of modern food science, compression testing enables researchers and product developers to quantify key textural parameters, predict sensory outcomes, and design foods for specific demographic needs and industrial applications. The precise instrumentation and standardized protocols now available have transformed compression testing into an indispensable tool across the food industry, driving innovations in product development, quality assurance, and specialized nutrition.

The following application notes and protocols detail how compression testing, particularly Texture Profile Analysis (TPA), is employed to solve real-world challenges—from ensuring batch-to-batch consistency in quality control laboratories to engineering novel food matrices for populations with specific mastication and swallowing needs. By correlating instrumental measurements with human sensory evaluation, this approach provides a robust framework for developing products that are not only safe and shelf-stable but also deliver superior consumer experiences.

Key Applications of Compression Testing

Compression testing, and specifically TPA, provides actionable data across the food product lifecycle. The table below summarizes the primary industry applications, key objectives, and the textural parameters most critical to each domain.

Table 1: Key Industry Applications of Compression Testing in Food Texture Research

Application Domain	Primary Objectives	Critical Textural Parameters	Relevant Food Products
Quality Control & Assurance	Ensure batch-to-batch consistency; verify compliance with internal or external standards; detect product defects.	Hardness, Fracturability, Springiness, Cohesiveness [15]	Biscuits, cakes, cheeses, processed meats, fresh produce [3]
Novel Food Development	Engineer tailored textures; optimize processing parameters; achieve target sensory profiles.	Hardness, Gumminess, Chewiness, Adhesiveness [15]	Plant-based analogues, senior-friendly foods, functional foods, reduced-fat products
Dysphagia & Clinical Nutrition	Ensure swallowing safety; create palatable, nutrient-dense foods that require minimal mastication.	Hardness, Adhesiveness, Cohesiveness [25] [4]	Pureed meats, thickened liquids, soft-solid gels, shape-retaining purees
Shelf-Life & Stability Studies	Monitor and predict textural changes over time; determine optimal packaging and storage conditions.	Firmness, Hardness, Springiness, Resilience [3]	Bakery products, fresh fruits, dairy products, prepared meals

Application in Action: Senior-Friendly and Dysphagia Foods

The development of safe and appealing foods for older adults with chewing or swallowing difficulties exemplifies a targeted application of compression testing. Research focuses on semi-solid foods, where texture is a critical safety and acceptability factor [25]. Instrumental texture analysis measures parameters like hardness and viscosity, which are correlated with sensory data to establish scientific profiles for these foods [25]. For instance, studies use TPA to ensure that pureed meats for hospital patients meet the hardness standards for "Universal Design Foods" Stage 4, signifying a texture that is safe for individuals with severe dysphagia [4]. This objective measurement is crucial, as visual and subjective assessments can be inconsistent; one study found that only 33% of pureed meat dishes passed the IDDSI (International Dysphagia Diet Standardisation Initiative) Level 4 criteria in their original form, a figure that rose to 100% after adding a food-shaping agent, a change verified by TPA [4].

Experimental Protocols

Core Protocol: Texture Profile Analysis (TPA) for Solid Foods

Texture Profile Analysis is a two-bite compression test that simulates the action of the jaw and provides multiple textural parameters from a single test.

1. Principle A texture analyzer compresses a bite-sized sample twice in a reciprocating motion, mimicking the chewing action. The resulting force-time curve is analyzed to extract parameters including hardness, cohesiveness, springiness, gumminess, and chewiness [15].

2. Equipment & Reagents

• Texture Analyzer: Equipped with a 50 kg or 100 kg load cell, depending on sample firmness.
• Compression Probe: A flat-faced cylindrical platen or probe with a diameter larger than the sample to ensure pure compression (e.g., 75 mm diameter for a 20 mm sample) [15] [3].
• Heavy-Duty Platform: A stable, flat base for sample placement.
• Calipers: For precise measurement of sample dimensions.
• Software: For controlling the instrument and analyzing the force-time curve.

3. Sample Preparation

• Prepare samples with uniform geometry (typically cylinders or cubes). A common size is 20 mm height x 20-30 mm diameter [15].
• Control for sample-to-sample variability by preparing a minimum of 6-12 replicates for homogeneous products and more for heterogeneous foods [3].
• Maintain consistent sample temperature, as it can significantly affect texture.

4. Instrumental Settings Key test parameters must be standardized for meaningful results. The following table provides a typical setup and the rationale for each parameter.

Table 2: Standard TPA Test Parameters and Rationale

Parameter	Typical Setting	Rationale & Impact
Test Mode	Compression	Uniaxial compression for bulk deformation [15]
Pre-test Speed	1.0 - 2.0 mm/s	Slow enough to accurately detect the sample surface and avoid "overshooting" the trigger force [15]
Test Speed	1.0 - 2.0 mm/s	Should mimic oral processing speed; faster speeds yield higher measured hardness [2] [15]
Post-test Speed	1.0 - 2.0 mm/s	Should be set equal to the test speed for accurate calculation of cohesiveness [15]
Target Deformation	70-80% of original height	Sufficient to cause significant structural breakdown, simulating mastication. Lower deformations (20-50%) are sometimes used but deviate from the original TPA principle [15]
Time Between Cycles	3 - 5 seconds	Simulates the pause between chews; affects springiness and cohesiveness measurements [15]
Trigger Force	0.05 N (5 g)	Ensures the probe contacts the sample before data recording begins [15]

5. Data Analysis Analyze the resulting force-time curve to calculate the following parameters [15]:

• Hardness: The peak force during the first compression cycle.
• Cohesiveness: The ratio of the positive area under the second compression cycle to that under the first cycle (Area 2 / Area 1). It represents how well the product withstands a second deformation.
• Springiness: The distance the sample recovers in height between the end of the first bite and the start of the second bite. It is a measure of elasticity.
• Gumminess: Hardness × Cohesiveness (for semi-solid foods).
• Chewiness: Hardness × Cohesiveness × Springiness (for solid foods).

6. Troubleshooting and Best Practices

• Probe Selection: A probe larger than the sample ensures uniaxial compression. A smaller probe introduces shear and puncture forces, altering the results [15].
• Deformation Level: The method should be developed using the hardest sample in a test set to ensure the instrument can achieve the required deformation for all samples [15].
• Parameter Relevance: Not all TPA parameters are relevant for every product. Researchers should identify which parameters are meaningful for their specific product rather than reporting all values indiscriminately [15]. For example, springiness is not a key characteristic of chocolate.

Advanced Protocol: Correlating Instrumental and Sensory Data

To ensure instrumental measurements predict human perception, a correlation protocol is essential.

1. Sensory Descriptive Analysis

• Train a panel (typically 6-12 individuals) to identify and quantify specific texture attributes using a 15-point scale [25] [26].
• Develop attributes for all stages of ingestion (e.g., first bite, chew down, swallow). One study on semi-solid foods for older adults developed 18 such attributes [25].

2. Statistical Correlation

• Perform TPA on the same set of samples evaluated by the sensory panel.
• Use multivariate statistical analyses, such as Multiple Factor Analysis (MFA), to identify correlations between instrumental TPA parameters (e.g., hardness, adhesiveness) and sensory attributes (e.g., sensory hardness, stickiness) [25].
• This correlation validates the instrumental method and allows it to be used as a proxy for sensory testing in future product development.

The Scientist's Toolkit: Essential Research Reagents and Materials

Successful compression testing requires careful selection of both equipment and consumables. The following table details key solutions and their functions.

Table 3: Essential Materials and Reagents for Food Compression Testing

Item	Function/Application	Examples & Notes
Texture Analyzer	Applies controlled compression/decompression cycles and measures force-distance-time data.	Stable Micro Systems TA.XT Plus; must be equipped with a calibrated load cell suitable for the expected force range [15] [3]
Compression Platens	Apply uniform force across the sample surface; used for bulk compression.	Available in various diameters (e.g., 50-100 mm) and materials (e.g., stainless steel, Delrin) [3]
Cylinder Probes (e.g., 25-50mm)	Used for general compression of soft solids like cakes, gels, and cheeses.	Should have a surface area equal to or larger than the sample [3]
Ottawa Cell	Provides a repeatable method for bulk compression of multi-particle or irregular samples (e.g., nuts, rice).	Overcomes variability between individual pieces [3]
Food-Shaping Agents	Polysaccharide-based additives to modify texture and shape retention in soft foods.	Used at ~1% (w/w) to help pureed meals pass dysphagia criteria by increasing hardness and cohesiveness [4]
Model Food Gels (Agar, Gelatin)	Standardized samples for method development and fundamental studies of food structure and breakdown.	Allow for controlled variation of texture by adjusting polymer concentration (e.g., 0.45%, 0.60%, 1.00% agar) [27]
Temperature Control System	Maintains consistent sample temperature during testing, a critical factor for many food textures.	Peltier cabinets or temperature-controlled rooms.

Experimental Workflow and Logical Relationships

The process of conducting and applying compression testing research follows a logical sequence from foundational setup to data interpretation and application. The diagram below outlines this comprehensive workflow.

Diagram 1: Compression Testing Research Workflow

The logical relationships between different test types and the resulting data can be complex. The following diagram maps how fundamental compression principles give rise to specific tests and, ultimately, to actionable product insights.

Diagram 2: From Test Type to Product Insight

Methodologies and Protocols: Implementing Compression Tests in the Lab

Texture Profile Analysis (TPA) is a fundamental instrumental technique that simulates the human mastication process to quantitatively characterize the textural properties of solid foods and pharmaceutical formulations. Originally developed by food scientists, TPA has become an essential methodology in both food science and pharmaceutical development for evaluating product performance, stability, and consumer acceptance. The two-bite test, a specific TPA protocol, mechanically reproduces the action of double chewing by applying two consecutive compression cycles to a sample, generating force-time curves from which multiple textural parameters are derived [28]. This approach provides researchers with objective, quantifiable data that correlates with sensory perceptions, enabling standardized quality control and product development across diverse applications from food design to medicated chewing gum (MCG) optimization [29] [4].

The significance of TPA extends beyond basic texture measurement to encompass critical aspects of product functionality and safety. In pharmaceutical applications, particularly for medicated chewing gums (MCGs), the efficiency of mastication directly impacts drug release profiles and bioavailability [29]. The mechanical action of chewing disrupts mechanical bonds within the gum matrix, facilitating the release of active pharmaceutical ingredients (APIs). Consequently, understanding and controlling textural properties through TPA is essential for ensuring consistent dosing and therapeutic efficacy in MCG products [29].

Theoretical Principles of the Two-Bite Test

Fundamental Mechanics

The two-bite test operates on the principle of simulating oral processing through controlled mechanical deformation. During testing, a sample undergoes two consecutive compression cycles with a defined pause between them, mimicking the human chewing action where food or gum is compressed between teeth, released, and compressed again [28]. This cyclic loading generates a characteristic force-time curve that encapsulates the material's response to mechanical stress, from which specific textural parameters are calculated. The test fundamentally measures how a material resists deformation and how it recovers between compressions, providing insights into its structural integrity and breakdown pattern.

The biomechanical basis of the test incorporates principles of mastication dynamics, including the crush/shear ratio that occurs during natural chewing. As the mandible moves with specific anatomical constraints, teeth apply complex forces combining compression and shear to comminute food or manipulate gum [29]. The Frankfort-mandibular plane angle (FMA) and Bennett angle (BA) are critical cephalometric measurements that influence actual chewing biomechanics by affecting jaw movement and occlusal relationships [29]. While simplified in instrumental TPA, the two-bite test effectively captures the essential mechanical actions that dominate the early stages of oral processing.

Key TPA Parameters Derived from the Two-Bite Test

The force-time curve generated during the two-bite test yields multiple quantitative parameters that define material texture. The table below summarizes the primary TPA parameters, their definitions, and sensory correlations:

Table 1: Fundamental TPA Parameters Derived from the Two-Bite Test

Parameter	Definition	Sensory Correlation	Typical Units
Hardness	Peak force during first compression cycle	Firmness perception	Force (N)
Fracturability	Force at first significant break	Crispness, brittleness	Force (N)
Cohesiveness	Ratio of second to first compression areas	Degree of structural integrity	Ratio (0-1)
Springiness	Distance recovered between cycles	Elastic recovery, rubberiness	Distance (mm)
Gumminess	Product of hardness and cohesiveness	Energy required to disintegrate	Force (N)
Chewiness	Product of gumminess and springiness	Work to masticate for swallowing	Energy (J)
Resilience	How quickly material recovers from deformation	Initial bounce-back	Ratio (0-1)

These parameters provide a comprehensive textural fingerprint of the tested material. For pharmaceutical applications, hardness and chewiness are particularly critical as they influence the chewing effort required and consequent drug release kinetics from MCGs [29]. In food science, cohesiveness and springiness are vital for designing products for specific populations, such as older adults with masticatory limitations [25].

Experimental Protocols

Standard Two-Bite Test Protocol for Solid Foods

The following protocol details the standard methodology for conducting TPA using a two-bite test approach for solid food samples, based on established procedures in food research [28]:

Sample Preparation

• Prepare samples with consistent dimensions (typically cubes or cylinders of 10-20mm height) to ensure uniform compression.
• For baked goods or porous solids, maintain consistent sample height to control for density variations.
• Condition samples to standard temperature (20°C ± 1°C) in a temperature-controlled room before testing [28].
• For heterogeneous products, ensure sampling from representative locations and maintain consistent orientation during testing.

Instrumental Configuration

• Use a texture analyzer (e.g., TA.XTplus/Stable Micro Systems) equipped with a load cell appropriate for expected force range (typically 5-50kg for solid foods).
• Employ a flat cylindrical plate probe (diameter 35-75mm, depending on sample size) for compression.
• Set crosshead speed to 1-2 mm/s for the approach stroke to simulate natural chewing speed.
• Program compression distance as a percentage of original sample height (typically 25-75%, depending on sample type).
• Include a pause of 1-5 seconds between compression cycles to simulate jaw opening between chews.

Test Execution

• Position sample centrally on the base plate of the texture analyzer.
• Initiate test program; the probe descends at defined speed until trigger force is achieved (typically 0.05-0.1N).
• First compression cycle proceeds to target strain level, then retracts to starting position.
• After defined pause period, second compression cycle repeats identical motion path.
• Minimum of eight replicates per sample type recommended for statistical significance [28].
• Record force-time data at minimum 200Hz sampling rate to capture detailed curve characteristics.

Specialized Protocol for Medicated Chewing Gum (MCG) Analysis

The evaluation of MCGs requires modifications to standard TPA protocols to account for their unique viscoelastic properties and the specific need to simulate mastication for drug release assessment [29]:

Sample Preparation and Conditioning

• Use complete gum pieces of standardized weight and dimensions (typically rectangular or cylindrical forms).
• Condition gums at 37°C for 30 minutes prior to testing to simulate oral temperature.
• For time-dependent analyses, test gums at multiple time points (0, 1, 5, 10 minutes) to simulate texture changes during chewing.

Instrumental Configuration for MCG

• Configure texture analyzer with a flat plate probe sized to exceed gum diameter.
• Set compression strain to 70-90% to simulate substantial deformation during chewing.
• Use higher crosshead speed (2-5 mm/s) to simulate vigorous chewing action.
• Program minimal pause between cycles (0.5-1 second) to simulate rapid chewing rhythm.
• Consider using artificial saliva immersion at 37°C for more biologically relevant conditions.

Mastication Simulation Parameters

• Implement multiple compression cycles (up to 50-100 cycles) to simulate extended chewing for drug release studies.
• Incorporate lateral motion or shear components if equipment permits, to better simulate natural mandibular movement.
• Vary compression parameters to simulate different chewing intensities based on population (e.g., elderly vs. adult chewing patterns).
• Correlate specific texture parameter changes (particularly hardness and chewiness) with API release rates measured through parallel dissolution testing.

Data Analysis and Interpretation

Quantitative Parameter Calculation

TPA parameters are calculated from the force-time curve generated during the two-bite test. The diagram below illustrates the key features of a typical TPA curve and the parameters derived from it:

Diagram 1: TPA curve analysis and sample classification

The mathematical relationships between these parameters allow for comprehensive material characterization. For MCGs, the chewing effort index, derived from the area under the first three compression cycles, correlates with the mechanical work required to initiate drug release [29].

Correlation with Sensory Evaluation

Instrumental TPA parameters must be validated against human sensory perception to establish their practical relevance. The table below demonstrates typical correlations between instrumental measurements and sensory evaluations:

Table 2: Correlation Between TPA Parameters and Sensory Attributes

TPA Parameter	Sensory Attribute	Correlation Strength (Typical R²)	Application Consideration
Hardness	Firmness	0.85-0.95	Critical for elderly nutrition [25]
Fracturability	Crispness/Crunchiness	0.75-0.90	Affects freshness perception [30]
Cohesiveness	Uniformity/Breakdown	0.70-0.85	Predicts bolus formation
Springiness	Elasticity/Rubberiness	0.80-0.92	Key for MCG acceptance [29]
Gumminess	Denseness/Heaviness	0.75-0.88	Important for satiety prediction
Chewiness	Chewing Effort/Masticatory Time	0.80-0.95	Directly impacts drug release from MCGs [29]

Multiple factor analysis (MFA) techniques have successfully demonstrated significant correlations between instrumental TPA data and sensory texture profiles, particularly for semi-solid foods tailored for older adults [25]. This validation is essential for utilizing TPA as a predictive tool for consumer acceptance and product optimization.

Research Applications and Case Studies

Pharmaceutical Applications: Medicated Chewing Gum (MCG) Development

TPA and the two-bite test play a crucial role in the development and quality control of MCGs, where texture directly influences drug release kinetics and patient compliance. The mechanical action of chewing disrupts the gum's structural integrity, facilitating API release through a combination of crushing and shearing actions [29]. Research has demonstrated that specific TPA parameters, particularly hardness and chewiness, correlate with API release rates, enabling formulators to optimize gum composition for desired release profiles.

Advanced research incorporates the two-bite test within a bionics product lifecycle management (PLM) framework to simulate human chewing more accurately. This approach considers critical anatomical factors including dental morphology, Frankfort-mandibular plane angle (FMA), and Bennett angle (BA) that influence natural mastication dynamics [29]. By simulating different population-specific chewing patterns (e.g., elderly vs. adult), researchers can design MCGs that ensure consistent drug release across diverse patient groups, potentially reducing API requirements by 10-20% while maintaining therapeutic efficacy [29].

Food Science Applications: Product Development and Quality Control

In food science, the two-bite test provides critical data for multiple applications:

Texture-Modified Foods for Dysphagia Management

TPA is essential for developing safe, texture-modified foods for individuals with swallowing difficulties. Research on pureed meat dishes demonstrates how TPA validates compliance with International Dysphagia Diet Standardisation Initiative (IDDSI) guidelines [4]. The addition of food-shaping agents (1% w/w) significantly increases hardness and adhesiveness while maintaining cohesiveness, ensuring products meet safety standards for dysphagia patients while maintaining aesthetic appeal [4].

Senior-Friendly Food Development

For older adult populations with masticatory limitations, TPA enables the development of foods with optimized texture properties. Studies on semi-solid foods correlate instrumental TPA measurements with sensory analysis, establishing reference standards for senior-friendly products [25]. All tested samples satisfied Korean Industrial Standards (KIS) level three criteria ("masticatable with tongue"), demonstrating the utility of TPA in standardizing textures for specific demographic needs.

Product Optimization and Shelf-Life Studies

The two-bite test provides quantitative data on textural changes during processing and storage. For example, TPA documents how mechanical (grinding, mincing), thermal (baking, frying), and chemical (marination, fermentation) processing methods alter food texture [31] [30]. This enables manufacturers to optimize processing parameters to achieve desired textural properties and monitor quality changes during storage, ensuring consistent consumer experience.

The Scientist's Toolkit: Essential Research Reagents and Materials

Successful implementation of TPA and the two-bite test requires specific instrumentation, reagents, and analytical tools. The following table details essential components for establishing this methodology in research settings:

Table 3: Essential Materials for TPA Two-Bite Test Research

Category	Specific Items	Function/Application	Technical Specifications
Core Instrumentation	Texture Analyzer (TA.XTplus/Stable Micro Systems)	Applies controlled compression and measures force response	5-500N load cell capacity [28]
Test Accessories	Cylindrical Plate Probe	Applies compression to samples	35-100mm diameter, depending on sample
Calibration Standards	Weight Set (certified)	Verifies force measurement accuracy	ISO 9001 traceable
Sample Preparation	Sample Corer/Cutter	Creates uniform geometries	Cylindrical or rectangular shapes
Temperature Control	Incubator/Environmental Chamber	Maintains sample temperature	20-37°C range, ±0.5°C stability [28]
Data Analysis	Texture Expert/TPA Software	Analyzes force-time curves, calculates parameters	Peak detection, area calculation algorithms
Specialized Reagents	Food-Shaping Agents (polysaccharide-based)	Modifies texture properties for specific applications	Xanthan gum, dextrin, glucomannan [4]
Simulated Fluids	Artificial Saliva (pH 7.4)	Simulates oral environment for MCG testing	Electrolyte composition matching human saliva

This toolkit enables researchers to establish standardized TPA methodologies across diverse applications. For pharmaceutical research on MCGs, additional specialized equipment may include humanoid chewing robots that incorporate more complex mandibular movements and anatomical factors for enhanced biological relevance [29].

Texture Profile Analysis using the two-bite test represents a sophisticated yet practical methodology for quantifying textural properties of solid foods and pharmaceutical formulations. By simulating the mastication process through controlled mechanical compression, this technique generates reproducible, quantitative data that correlates with sensory perception and product functionality. The continuing refinement of TPA protocols, including incorporation of anatomical factors such as FMA and BA angles in advanced simulations, enhances its predictive capability for real-world performance [29]. As product development increasingly prioritizes textural optimization for specific consumer populations, particularly in nutraceutical and pharmaceutical applications, TPA and the two-bite test will remain indispensable tools for researchers seeking to correlate mechanical properties with functional performance and consumer acceptance.

In the field of solid food texture research, compression testing serves as a fundamental technique for quantifying mechanical properties that correlate with sensory perception, quality, and safety. The selection of an appropriate probe or fixture is not merely a procedural step but a critical methodological decision that directly influences data accuracy, reproducibility, and biological relevance. The probe acts as the primary interface between the testing instrument and the food sample, defining the stress application and deformation mode. An ill-suited probe can lead to misrepresentation of material properties, while a properly selected one ensures that the measured parameters—such as hardness, fracturability, and elasticity—truly reflect the textural attributes experienced during human consumption. This document establishes application notes and protocols for selecting from three primary categories of compression interfaces: cylindrical probes, compression platens, and bespoke (custom-designed) fixtures, with specific application to solid food research within academic, industrial, and clinical development settings.

Understanding Probe Types and Their Fundamental Principles

The choice of probe geometry dictates the stress distribution within a food sample and the type of deformation it undergoes. Understanding the underlying principles of each probe category is essential for selecting the correct tool for a given research question.

Cylindrical Probes

Cylindrical probes are characterized by their circular cross-section and are typically used for penetration or compression tests. The key principle is that the small cross-sectional area relative to the sample results in high localized stress, causing the probe to penetrate or fracture the material.

• Principle: Penetration and localized compression.
• Stress Type: High, localized stress leading to sample yield or fracture.
• Typical Measurements: Firmness (e.g., of fruits, gels), yield point, fracture force, bloom strength.
• Ideal for: Samples with a skin or crust, soft solids, homogeneous gels, and when mimicking biting with incisors.

Compression Platens

Compression platens are flat, rigid plates, often larger than the sample itself, used to apply a compressive force across the entire sample surface. The principle is one of bulk compression, where the force is distributed evenly.

• Principle: Bulk compression and uniform deformation.
• Stress Type: Distributed, uniform compressive stress.
• Typical Measurements: Hardness, springiness, compressibility, elastic modulus, compactability.
• Ideal for: Self-supporting, regularly shaped solids like cheese blocks, bread loaves, biscuits, and tablets [3]. Spherically seated platens are available to compensate for minor non-parallelism in sample surfaces, ensuring uniform force application [32] [33].

Bespoke Fixtures

Bespoke fixtures are custom-engineered probes or assemblies designed for specific, non-standard testing scenarios where off-the-shelf probes are inadequate.

• Principle: Application-specific simulation.
• Stress Type: Defined by the custom geometry to mimic real-world conditions.
• Typical Measurements: Varies widely (e.g., crispiness from a multiple blade fixture, shell strength from a dome fixture).
• Ideal for: Mimicking a specific oral processing action (e.g., molar action), testing products with unique geometries, or adhering to a standardized test method that requires a specialized fixture [3] [34]. These are typically manufactured from stainless steel, aluminum, or engineering plastics to precise specifications [34].

Comparative Analysis of Probe Types

The following table provides a structured comparison of the three primary probe types for easy reference and selection.

Table 1: Comparative Analysis of Probe Types for Food Texture Analysis

Feature	Cylindrical Probes	Compression Platens	Bespoke Fixtures
Primary Mechanism	Penetration & Localized Compression	Bulk Compression & Uniform Deformation	Application-Specific Simulation
Stress Distribution	High & Localized	Distributed & Uniform	Defined by Custom Geometry
Typical Food Applications	Fruit firmness, gel strength, cheese hardness	Bread springiness, biscuit fracturability, tablet hardness	Chip crispiness, pasta firmness, confectionery stickiness
Key Measured Parameters	Firmness, Yield/Fracture Force	Hardness, Springiness, Compressibility, Elastic Modulus	Varies (e.g., Crispiness, Chewiness, Adhesiveness)
Sample Considerations	Samples with skin/crust; soft solids	Self-supporting, regular shapes	Irregular shapes; specific oral processing simulation
Relative Cost	Low	Low to Medium	High (due to custom design and manufacturing)

Experimental Protocols for Probe-Based Texture Analysis

Protocol 1: Texture Profile Analysis (TPA) using Compression Platens

Objective: To determine the textural properties of a solid food (e.g., cheese, bread, gel) through a two-bite compression test, simulating mastication.

Materials and Reagents:

• Texture Analyzer equipped with a 50-100 kg load cell.
• Flat compression platen (diameter larger than the sample) [3].
• Heavy-duty platform.
• Sample preparation tools (cork borer, knife, ruler).
• Solid food sample, prepared as a cylinder or cube with parallel surfaces.

Methodology:

• Sample Preparation: Prepare at least 10 replicates of the food sample with uniform dimensions (e.g., 20mm height x 20mm diameter cylinder). Record the exact dimensions for each sample.
• Instrument Setup:
  • Mount the compression platen to the texture analyzer's arm.
  • Calibrate the instrument for force and height.
  • Set the test parameters based on preliminary trials [35]:
    • Test Mode: Compression
    • Pre-test Speed: 1.0 mm/s
    • Test Speed: 1.0-5.0 mm/s (e.g., 2.0 mm/s for soft solids)
    • Post-test Speed: 5.0 mm/s
    • Target Mode: Strain (typically 50-75% of original height)
    • Trigger Force: 5 g (use a lower force, e.g., 0.5 g, for very soft samples) [35]
    • Data Acquisition Rate: 200 points per second (pps)
• Testing Procedure:
  • Center the sample on the heavy-duty platform beneath the platen.
  • Initiate the test. The platen will descend, compress the sample to the target strain, retract, and then perform a second compression cycle.
  • Remove the sample and record observations (e.g., fracture, recovery).
  • Repeat for all replicates.
• Data Analysis:
  • From the force-time curve, calculate the following TPA parameters [3]:
    • Hardness: Peak force of the first compression cycle.
    • Fracturability: First significant peak in the first compression cycle (if present).
    • Cohesiveness: Ratio of the area under the second compression curve to the area under the first compression curve (Area2/Area1).
    • Springiness: The distance the sample recovers between the end of the first cycle and the start of the second cycle.
    • Gumminess: Hardness × Cohesiveness (for semi-solid foods).
    • Chewiness: Gumminess × Springiness (for solid foods).

Protocol 2: Firmness and Puncture Strength using a Cylindrical Probe

Objective: To measure the firmness and skin strength of fruits and vegetables (e.g., apple, tomato).

Materials and Reagents:

• Texture Analyzer with a 5-50 kg load cell.
• Cylindrical probe (e.g., 2-8 mm diameter P/2 or P/8 probe).
• Heavy-duty platform.
• Sample preparation tools (knife, corer).
• Whole or prepared fruit/vegetable sample.

Methodology:

• Sample Preparation: For whole fruits, use intact skin. For prepared samples, create uniform cylinders or cubes with skin on one surface. A minimum of 15 replicates is recommended due to high biological variability.
• Instrument Setup:
  • Mount the cylindrical probe.
  • Set the test parameters:
    • Test Mode: Compression
    • Pre-test Speed: 2.0 mm/s
    • Test Speed: 1.0-2.0 mm/s (e.g., 1.0 mm/s to capture fracture event)
    • Post-test Speed: 10.0 mm/s
    • Target Mode: Distance (sufficient to penetrate the flesh, e.g., 8-10 mm)
    • Trigger Force: 5 g
    • Data Acquisition Rate: 500 pps (to accurately capture the fracture point) [35].
• Testing Procedure:
  • Place the sample on the platform so the probe will contact the skin or flesh surface perpendicularly.
  • Initiate the test. The probe will descend and puncture the sample.
  • Repeat for all replicates, ensuring consistent probe placement.
• Data Analysis:
  • From the force-distance curve, identify:
    • Firmness/Hardness: The maximum force (N) required to penetrate the sample.
    • Bioyield Point: The force at which the tissue initially yields, often seen as a small peak before the major rupture.
    • Puncture Strength: The force at the point of rupture of the skin or flesh.

The workflow for selecting and applying the appropriate probe and protocol is summarized in the following diagram:

The Scientist's Toolkit: Essential Research Reagent Solutions

Successful texture analysis relies on more than just the probe. The following table details key materials and reagents essential for conducting the experiments described in this document.

Table 2: Essential Research Reagents and Materials for Food Texture Analysis

Item	Function/Application	Specification Notes
Food-Shaping Agents	Polysaccharide-based additives (e.g., dextrin, xanthan gum) used to standardize texture and shape in dysphagia food research, enabling passage of IDDSI tests [4].	Typically applied at 1% (w/w) to pureed meats to achieve IDDSI Level 4 consistency and improve textural stability.
Standard Reference Materials	Certified food samples (e.g., specific cheese, gelatine gels) with known texture properties.	Used for instrument calibration and method validation to ensure inter-laboratory reproducibility and data accuracy.
Texture Analyzer Calibration Weights	Certified masses used to calibrate the load cell of the texture analyzer, ensuring force measurement traceability to international standards.	Weights should cover the entire operational range of the load cell and be recalibrated annually.
Rigid Plastic Specimens	Standardized cubes or cylinders (e.g., as per ASTM D695) used for verifying the mechanical alignment and accuracy of the testing system [32].	Helps distinguish between instrument error and probe/sample interaction effects.
Silicone Lubricants	High-vacuum grease or similar, applied minimally to compression platen surfaces.	Reduces friction between the platen and sticky samples, minimizing unwanted shear forces and ensuring pure compression.

The selection of the correct probe—be it a cylinder, platen, or bespoke fixture—is a foundational element of robust and meaningful food texture research. This selection, guided by the sample's physical properties and the specific textural attribute of interest, directly dictates the validity of the resulting data. The protocols and guidelines provided here, from experimental setup and parameter optimization to data interpretation, offer a framework for standardizing texture measurement practices. Adhering to these principles ensures that research outcomes are not only precise and reproducible but also relevant to real-world sensory experiences and clinical needs, such as the development of safer foods for populations with specific requirements like older adults [25] [4]. As the field advances, the synergy between standardized testing protocols and innovative custom fixture design will continue to deepen our understanding of food texture and its critical role in nutrition and health.

Compression testing serves as a fundamental methodology in solid food texture research, providing critical insights into the mechanical and rheological properties of food materials. Within this domain, three specialized procedures—Distance-to-Force, Stress Relaxation, and Creep Recovery—offer distinct and complementary data for characterizing material behavior. These tests are indispensable for researchers and product development professionals who require objective, quantifiable data to predict sensory outcomes, optimize formulations, and ensure product quality and consistency. This document details the application and protocol for each method, framed within the rigorous context of academic and industrial food science research.

Compression Testing in Food Texture Research

Fundamental Principles

Compression testing, in its basic form, involves applying a compressive force to a sample and measuring its response. A Texture Analyser or Universal Testing Machine (UTM) is typically used to lower a probe or platen onto a sample to a defined distance or force, while the instrument records the sample's resistance and deformation [3]. This process generates force-versus-distance or force-versus-time curves, from which key textural properties are derived.

The choice of test depends on the specific material property of interest. Distance-to-Force is ideal for measuring compactability, Stress Relaxation probes the time-dependent decay of force under constant strain, revealing a material's viscoelastic balance, and Creep Recovery examines the time-dependent recovery after the removal of a constant load, quantifying a material's ability to regain its original shape [3].

Key Concepts: Hardness vs. Firmness

A critical conceptual foundation for these tests is the clear distinction between hardness and firmness, as the terms are often misused.

• Hardness should be reserved for the stress or force required to cause a food structure to break or fracture, typically measured at high strain (large deformation) [2]. It is a property associated with destructive testing.
• Firmness describes an intermediate level of hardness, associated with non-destructive compression at low strains (typically around 0.1) where the material behavior is largely elastic [2]. This is the property assessed when gently squeezing fruit to gauge ripeness.

Instrumentally, results for these properties can be reported as force (Newtons, N) or stress (Pascals, Pa), with the latter taking the contact area into account [2]. This distinction is vital for accurate communication and interpretation of test results.

Test Procedure 1: Distance-to-Force Compression

Application and Objective

The Distance-to-Force test is primarily used to measure the compactability or compressibility of a material [3]. The objective is to determine the distance a probe must travel to achieve a pre-defined target force. This protocol is particularly suitable for powdered, granular, or soft solid foods where the degree of volume reduction under a standard load is a critical quality parameter, such as in the production of tablets from granules or the assessment of foam resilience [3].

Experimental Protocol

1. Equipment and Reagents:

• Texture Analyser/Universal Testing Machine (UTM): Configured for compression, with a calibrated load cell appropriate for the expected force range [3] [36].
• Compression Platens/Probes: Typically flat, cylindrical probes or platens with a surface area equal to or larger than the sample's surface to ensure pure compression principles apply [3].
• Heavy-Duty Platform: A flat, stable base to support the sample [3].
• Software: System software for controlling the test and acquiring data (e.g., VectorPro) [19].

2. Sample Preparation:

• Prepare samples with controlled dimensions to minimize variability. The sample must be smaller than the surface area of the probe and the test base for the duration of the test [20].
• For heterogeneous materials, a larger number of replicates (8-12) is recommended to achieve statistical confidence [3].
• Condition samples to a uniform temperature, as temperature can significantly affect mechanical properties.

3. Test Parameters:

• Set the test mode to "Compression to a Target Force."
• Define the target force based on the material's properties and the research objective.
• Set the test speed (e.g., 1-3 mm/s) and a trigger force (e.g., 5 g) to define the start of the test [37].
• The test concludes automatically when the specified force is achieved.

4. Data Acquisition and Analysis:

• The primary output is the distance (or deformation) traveled by the probe to reach the target force.
• This distance is a direct indicator of compactability; a larger distance indicates a more compressible material.
• The slope of the force-distance curve before the target force can also be analyzed as a modulus of deformability [3].

Table 1: Key Parameters for Distance-to-Force Compression Test

Parameter	Typical Value/Range	Notes
Target Force	Variable (e.g., 100 N)	Must be within the linear range for the material; prevents sample fracture.
Test Speed	1-3 mm/s	Constant speed is maintained throughout the test.
Probe Type	Cylindrical Probe, Platen	Must have larger surface area than the sample.
Primary Output	Distance (mm)	The measure of compactability.

Figure 1: Workflow for a Distance-to-Force Compression Test

Test Procedure 2: Stress Relaxation

Application and Objective

Stress Relaxation tests are used to study the time-dependent force decay in a material when it is subjected to a sudden deformation that is then held constant [3] [37]. The objective is to quantify the viscoelastic nature of the material by observing how the internal stresses dissipate over time. This is a key method for assessing the "freshness" of bakery products (e.g., bread staling) or the relaxation behavior of recoverable materials like gels and certain cheeses [3].

Experimental Protocol

1. Equipment and Reagents:

• Texture Analyser/UTM: Must have the capability to hold a constant distance and record force over time.
• Compression Probe/Platen: Selected based on sample geometry (e.g., a large flat platen for uniform compression) [3].
• Software: For controlling complex test cycles and recording high-frequency force-time data.

2. Sample Preparation:

• Samples should be prepared to uniform dimensions and conditioned to a stable temperature.
• The test is often performed within the material's linear viscoelastic region, which may require preliminary strain sweeps to determine [37].

3. Test Parameters:

• Set the test mode to "Compression with Hold."
• Define the target strain or compression distance (e.g., compress to 30% of the sample's original height) [37].
• Set the hold time (e.g., 60 seconds) during which the deformation is maintained [3] [37].
• Set the test speed for the compression and return phases.

4. Data Acquisition and Analysis:

• The primary output is a force-time curve during the hold period.
• Key metrics include:
  • Initial Peak Force (F₀): The force immediately after compression.
  • Force at End of Hold (Fₜ): The force after a specified time (t).
  • Percent Stress Relaxation: Calculated as [(F₀ - Fₜ) / F₀] × 100.
• A higher percentage of relaxation indicates a more pronounced viscous component, as the material's structure yields and flows to relieve the stress.

Table 2: Key Parameters for Stress Relaxation Test

Parameter	Typical Value/Range	Notes
Target Strain	20-50%	Often determined from preliminary linearity tests.
Hold Time	30-300 s	Must be sufficient for the force to plateau or decay significantly.
Primary Output	Force Decay (N)	Plotted over time; used to calculate % Relaxation.
Key Metric	% Stress Relaxation	Quantifies the viscous component of the material.

Test Procedure 3: Creep and Recovery

Application and Objective

Creep and Recovery testing investigates how a material deforms over time under a constant load (creep) and how much of that deformation is recovered once the load is removed (recovery) [3] [38]. This test is crucial for characterizing the viscoelastic properties of semi-solid foods like dressings, creams, and processed meats [38] [37]. It quantifies a material's ability to retain shape under a sustained load, which relates to sensory attributes like "spreadability" and "mouthfeel."

Experimental Protocol and Theoretical Modeling

1. Equipment and Reagents:

• Texture Analyser/UTM: Capable of applying a constant force and monitoring deformation with high precision.
• Compression Probe/Platen: As required by the sample.
• Software: For complex test sequencing and data analysis.

2. Sample Preparation:

• Uniform sample preparation and temperature control are critical.
• The applied stress should be within the linear viscoelastic range to allow for meaningful model fitting.

3. Test Parameters:

• Set the test mode to "Creep" or a custom sequence.
• Define the creep phase: Apply a constant force for a set time (e.g., 200 g for 60 seconds) [37].
• Define the recovery phase: Remove the force completely and monitor the sample's deformation for an equal or longer period.

4. Data Acquisition, Analysis, and Modeling:

• The output is a deformation-time (or compliance-time) curve spanning both creep and recovery phases.
• Compliance, J(t) = γ/σ (deformation per unit stress), is often plotted to normalize the data [38].
• The data is frequently modeled using the Burger model (a series combination of a Maxwell unit and a Kelvin-Voigt unit), which provides a mechanical analog for the material's behavior [38] [39].

The compliance for the Burger model during creep is given by: J(t) = 1/G₀ + 1/G₁[1 - exp(-t·G₁/η₁)] + t/η₀ [38] Where:

• G₀ is the instantaneous elastic modulus (Maxwell spring).
• η₀ is the residual viscosity (Maxwell dashpot).
• G₁ and η₁ are the elastic modulus and internal viscosity of the Kelvin-Voigt unit, representing the retarded elastic response.

Key metrics derived from the test and model include:

• Instantaneous Compliance (1/G₀): The immediate elastic deformation.
• Retarded Compliance (1/G₁): The time-dependent elastic deformation.
• Viscous Flow (t/η₀): The permanent, non-recoverable deformation.
• Final Percentage Recovery: Calculated as [(Jmax - J∞) / Jmax] × 100, where Jmax is the maximum compliance at the end of creep and J_∞ is the residual compliance after infinite recovery time [38].

Table 3: Key Parameters and Model Elements for Creep and Recovery Test

Parameter/Element	Description	Sensory Correlation
Applied Stress	Constant force applied during creep phase.	Magnitude of the "load" (e.g., spreading, biting).
Maxwell Spring (G₀)	Represents instantaneous elastic deformation.	Initial "firmness" or resistance.
Kelvin-Voigt (G₁, η₁)	Represents delayed, recoverable deformation.	"Springiness" or "chewiness".
Maxwell Dashpot (η₀)	Represents irreversible, viscous flow.	"Spreadability" or permanent deformation.
% Recovery	Overall ability to regain shape.	Product "resilience" and "freshness".

Figure 2: Mechanical Analog of the Burger Model for Creep and Recovery

The Scientist's Toolkit: Research Reagent Solutions

The following table compiles essential equipment and fixtures required for implementing the compression test procedures described in this document.

Table 4: Essential Research Equipment and Fixtures for Compression Testing

Item	Function/Description	Example Applications
Texture Analyser/UTM	Electromechanical instrument that applies controlled forces/displacements and records data.	Core instrument for all compression tests; available as table-top (e.g., zwickiLine) or floor models (e.g., AllroundLine) with varying force capacities [36] [19].
Cylindrical Probes	General-purpose compression probes for soft solids.	Measuring firmness of cakes, gels, and cheeses [3].
Compression Platens	Flat, rigid plates for uniform compression.	Testing packaging materials, foams, and flat food samples [3] [36].
Ottawa Cell	A specialized fixture for bulk compression of multi-particle or irregular samples.	Providing a repeatable method for testing non-uniform pieces like nuts, berries, or grains [3].
Powder Compaction Rigs	Fixtures designed to assess the compressibility of granules or powders.	Common in pharmaceuticals and food R&D for tablet formation [3].
Temperature Control System	Accessory to maintain sample temperature during testing.	Essential for testing temperature-sensitive samples like fats and chocolate [3].
Calibrated Load Cells	Sensor that measures the applied force; must be sized appropriately for the test.	Accurate force measurement is fundamental; range should be selected so the test force falls between 20-80% of cell capacity [3] [40].

In the field of solid food texture research, instrumental compression testing serves as a cornerstone for quantifying critical mechanical properties that dictate sensory perception, product stability, and quality control. The reliability and reproducibility of these measurements are fundamentally governed by three key test parameters: approach speed, strain, and dwell time. These parameters directly influence the material's response during testing, affecting outcomes such as hardness, cohesiveness, and springiness [16]. Precise control and understanding of these variables are therefore essential for generating meaningful data that can correlate with human sensory evaluation and predict in-mouth behavior [41]. This application note provides a detailed framework for researchers and scientists to standardize compression testing methodologies, complete with quantitative guidelines and experimental protocols.

Theoretical Foundations of Key Parameters

Texture Profile Analysis (TPA) is a widely used double-compression test that simulates the action of biting and chewing food [16]. The resulting force-time curve provides quantitative data on multiple textural attributes. The values of these attributes are highly sensitive to the chosen test settings, making parameter selection a critical step in method development.

• Approach Speed: This parameter, often related to strain rate, defines the velocity at which the probe approaches and compresses the sample. It influences how a material deforms. For visco-elastic materials, a higher strain rate can result in increased resistance to deformation, making the material appear harder or more brittle [42].
• Strain and Deformation: Strain refers to the degree of compression applied to the sample, typically expressed as a percentage of its original height. It directly determines the stress level within the material and is crucial for triggering specific structural failures, such as fracture.
• Dwell Time: This is the period during which the compressive force is held at a constant level (or at a constant strain) between the compression and decompression cycles. Its primary role is to allow for stress relaxation and de-aeration within the sample, which can impact the material's recovery and the measured values in the second compression cycle [16] [42].

The independent manipulation of these parameters is crucial for decoupling their individual effects on material properties. Studies using compaction simulators have shown that while strain rate significantly impacts tablet tensile strength across different material classes, the effects of dwell time are often marginal at the timescales relevant to industrial manufacturing (e.g., 10–100 ms), except for highly visco-elastic materials or at impractically long durations [42].

Quantitative Data and Parameter Specifications

The following tables summarize the typical ranges and effects of the key test parameters for different food material classes.

Table 1: Recommended Parameter Ranges for Common Solid Food Categories

Food Category	Approach Speed (mm/s)	Typical Strain (%)	Dwell Time (s)	Key Measured Attributes
Hard & Brittle (e.g., Hard Biscuits)	1 - 2	50 - 75 [16]	0 - 1	Hardness (Fracturability), Low Cohesiveness [16]
Soft & Elastic (e.g., Gelatin Gels, Cake)	1 - 2	70 - 80 [16]	1 - 5	Hardness, High Springiness, High Cohesiveness [16]
Gummy & Sticky (e.g., Cheese, Marshmallow)	0.5 - 1	70 - 75	1 - 3	High Adhesiveness, Gumminess, Chewiness [16]
Powder Compacts (e.g., Pharmaceutical Tablets)	Varies (Strain Rate: 1 - 100 s⁻¹) [42]	N/A	0.01 - 0.1 [42]	Tensile Strength, Cracking Propensity [42]

Table 2: Impact of Parameter Variation on TPA Results

Parameter	Increase	Primary Effect on TPA Results
Approach Speed / Strain Rate	Increase	Increased measured hardness for visco-elastic materials; higher risk of brittle fracture [42].
Strain (Deformation)	Increase	Increased hardness and fracturability; may reduce springiness if material structure is compromised.
Dwell Time	Increase (to extreme durations)	Allows for stress relaxation and de-aeration, which can reduce springiness and potentially increase cohesiveness in specific materials [42].

Experimental Protocols

General Texture Profile Analysis (TPA) Protocol

This protocol outlines the standard double-compression test for solid foods using a texture analyzer.

4.1.1 Research Reagent Solutions and Essential Materials

Table 3: Key Equipment and Consumables for TPA

Item	Function/Description
Texture Analyzer	Universal testing machine capable of controlled compression and data acquisition (e.g., Stable Micro Systems, Mecmesin models) [16] [19].
Compression Plates/Probes	Typically a flat cylindrical plate (e.g., 50-100 mm diameter) for uniform compression. A spherical probe may be used for penetration tests [19].
Load Cell	Sensor to measure force; capacity should be matched to sample hardness (e.g., 50N for soft foods, 1kN for hard foods) [19].
Sample Preparation Tools	Coring tools, blades, and rulers for preparing samples with uniform geometry (e.g., cylinders 20mm height x 20mm diameter).
Temperature Control Chamber	(Optional) For tests requiring precise temperature control.

4.1.2 Step-by-Step Methodology

• Sample Preparation: Prepare a minimum of six replicates. For solid foods, cut into uniform cylinders or cubes. Control sample temperature as it significantly affects texture [19].
• Instrument Setup:
  • Mount the appropriate flat plate probe and load cell.
  • Calibrate the instrument for force and distance according to the manufacturer's instructions.
  • Set the test parameters based on the food category (refer to Table 1). A typical setting is:
    • Pre-test speed: 1.0 mm/s
    • Test speed: 1.0 mm/s
    • Post-test speed: 1.0 mm/s
    • Target strain: 75% (or 25% compression of original height)
    • Dwell time: 1-5 seconds (depending on material visco-elasticity)
    • Trigger force: 0.1 N (to identify the sample surface) [16] [11].
• Test Execution: Place the sample centrally on the base plate. Initiate the test. The probe will perform two complete compression cycles with a defined dwell time between them, generating a force-time curve.
• Data Analysis: Analyze the resulting TPA curve to extract quantitative parameters [16]:
  • Hardness: Peak force of the first compression cycle.
  • Cohesiveness: Ratio of the positive area under the second compression to the first compression (Area2 / Area1).
  • Springiness: The distance the sample recovers between the end of the first bite and the start of the second bite (Time2 / Time1).
  • Adhesiveness: The negative area of the first cycle, representing the work required to pull the probe away from the sample.

Protocol for Isolating the Effects of Dwell Time and Strain Rate

This advanced protocol, adapted from pharmaceutical research, is ideal for investigating the fundamental sensitivity of a material.

4.2.1 Workflow Diagram

The following diagram illustrates the logical workflow for a systematic study of these parameters.

4.2.2 Key Steps:

• Experimental Design: Create a full-factorial experimental matrix that independently varies dwell time and strain rate (punch velocity) over a wide range. For example, dwell times from 10 ms to 100 s and strain rates from 1 s⁻¹ to 100 s⁻¹ [42].
• Equipment: Use a hydraulic compaction simulator or a texture analyzer that allows for custom, programmable compression profiles to decouple these parameters [42].
• Analysis: For each test, eject the compact and measure its mechanical strength (e.g., via diametrical compression for tensile strength) and visually inspect for defects like capping or lamination [42].

The rigorous control of approach speed (strain rate), strain, and dwell time is non-negotiable for generating accurate, reproducible, and meaningful texture data in solid food research. While general guidelines provide a starting point, the optimal parameters are inherently material-dependent. The protocols outlined herein empower researchers to not only apply standard methods but also to deconstruct and understand the fundamental rheological behavior of their specific materials. By adopting this systematic approach, scientists and product developers can enhance the reliability of their data, improve correlations with sensory outcomes, and accelerate the development of high-quality food and pharmaceutical products.

Texture Profile Analysis (TPA) is a fundamental empirical method in food science that objectively quantifies the textural properties of solid foods through a double compression test, simulating the action of teeth during mastication [41] [43]. For the emerging cultured meat industry, TPA provides a critical tool for benchmarking products against conventional meat, establishing key targets for consumer acceptance, and guiding process optimization during scalable production [43] [44]. This case study details the application of TPA to evaluate the mechanical and textural properties of cultured meat products against traditional meat benchmarks, providing standardized protocols and analytical frameworks for researchers and product developers.

The imperative for this research stems from clear market dynamics: consumer acceptance of cultured meat is highly dependent on its ability to replicate the sensory experience of conventional meat, with texture and flavor being the two most critical factors [43]. Quantitative texture analysis allows producers to identify gaps in product development and refine processes before market launch. Research indicates that when cultivated meat matches conventional meat's texture, consumer willingness to adopt increases significantly, with one study showing participants rated their willingness to replace farm-raised chicken with cultivated chicken at 8 out of 10 [43].

Experimental Objectives and Design

Primary Research Objectives

This study aims to:

• Quantify and compare key textural parameters (hardness, cohesiveness, springiness, chewiness) between cultured meat products and conventional meat benchmarks using standardized TPA methodology.
• Establish quantitative target ranges for mechanical properties that cultured meat should achieve to successfully mimic conventional meat products.
• Evaluate the influence of different scaffold technologies and cell culture processing parameters on the resultant textural properties of cultured meat.

Experimental Workflow

The following workflow diagrams the logical progression of the TPA benchmarking process, from sample preparation through data interpretation.

Materials and Methods

Research Reagent Solutions and Essential Materials

The following table details key reagents, materials, and instrumentation essential for conducting TPA on cultured meat products.

Table 1: Essential Research Reagents and Materials for Cultured Meat TPA

Item Category	Specific Examples & Specifications	Primary Function in TPA Workflow
Cultured Meat Samples	Engineered tissue on edible scaffolds (e.g., alginate-based, TVP/soy-protein, nanofiber scaffolds) [45] [43]	Primary test material whose mechanical properties are being characterized and benchmarked.
Conventional Meat Controls	Commercial beef burgers, chicken breast, Frankfurt-style sausages (varying fat % and quality grades) [44] [46]	Provides the benchmark textural profile that cultured meat aims to replicate.
Texture Analyzer	Stable Micro Systems TA.XT Plus or equivalent; calibrated load cell (e.g., 5 kg to 50 kg capacity) [47] [3]	Core instrument that performs the double compression test and records force-time data.
Compression Attachments	Flat cylindrical platen or rectangular platen; surface area larger than sample [3]	Applies uniform compressive force to the sample during testing.
Sample Preparation Tools	Cylindrical cork borers or custom dies (e.g., 10-20 mm diameter); precision balance; temperature probe [44] [3]	Creates standardized, dimensionally consistent samples for reproducible testing.

Detailed TPA Protocol for Cultured Meat

Sample Preparation Protocol

• Thermal Processing: Cook all meat samples (cultured and conventional) to a predetermined safe internal temperature using a standardized method (e.g., grilling, water bath). Record the final internal temperature. For burger samples, cooking to 71°C internal temperature is typical [44].
• Cooling and Equilibration: Allow cooked samples to cool to room temperature (approx. 25°C) for 30 minutes to stabilize texture and prevent excess steam during testing.
• Standardized Coring: Using a cylindrical cork borer or sharp-edged metal die, extract cores from consistent locations within each sample. Target a uniform height-to-diameter ratio to minimize shear effects; a 1:1 ratio is often ideal [3].
• Dimension and Weight Measurement: Precisely measure the diameter and height of each core using digital calipers. Record the weight of each core using a precision balance.

Texture Analyzer Configuration

• Instrument Setup: Install a flat compression platen on the texture analyzer. Ensure the instrument is calibrated for force and distance according to manufacturer specifications [3].
• Test Parameter Programming:
  • Test Type: Two-cycle compression (Texture Profile Analysis).
  • Pre-Test Speed: 1.0 mm/s
  • Test Speed: 1.0 mm/s
  • Post-Test Speed: 1.0 mm/s
  • Target Strain: 50% or 70% of original sample height (this must be consistent for all samples in a study) [43] [44].
  • Time Between Compressions: 3 seconds (allows for partial sample recovery).
  • Trigger Force: 0.05 N (ensances the test starts upon slight contact with the sample).
  • Data Acquisition Rate: 200 points per second.

TPA Execution and Data Acquisition

• Sample Placement: Position a single sample core centrally on the heavy-duty platform of the texture analyzer, directly beneath the compression platen.
• Test Initiation: Initiate the TPA sequence via the instrument software. The platen will descend, compress the sample to the predefined strain, retract during the pause period, and then compress a second time.
• Replication: Conduct a minimum of 8-12 replicates per sample type to account for biological and processing heterogeneity [3]. Use fresh sample cores for each replicate.

Results and Data Analysis

TPA Parameter Calculation

The force-time curve generated from the two-cycle compression is analyzed to extract fundamental TPA parameters. The diagram below illustrates the relationship between the curve and the calculated parameters.

• Hardness: The peak force (N or Pa) during the first compression cycle (F₁) [2]. Note that reporting in stress (Pa) requires accurate knowledge of the initial contact area [2].
• Cohesiveness: The ratio of the area of work during the second compression to the area of work during the first compression (A₂/A₁). It represents the sample's internal bond strength [44].
• Springiness: The ratio of the distance the sample recovers during the pause between compressions to the original deformation (T₂/T₁). It measures elasticity and how well the sample springs back after deformation [3].
• Chewiness: The product of Hardness × Cohesiveness × Springiness. For semisolid foods, this parameter represents the energy required to masticate the sample for swallowing [44].

Benchmarking Data: Cultured vs. Conventional Meat

Quantitative TPA data from recent studies demonstrates the progress and challenges in replicating conventional meat textures. The following tables consolidate key findings.

Table 2: TPA Parameter Comparison for Burger Products (Cooked) [44]

Sample Type	Hardness (N)	Chewiness (N)	Cohesiveness (Ratio)	Springiness (Ratio)
High Beef Content Burger (>95%)	40 - 60	18 - 30	0.50 - 0.65	0.70 - 0.85
Low Beef Content Burger (<81%)	55 - 80	15 - 25	0.40 - 0.55	0.60 - 0.75
Plant-Based Burger Analogue	65 - 100	10 - 20	0.30 - 0.45	0.50 - 0.70
Target for Cultured Beef Burger	40 - 60	18 - 30	0.50 - 0.65	0.70 - 0.85

Table 3: Texture Comparison for Frankfurt-Style Sausages [46]

Texture Parameter	Conventional Sausage	Cultured Meat Sausage	Statistical Significance
Hardness	Within comparable range	Within comparable range	No significant difference (p > 0.05)
Chewiness	Within comparable range	Within comparable range	No significant difference (p > 0.05)
Stiffness (Young's Modulus)	Lower	Higher	Statistically significant (p < 0.05)

Discussion

Interpretation of Benchmarking Results

The data reveals that cultured meat products are achieving significant milestones in texture replication. The study on Frankfurt sausages found that key TPA parameters like hardness and chewiness showed no significant difference from conventional products, which is critical for initial consumer "bite" perception [46]. This indicates that for comminuted, processed meat products like sausages and burgers, cultured meat is nearing the required textural benchmarks.

However, the same study noted a higher stiffness (Young's Modulus) in the cultured sample, suggesting differences in the microstructural organization of the proteins and the interplay between cells and scaffold that may affect the mouthfeel beyond the first bite [46]. The benchmarking of burgers clearly shows that chewiness and hardness are the most pertinent properties for distinguishing between high-quality traditional meat and alternatives, providing cultured meat developers with clear target ranges [44].

Technical and Methodological Considerations

A significant challenge in TPA is the inconsistency in reporting, with some studies using force (N) and others stress (Pa) for parameters like hardness [2]. Researchers must clearly state which unit they are using and ensure the contact geometry is well-defined if reporting stress. Furthermore, test parameters like compression speed and degree of deformation (strain) must be carefully standardized and reported, as they directly impact the results [2]. A strain of 50-75% is common, but it must be consistent within a study to allow valid comparisons.

For cultured meat specifically, the choice of scaffold material is a major determinant of texture. Scaffolds must provide the necessary structure for cell growth and, upon processing, yield the desired fibrousness and mechanical properties [45]. Innovations in edible, food-grade scaffolds derived from plant proteins or other bio-compatible materials are key to advancing this field [45] [43].

This application note demonstrates that Texture Profile Analysis is an indispensable, quantitative tool for guiding the development of cultured meat. By providing objective measurements of key textural attributes, TPA allows researchers to benchmark their products against conventional meat, identify specific gaps in hardness, chewiness, or cohesiveness, and systematically optimize cell culture parameters and scaffold designs to close those gaps. The established target values for beef burgers and the successful replication of sausage texture profiles mark significant progress for the industry.

Future work should focus on correlating these instrumental TPA measurements with data from trained human sensory panels to build predictive models of consumer acceptance. Furthermore, as the field advances towards more structured, whole-muscle cuts, TPA protocols will need to evolve to characterize the anisotropic and layered textures of products like steaks and chicken breasts, pushing the boundaries of solid food texture research.

Texture modification is a critical intervention for managing oropharyngeal dysphagia (OD), a condition affecting over 60% of institutionalized older adults [48]. The International Dysphagia Diet Standardisation Initiative (IDDSI) framework provides standardized classifications for texture-modified foods (TMFs), yet its subjective assessment methods can lead to inconsistencies in food preparation and safety risks [11] [49]. This case study explores the application of back-extrusion testing (BET) as an instrumental method to objectively characterize semi-solid foods within the broader context of compression testing for solid food texture research.

Back-extrusion provides quantitative texture profiling that correlates with sensory perception, enabling more reliable development of safe, appealing foods for older adults with chewing and swallowing difficulties [50] [51]. This paper presents standardized protocols, analytical frameworks, and practical applications of BET for classifying TMFs according to IDDSI levels, with particular focus on ensuring dietary safety through reproducible texture measurement.

Background and Significance

The Challenge of Texture Modification in Dysphagia

Semi-solid foods for older adults must balance swallowing safety with nutritional adequacy and sensory appeal. Traditional puréed diets often result in reduced energy intake of 17-37% compared to regular diets due to poor visual appeal and monotony [48]. Texture-modified foods are typically classified as IDDSI Level 4 (puréed) or Level 3 (liquidized), requiring sufficient cohesion to form a safe bolus without crumbling or separating [48] [11].

Current clinical practice relies heavily on subjective IDDSI tests including fork pressure, fork drip, and spoon tilt tests. However, recent research found only 33% (6 of 18) of hospital puréed meat dishes met all IDDSI Level 4 criteria in their original form, highlighting significant consistency problems in texture modification [11].

The Role of Instrumental Texture Analysis

Instrumental texture analysis addresses key limitations of subjective evaluations by providing:

• Quantitative parameters for standardization across production batches
• Correlations with sensory perception for predictive food design
• Objective quality control for clinical food safety
• Reproducible measurements for research and development [11] [49]

Back-extrusion testing specifically measures resistance to flow, making it ideal for semi-solid foods that cannot be tested through conventional compression methods [50].

Back-Extrusion Methodology

Fundamental Principles

Back-extrusion testing simulates the mechanical action of swallowing by measuring the force required to extrude a semi-solid food upward through an annular gap between a disc plunger and container wall [50]. This method characterizes the consistency and flow resistance of viscous products, which are key determinants of swallowing safety [50] [49].

The test employs a cylindrical probe with a flat disc that compresses the sample, forcing it to flow backward through the gap between the disc and container walls. The resulting force-time curve provides multiple texture parameters that can be correlated with sensory properties [50].

Key Texture Parameters from Back-Extrusion

Table 1: Key Parameters Measured in Back-Extrusion Testing

Parameter	Definition	Sensory Correlation	Clinical Significance
Firmness	Maximum force required to extrude sample	Perceived hardness	Indicates bolus formation effort
Consistency	Area under compression curve	Thickness perception	Relates to swallowing effort
Cohesiveness	Maximum negative force during probe return	Structural integrity	Affects bolus cohesion during swallowing
Work of Cohesion	Area of negative region during return	Resistance to breakdown	Predicts oral processing requirements

Research has demonstrated significant correlations between these instrumental measurements and sensory texture attributes. A study profiling semi-solid foods for older adults found positive correlations between force-related parameters and sensory hardness, enabling predictive texture design [51].

Experimental Protocol for Back-Extrusion Testing

Equipment and Setup

Table 2: Research Reagent Solutions for Back-Extrusion Testing

Item	Specifications	Function	Application Notes
Texture Analyzer	Stable Micro Systems TA.XT Plus with 5kg load cell	Applies controlled compression	Calibrate with 2kg weight before use [49]
Back-Extrusion Rig	35mm diameter aluminum disc (P/35)	Extrudes product through annular gap	Smaller discs for thicker products [50]
Sample Container	Methacrylate cell, 50mm inner diameter	Holds sample during testing	Fill to 50mm height for consistency [49]
Temperature Control	Environmental chamber or water bath	Maintains test temperature	Test at 5°C, 20°C, and 40°C to simulate consumption [49]

Sample Preparation Protocol

• Sample Collection: Obtain commercial TMFs or prepare laboratory formulations. For pureed meats, use food processor with consistent processing time [11].
• Shaping Agent Incorporation: Where applicable, incorporate polysaccharide-based shaping agents (e.g., 1% w/w mousse and jelly powder) to improve texture stability [11].
• Temperature Equilibration: Temper samples to testing temperature (5°C, 20°C, or 40°C) using water bath or environmental chamber [49].
• Container Filling: Transfer sample to methacrylate container, filling to height of 50mm. Avoid air bubbles during transfer [49].
• Replication: Prepare minimum of three replicates per sample to ensure statistical reliability [49].

Instrumental Parameters

The following standardized testing parameters should be used for semi-solid food characterization:

• Trigger Force: 0.049 N
• Test Distance: 30 mm (60% strain level)
• Pre-test Speed: 10 mm/s
• Test Speed: 5 mm/s
• Post-test Speed: 10 mm/s
• Data Acquisition Rate: 200 points per second [49]

Data Analysis and Interpretation

The force-time curve generated during back-extrusion testing provides the fundamental data for texture characterization. Analysis should focus on:

• Firmness: Peak force (N) during compression phase - indicates initial resistance to deformation
• Consistency: Positive area under curve (N×mm) - represents work required to extrude sample
• Cohesiveness: Maximum negative force (N) during probe return - measures structural recovery
• Work of Cohesion: Negative area under curve (N×mm) - quantifies energy needed to overcome sample adhesion [50] [49]

Recent research demonstrates that BET can achieve 76.8% classification accuracy for IDDSI levels when using multiple texture parameters (BET2 method), significantly outperforming single-parameter approaches (66.1% with BET1) [49].

Applications in Food Texture Research

IDDSI Level Classification

Back-extrusion testing enables quantitative classification of TMFs according to IDDSI framework. Studies show a progressive increase in firmness and consistency values as IDDSI level increases, with significant differences between levels [49]. This allows for objective verification of texture levels, reducing reliance on subjective assessments that can vary between clinicians and facilities.

Table 3: Back-Extrusion Texture Ranges for IDDSI Food Levels

IDDSI Level	Food Texture Description	Firmness Range (N)	Consistency Range (N×mm)	Clinical Application
Level 3	Liquidized	0.5-1.5	50-150	Moderate swallowing impairment
Level 4	Puréed	1.5-3.0	150-300	Significant oral processing difficulty
Level 5	Minced & Moist	3.0-5.0	300-500	Transition to more solid textures

Formulation Optimization

Back-extrusion provides critical data for optimizing food-shaping agents and thickening systems. Research demonstrates that adding 1% food-shaping agent significantly increases hardness and adhesiveness (p<0.001) while maintaining cohesiveness, ensuring IDDSI compliance [11]. This enables precise formulation adjustments to achieve target texture profiles while maintaining palatability.

Correlation with Sensory Perception

Instrumental texture measurements must correlate with sensory experience to be clinically meaningful. Studies establishing correlations between BET parameters and sensory attributes enable predictive texture design without extensive sensory panels [51]. For example, firmness values show strong correlation with perceived hardness, while cohesiveness relates to structural integrity during oral processing.

Back-extrusion testing represents a robust methodological approach within the broader compression testing framework for solid food texture research. Its ability to provide quantitative, reproducible texture characterization makes it particularly valuable for developing safe, appealing foods for older adults with dysphagia.

The standardized protocol outlined in this paper enables researchers to:

• Objectively classify TMFs according to IDDSI framework
• Optimize formulation with food-shaping agents and thickeners
• Establish correlations between instrumental measurements and sensory perception
• Ensure batch-to-batch consistency in clinical food production

Integration of back-extrusion testing into routine quality control and research protocols will advance the development of personalized nutrition solutions for aging populations, addressing both safety and acceptability challenges in dysphagia management. Future research should focus on expanding database correlations between instrumental measurements and clinical swallowing outcomes to further refine texture modification strategies.

Optimizing Test Protocols and Overcoming Common Challenges

In the field of solid food texture research, compression testing is a fundamental technique for quantifying mechanical properties such as hardness, fracturability, and elasticity. The reliability of this data is paramount, as it influences product development, quality control, and scientific conclusions. Achieving high levels of repeatability and reproducibility is a central challenge, heavily dependent on rigorous standardization of experimental conditions. This application note details the critical protocols for controlling three pivotal factors—sample preparation, temperature, and instrument alignment—to ensure the generation of consistent and reliable compression data in food texture research.

Critical Factor 1: Sample Preparation

Sample preparation is the primary source of variability in texture analysis if not properly controlled. Standardizing the size, shape, and geometry of test specimens is essential because these parameters directly determine stress distribution and fracture properties within the material [52].

Guidelines for Representative and Consistent Samples

• Reproducible Preparation: Samples must be prepared in a consistent manner using very sharp instruments to minimize pre-test deformation [52].
• Handling and Defects: Minimize handling to prevent altering the sample's surface or internal structure. Samples with structural defects should be avoided, as they lead to high result variation [52].
• Anisotropy Consideration: Many biological materials are anisotropic, meaning their mechanical properties vary with the direction of loading (e.g., meat fibers, vegetable tissues). The orientation of the test specimen must be consistent across all replicate tests [52].
• Moisture Loss: For many food materials (fleshy plant material, meat, bakery products), rapid moisture loss can significantly alter mechanical properties. This can be minimized by reducing exposure to air, sealing the specimen, or testing in a constant humidity environment [52].
• Testing Schedule: Test all samples within a short timeframe to avoid changes in properties over time due to aging or drying out [52].

Standardization of Sample Dimensions

The size and shape of test specimens are critical. Specimens that are too small can yield different results from larger ones due to the "size effect" [52].

Table 1: Impact of Sample Dimension Variability on Cross-Sectional Area

Target Surface Area	Actual Surface Area	Increase in Cross-Sectional Area	Expected Force Increase
10 mm x 10 mm	11 mm x 11 mm	21%	≈20%

Larger samples generally have a lower relative effect from minor dimension differences. The use of templates, moulds, or cutting guides (e.g., a Twin Blade Sample Preparation Tool) is strongly recommended to standardize dimensions [52].

Approach for Different Product Types

Table 2: Sample Preparation Strategies for Different Food Types

Food Type	Characteristics	Recommended Preparation Strategy
Natural/Non-Homogeneous (e.g., fruit, meat)	High inherent variability, anisotropic structures	Test in 'bulk' (a defined weight or number of pieces) to achieve an averaging effect. Cut reproducible geometric shapes (cylinders, cubes) to eliminate shape as a variable [52].
Formulated/Processed (e.g., cheese, biscuits, gels)	Consistent size and shape, more homogeneous	Individual specimen testing is suitable. Ensure consistent dimensions and avoid structural defects [52].
Multi-Particulate/Bulk (e.g., cereals, irregular snacks)	Pieces differ in size and shape	Use a bulk compression test (e.g., with an Ottawa Cell or Kramer Shear Cell) to get an averaging effect [52] [18].

Critical Factor 2: Temperature Control

Temperature has a profound influence on the rheological and fracture properties of most food materials. Even minor fluctuations in ambient temperature can affect the stiffness of plant and animal tissues, while larger fluctuations impact the brittleness of products like pasta, snacks, and bakery items [52].

Impact of Temperature on Material Properties

• Biological Tissues: Stiffness is affected by minor ambient temperature changes [52].
• Fat-Based Products (e.g., butter, margarine): The fat crystal network is highly temperature-sensitive, directly influencing spreadability and firmness [19] [53].
• Gels (e.g., gelatine, hydrocolloids): Gel strength (Bloom strength) is critically dependent on precise temperature control during conditioning and testing [19].
• Frozen Products: Small temperature changes can dramatically affect ice crystal size and the resulting mechanical damage to the material, leading to large variations in results [52].

Protocols for Temperature Management

• Pre-conditioning: Samples should be equilibrated to the target test temperature in a controlled environment (e.g., incubator, water bath) prior to testing. This is critical for temperature-sensitive products like fats and gels [52] [19].
• Controlled Test Environment: Perform tests in a temperature-controlled room or use a Temperature Control Cabinet (TCC) that encloses the test platform and sample. This is essential for tests that are lengthy or for highly sensitive products [52].
• Reporting: Always report the temperature at which tests were conducted to allow for proper interpretation and replication [52].

Critical Factor 3: Instrument and Fixture Alignment

Proper mechanical alignment of the texture analyzer, its probes, and fixtures is fundamental to ensuring that the applied force is axial and that the probe contacts the sample as intended. Misalignment can cause off-center force application, leading to bending moments, uneven stress distribution, and significant variability in results.

Consequences of Misalignment

• Non-axial loading induces shear forces, invalidating fundamental compression assumptions.
• Irregular fracture or yielding of the sample.
• Increased coefficient of variation in replicate measurements.

Alignment Verification and Best Practices

• Use of Accessories: Employ a heavy-duty platform with concentric alignment rings to centralize the sample [18]. Use magnetic or quick-twist probe adapters for secure and repeatable attachment, ensuring the probe is perpendicular to the test surface [18].
• Visual Inspection: Before testing, visually confirm that the probe is parallel to the sample surface and that the sample is centered.
• Regular Calibration: The instrument should be regularly calibrated for force and distance using certified weights and distance standards to maintain measurement accuracy [54].

Experimental Protocol: Texture Profile Analysis (TPA) of a Solid Food Gel

This protocol outlines a standardized TPA test, a double compression cycle test, which is widely used to simulate mastication and characterize multiple textural properties [55].

Research Reagent Solutions

Table 3: Essential Materials and Equipment for TPA

Item	Function/Justification
Texture Analyzer	A system capable of controlled compression and data acquisition (e.g., TA.XTplus, ZwickiLine) [54] [55].
50 N Load Cell	Suitable for measuring the expected force range of a soft to medium-firm gel [55].
Cylindrical Compression Platen (e.g., 50-75 mm diameter)	A flat, rigid plate larger than the sample to ensure uniform compression [18].
Sample Preparation Tools	Sharp cork borers or custom cutting dies to create cylindrical samples with consistent dimensions.
Ruler/Digital Caliper	For precise measurement of sample height and diameter.
Temperature-Controlled Incubator/Water Bath	For pre-conditioning gels to a standardized temperature (e.g., 10°C) prior to testing [52] [19].

Detailed Methodology

• Sample Preparation:

  • Prepare a homogenous gel according to the formulation.
  • Using a sharp cylindrical cutter, extract at least six (6) replicate samples from the gel batch [55].
  • Trim the samples to a consistent height (e.g., 20 mm) using a microtome blade or a sharp knife with a guiding template [55].
  • Measure and record the exact diameter and height of each sample.

• Pre-conditioning:

  • Place all samples in a temperature-controlled incubator or water bath at 10°C for a minimum of 2 hours to ensure thermal equilibrium [52].

• Instrument Setup:

  • Calibrate the texture analyzer for force and distance as per the manufacturer's instructions.
  • Attach the selected cylindrical platen to the instrument's load cell.
  • In the instrument's software, configure a "TPA" or "Double Compression" test with the following typical parameters [55]:
    • Test Speed: 1.0 mm/s
    • Target Strain: 50% (The probe will compress the sample to 50% of its original height)
    • Time Between Cycles: 5 seconds
    • Trigger Force: 0.05 N (to define the point of initial contact)

• Test Execution:

  • Quickly transfer one pre-conditioned sample from the incubator to the center of the texture analyzer's base platform.
  • Initiate the test. The instrument will perform two complete compression cycles with a 5-second pause between them.
  • Repeat for all sample replicates.

• Data Analysis:

  • The software will generate a force-time curve from which key parameters are derived [54] [55].
  • Hardness: Maximum force (N) during the first compression cycle (F1).
  • Cohesiveness: Ratio of the area under the second compression cycle (A2) to the area under the first compression cycle (A1). (A2/A1).
  • Springiness: The distance (mm) the sample recovers between the end of the first cycle and the start of the second cycle.
  • Chewiness: The product of Hardness × Cohesiveness × Springiness (N × mm).

The workflow for this experimental protocol is summarized in the following diagram:

Repeatable and reliable compression testing in food texture research is an achievable goal that demands meticulous attention to experimental detail. By implementing the standardized protocols outlined for sample preparation, temperature control, and instrument alignment, researchers can significantly reduce variability, strengthen the validity of their data, and ensure that results are comparable across different studies and laboratories.

Resolving the Hardness vs. Firmness Ambiguity in Data Reporting

In the field of solid food texture research, precise terminology is paramount for generating reproducible and meaningful data. The terms "hardness" and "firmness" are frequently used interchangeably in subjective sensory evaluation, yet they represent distinct mechanical properties in instrumental texture analysis. This ambiguity poses a significant challenge in data reporting, interpretation, and cross-study comparison within scientific literature. This application note, framed within a broader thesis on compression testing, provides explicit definitions, standardized protocols, and clear reporting frameworks to resolve this common terminology conflict. By establishing a unified lexicon and methodology, we aim to enhance the clarity and reliability of texture research for scientists and product developers.

Defining Mechanical Properties

In instrumental texture profile analysis (TPA), hardness and firmness are derived from a force-time curve generated during a compression test. Their distinct definitions are as follows:

• Hardness is a primary textural parameter obtained from the first compression cycle of a TPA. It is quantitatively defined as the peak force (Fₕ) achieved during the first compression cycle. It represents the maximum resistance of the food sample to deformation and is often synonymous with the sensory perception of "hardness" [56] [19].
• Firmness is a more general term often used to describe the force required to achieve a specific deformation rather than the absolute peak force. It is frequently measured in a single compression test (not necessarily a full TPA) or a puncture test. For instance, firmness can be reported as the force measured at a pre-defined strain (e.g., 25% or 50% compression) or the force required to puncture a gel or fruit surface [19].

The table below summarizes the key differences between these two parameters for easy comparison and correct application.

Table 1: Key Characteristics of Hardness vs. Firmness

Characteristic	Hardness	Firmness
Technical Definition	Peak force during the first compression cycle [56]	Force at a specified deformation or distance [19]
Typical Test	Texture Profile Analysis (TPA)	Single Compression or Puncture Test
Represents	Resistance to irreversible deformation (failure)	Resistance to reversible/elastic deformation (stiffness)
Data Reported As	Maximum force (Fₕ)	Force at X% compression or at X mm distance
Sensory Correlation	Force required to crush a food between molars	Force required to compress a food between tongue and palate

Experimental Protocols for Measurement

Standardized Texture Profile Analysis (TPA) for Hardness

This protocol details the measurement of hardness using a two-bite compression test, suitable for a wide range of solid foods.

1. Equipment and Reagents

• Texture Analyzer: System equipped with a load cell appropriate for the sample (e.g., 5-50 kg for candies or cheeses) [56].
• Software: System control and data analysis software (e.g., Exponent Connect, VectorPro) [57] [19].
• Probe: Flat-faced cylindrical plate (e.g., 50-100 mm diameter), typically made of aluminum or acrylic.
• Platform: Flat, rigid base plate.
• Sample Preparation Tools: Precision slicer, cork borers, and digital calipers for standardizing sample dimensions.

2. Sample Preparation

• Prepare samples with uniform size and shape (e.g., cylinders or cubes). A 1:1 height-to-diameter ratio is often recommended [56].
• Control sample temperature precisely, as it significantly impacts texture. For example, butter firmness is tested under controlled conditions per ISO 16305 [19].
• For porous or non-homogeneous products, ensure sample size is large enough to be representative.

3. Instrument Parameters The following parameters should be explicitly reported in all experimental methods [58] [56] [19].

Table 2: Standard TPA Instrument Parameters for Hardness Measurement

Parameter	Recommended Setting	Justification
Pre-test Speed	1.0 - 2.0 mm/s	Ensures consistent initial contact without impact.
Test Speed	1.0 - 2.0 mm/s	Standard rate simulating oral processing; faster speeds increase apparent hardness.
Compression Strain	50-75% of original height	Must be sufficient to initiate structural failure but not total collapse.
Post-test Speed	1.0 - 2.0 mm/s	Consistent retraction for second compression cycle.
Trigger Force	0.05 - 0.10 N	Ensures test initiation upon sample contact; avoids false starts.
Pause Between Cycles	3-5 seconds	Allows for partial sample recovery to measure springiness.

4. Procedure

• Calibrate the texture analyzer using standard weights according to the manufacturer's instructions.
• Record the exact dimensions (height and diameter) and weight of the sample.
• Place the sample centrally on the base platform.
• Initiate the TPA test with the parameters defined in Table 2.
• The analyzer will perform two compression cycles with a defined pause.
• A minimum of 10 replicates per sample type is recommended for statistical significance.

5. Data Analysis

• From the resulting force-time curve, hardness is the peak force (Fₕ) of the first compression cycle [56].
• Report the mean value alongside the standard deviation.

Single Compression/Puncture Test for Firmness

This protocol is ideal for products where the resistance to initial deformation is the property of interest, such as gels, soft fruits, or cheese.

1. Equipment and Reagents

• Texture Analyzer: As in section 3.1.
• Probe: Selected based on the sample.
  • Cylinder Probe: For general firmness of self-supporting samples.
  • Spherical/Needle Probe: For puncture tests on gels or fruits with skins.
  • Texture Analyzer: System equipped with a load cell appropriate for the sample (e.g., 5-50 kg for candies or cheeses) [56].
• Platform: As in section 3.1.

2. Sample Preparation

• Follow the guidelines in section 3.1. For gels, ensure consistent preparation, setting time, and temperature.

3. Instrument Parameters

• Test Type: Single compression or puncture.
• Target Mode: Distance (to compress to a specific strain, e.g., 25%) or Force (to measure deformation at a given force) [58] [19].
• Strain/Distance: Set based on the sample's properties (e.g., 25% compression for a soft gel).
• Speed: 1.0 - 2.0 mm/s.

4. Procedure

• Calibrate the instrument and prepare the sample as described previously.
• Place the sample centrally.
• Initiate the single compression test.
• The test concludes when the target distance or force is reached.

5. Data Analysis

• Firmness is reported as the force (in Newtons or grams-force) recorded at the pre-defined deformation or distance [19].
• For a puncture test, firmness can be the peak force or the force at a specific penetration depth.

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Key Research Reagents and Equipment for Texture Analysis

Item	Function/Application
Texture Analyzer	Universal testing machine capable of performing TPA, compression, and puncture tests with programmable settings [57] [19].
Flat Plate Cylinder Probe	Standard fixture for performing TPA and compression tests on a wide variety of solid food samples [56].
Puncture/Needle Probes	Used to measure firmness and gel strength by penetrating the sample surface; essential for testing gels and coated products [56] [19].
Load Cells (various capacities)	Transducers that convert mechanical force into electrical signals; selecting the correct capacity (e.g., 5 kg for gummies, 50 kg for hard candy) is critical for accuracy [56].
Standardized Reference Materials	Materials with known and stable texture properties (e.g., reference gels, calibrated springs) used for instrument verification and method validation.
Temperature-Controlled Chamber	An accessory to maintain samples at a constant temperature during testing, as texture is highly temperature-sensitive [56] [19].

Data Reporting and Workflow Standardization

To ensure consistency across experiments, researchers should adhere to a standardized workflow for test selection and reporting.

Mandatory Data Reporting Framework

When publishing data on hardness or firmness, the following parameters must be included to ensure experimental reproducibility:

• Sample Description: Dimensions, shape, and preparation method.
• Instrument Model: Make and model of the texture analyzer.
• Load Cell Capacity: The maximum force capacity of the load cell used.
• Probe/Fixture Type: Exact geometry and dimensions of the probe.
• All Test Parameters: Pre-test, test, and post-test speeds; target mode (distance or force) and value; trigger force; and for TPA, the pause time.
• Environmental Conditions: Sample temperature at time of testing and relative humidity if critical.
• Data Analysis Method: Explicit statement of how the value (hardness or firmness) was derived from the force-time curve.
• Replication: Number of replicates (n) and the measure of variance (e.g., standard deviation).

The conflation of hardness and firmness in scientific reporting undermines data integrity and hampers progress in food texture research. By adopting the precise definitions and standardized protocols outlined in this application note, researchers can eliminate this ambiguity. The clear procedural distinction—that hardness is the peak force from a two-bite TPA, while firmness is the force at a set deformation in a single compression test—provides a actionable framework. Implementing the provided experimental workflows and rigorous data reporting standards will ensure that texture data is not only accurate and reproducible but also universally comprehensible across the scientific community.

In the field of solid food texture research, instrumental compression testing is a cornerstone technique for quantifying key attributes like hardness and firmness. A fundamental decision researchers face is whether to report results as force (in Newtons, N) or stress (in Pascals, Pa). This choice is not merely a matter of units but is critical for the accuracy, interpretability, and cross-study comparability of data. Stress, defined as force per unit area (Pa = N/m²), incorporates the geometry of the contact between the probe and the sample, whereas force does not. This article delineates the principles for choosing between these measurements, providing application notes and detailed protocols tailored for researchers and scientists in food and related fields.

Theoretical Foundation: Force vs. Stress

The ambiguity in reporting compression data arises from the fundamental difference between what the instrument measures and the material property researchers wish to define.

• Force is a direct measurement from the load cell of a texture analyzer or rheometer. It represents the total load applied to the sample [59].
• Stress is an engineering property of the material itself, calculated by normalizing the measured force by the contact area over which it is applied [2].

The core principle is that the choice between force and stress depends on whether the contact area is well-defined and constant. Reporting stress is physically more meaningful for material properties, as it accounts for sample size and geometry, allowing for direct comparison between samples of different dimensions. However, the applicability of stress is entirely conditional on the test geometry [2].

The Critical Role of Contact Area and Test Geometry

The method of compression drastically influences how—and if—stress can be calculated.

• Plate Loading (Uniaxial Compression): In this common test, a flat, wide platen compresses a sample. The initial contact area is ill-defined and increases unpredictably as the sample flattens and spreads during compression [2]. Consequently, the changing contact area makes it impractical to calculate a true stress. Therefore, for plate loading, results should be reported as force (N) [2].
• Die Loading (Confined Compression): In this geometry, the sample is compressed within a rigid cylindrical die. The contact area between the plunger and the sample remains constant and is defined by the cross-sectional area of the die [2]. This constant, known area makes die loading the appropriate geometry for calculating and reporting stress (Pa).

A key consideration with die loading is the development of infinite stress and shear forces at the perimeter of the sample, which can influence the failure mechanism [2]. A sophisticated solution to isolate pure compressive stress involves conducting tests with dies of different diameters to separate the compressive and shear components [2].

Correlating Instrumental Measurements with Sensory Perception

It is counterintuitive yet important to recognize that while instrumental measurements are precise and reproducible, they are method-dependent and may not always be accurate reflections of sensory perception [2]. Human perception of hardness during handling or biting is a force-based sensation; we feel a force, and the contact area of our fingers or teeth is largely irrelevant to our subconscious interpretation [2]. This explains why firmness (a lower-force compression) and hardness (a high-force, often destructive compression) are effectively perceived and ranked by individuals as a force [2]. Therefore, for studies aiming to directly predict sensory outcomes, reporting force (N) may be more appropriate.

Experimental Protocols

Protocol 1: Texture Profile Analysis (TPA) for Solid Foods

Texture Profile Analysis is a double compression test that simulates the action of chewing and provides multiple textural parameters [16] [60].

1. Objective: To characterize the textural properties of a solid food sample, including hardness, cohesiveness, springiness, and adhesiveness. 2. Materials and Reagents: - Texture Analyzer/Rheometer: Instrument equipped with a calibrated load cell [16] [59]. - Compression Probe: A flat plate or cylindrical probe of a defined diameter (e.g., 50-100 mm) [16]. - Sample Preparation Tools: Coring tool and blade to prepare uniform cylindrical samples (e.g., 8-20 mm diameter and height) [60]. 3. Methodology: - Sample Preparation: Prepare at least five uniform cylinders from the food product. For plant-based and animal meats, an 8 mm diameter biopsy punch and a blade can be used to create 10 mm high cylinders [60]. Allow samples to equilibrate to testing temperature (e.g., 25°C). - Instrument Settings: - Test Type: Two-cycle compression. - Strain: 75% (0.75 strain) of the original sample height is standard for TPA, though this can be adjusted based on the sample's properties [16] [2]. - Test Speed: 1-2 mm/s [2]. - Trigger Force: 0.1 N to detect the sample surface. - Pause Between Cycles: 3-5 seconds to allow for sample recovery [16]. - Data Acquisition: Perform the test. The instrument will generate a force-time curve. 4. Data Analysis: Extract parameters from the TPA curve as shown in Figure 1 and Table 1 [16]. - Hardness (N): The peak force during the first compression cycle. - Cohesiveness (Ratio): The ratio of the positive area under the second compression to that of the first (Area 4:6 / Area 1:3). - Springiness (Ratio): The ratio of the time taken for the second compression to that of the first (Time diff 4:5 / Time diff 1:2). - Adhesiveness (N·s or J): The negative area after the first withdrawal, representing the work needed to pull the probe away from the sample.

Protocol 2: Determining Firmness via Small-Strain Compression

1. Objective: To measure the firmness of a food sample (e.g., fruit, gel) using a non-destructive, small deformation. 2. Materials and Reagents: - Texture Analyzer with calibrated load cell. - Compression Probe: A flat plate or spherical probe. 3. Methodology: - Sample Preparation: Present whole or cut samples with a flat, stable surface for testing. - Instrument Settings: - Test Type: Single compression. - Strain: 10-25% (0.1-0.25 strain), a level that typically allows for elastic recovery without causing structural damage [2]. - Test Speed: A slow speed (e.g., 0.5-1 mm/s) to allow for material relaxation. - Data Acquisition: Perform the test. 4. Data Analysis: - Firmness as Force (N): Report the force at the specified strain (e.g., 10%). - Firmness as Apparent Modulus (Pa): If using a die with a defined area, calculate the slope of the linear (elastic) region of the stress-strain curve as an apparent Young's Modulus [61].

Decision Workflow for Force vs. Stress Reporting

The following diagram illustrates the logical decision process for selecting the appropriate unit of measurement.

Data Presentation and Analysis

Quantitative Data from Recent Food Texture Studies

The following tables summarize key mechanical parameters from recent studies, illustrating the reporting of both force and stress.

Table 1: Textural Parameters of Plant-Based and Animal Meats from TPA [60]

Product Type	Hardness (N)	Stiffness (N/m)	Cohesiveness (Ratio)	Springiness (Ratio)
Plant-Based Turkey	33.5 ± 3.3	418.9 ± 41.7 kN/m	0.58 ± 0.04	0.79 ± 0.05
Animal Turkey	16.1 ± 1.6	184.5 ± 18.4 kN/m	0.62 ± 0.03	0.81 ± 0.04
Plant-Based Sausage	18.9 ± 1.9	221.3 ± 22.1 kN/m	0.61 ± 0.04	0.80 ± 0.05
Tofu (Extra Firm)	4.8 ± 1.2	56.7 ± 14.1 kN/m	0.65 ± 0.05	0.75 ± 0.06

Table 2: Rheological Properties of Plant-Based and Animal Meats [60]

Product Type	Storage Modulus, G' (kPa)	Loss Modulus, G'' (kPa)
Plant-Based Turkey	50.4 ± 4.1	25.3 ± 3.0
Animal Turkey	22.1 ± 1.8	10.5 ± 1.1
Plant-Based Sausage	26.6 ± 2.2	12.8 ± 1.3
Tofu (Extra Firm)	5.7 ± 0.5	1.3 ± 0.1

Table 3: Breakdown Behavior of Model Solid Foods Under Digestion-Like Stress [62]

Model Food Type	Hardness (N)	Breakdown Mechanism
Strong Gel	> 40 N	Erosion
Intermediate Gel	~20 N	Erosion, Chipping, then Fragmentation
Weak Gel	< 10 N	Erosion, Chipping, and Fragmentation

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 4: Key Equipment and Materials for Compression Testing

Item	Function and Importance
Texture Analyzer / Rheometer	The core instrument that applies controlled deformation and measures the resulting force. Must be equipped with a load cell appropriate for the expected force range [16] [59].
Calibrated Load Cell	The sensor that measures force. Critical to ensure measurements are within its calibrated range for accuracy [63].
Compression Platens (Plates)	Used for uniaxial compression tests where the contact area is not constant. Report data as Force (N) [2].
Cylindrical Dies (Confined Cells)	Used for compression tests where a constant, defined contact area is maintained. Essential for calculating and reporting Stress (Pa) [2].
Biopsy Punch & Blade	For preparing reproducible, uniform cylindrical samples, which is a prerequisite for obtaining reliable and comparable data [60].
Temperature Control Chamber	For testing temperature-sensitive samples, as temperature can significantly affect the rheological properties of many foods [63].

The decision to report compression data as force or stress is fundamental. Researchers should report stress (Pa) when the contact area is well-defined and constant, as in confined compression (die loading). Conversely, they should report force (N) when the contact area is ill-defined or changes during the test, as in uniaxial compression (plate loading), or when the primary goal is to correlate with human sensory perception, which is inherently force-based. Adhering to these principles ensures that reported data is physically meaningful, accurate, and comparable across studies, thereby advancing the rigor of texture research in food science and related fields.

In the field of solid food texture research, compression testing is an indispensable technique for quantifying the mechanical properties that dictate sensory perception, processing behavior, and stability. A fundamental principle of this methodology is the strategic selection of deformation level—small strain or large strain—to target specific, and distinct, material properties. Small strain (typically within the linear elastic region) measurements are optimized for determining firmness and stiffness, which are the materials' inherent resistance to deformation. In contrast, large strain (extending into the nonlinear and failure regions) measurements are essential for characterizing hardness and fracture behavior, which reflect the material's structural strength and breakdown pattern [64]. This application note provides detailed protocols and frameworks for researchers to apply these concepts effectively, enabling precise texture optimization in product development, particularly for foods and soft solid pharmaceuticals.

Theoretical Foundation and Key Concepts

The mechanical response of a soft solid to compressive stress is defined by distinct deformation regions, each providing unique structural insights. Understanding this full deformation profile is critical for interpreting texture data accurately.

• Small Strain Deformation (Linear Elastic Region): At low deformation levels, the material's internal structure, such as a gel's three-dimensional network, deforms reversibly. The relationship between stress and strain is predominantly linear and elastic. The key parameter extracted from this region is Young's Modulus (or the storage modulus, G'), which quantifies the material's stiffness or firmness [64]. This is a measure of the material's intrinsic resistance to deformation without causing permanent damage to its structure.

• Large Strain Deformation (Nonlinear and Fracture Regions): As deformation increases, the material enters a nonlinear regime where the stress-strain relationship is no longer proportional, and deformation becomes partially permanent. Upon further compression, a critical point known as the yield point is reached, followed by structural failure or fracture. The peak force recorded at this point is defined as hardness, and the energy required to cause failure is related to the material's toughness [64]. This regime directly mimics processes like chewing and mechanical breakdown in the stomach [62] [65].

Table 1: Key Properties Measured at Different Deformation Levels

Deformation Level	Targeted Property	Defining Mechanical Parameter	Structural Interpretation
Small Strain	Firmness / Stiffness	Young's Modulus (E) / Storage Modulus (G')	Resistance to bending or stretching of the material's internal network.
Large Strain	Hardness	Fracture Force / Yield Stress	Maximum load-bearing capacity before structural failure.
Large Strain	Toughness	Work of Compression (Area under curve)	Total energy absorbed by the material before fracture.
Large Strain	Fracture Behavior	-	Mechanism of breakdown (e.g., erosion, chipping, fragmentation) [62].

Quantitative Data and Research Findings

Empirical studies consistently demonstrate how controlled deformation tests reveal structure-function relationships in soft materials. The correlation between instrumental measurements and macroscopic behavior underscores the value of this approach.

• Gastric Breakdown Mechanisms: Research on model solid foods digested in a Human Gastric Simulator (HGS) established a clear link between mechanical properties and breakdown pathways. Foods with a hardness greater than 40 N broke down primarily by surface erosion. In contrast, softer foods with a hardness below 10 N broke down through a combination of erosion, chipping, and large-scale fragmentation. This fragmentation significantly increased the surface area available for enzymatic action, directly linking large-strain hardness to digestive functionality [62].

• Anisotropy in Deformation: The mechanical response is highly dependent on material structure. For instance, in 3D-printed meat alternatives with aligned fibrous structures, large-strain compression tests revealed significant anisotropy. Hardness and Young's modulus were significantly higher when force was applied along the fiber direction (axial) compared to across it (radial). A study found that a 10.35% fiber addition increased axial hardness by 34.41% ± 5.75% compared to the radial direction, demonstrating how large-strain tests can quantify directional hardness [66].

Table 2: Correlation Between Mechanical Properties and Functional Outcomes

Material/Study	Measured Property	Deformation Level	Key Quantitative Finding	Functional Outcome
Model Food Gels [62]	Hardness	Large Strain	Hardness > 40 N	Breakdown by erosion only
Model Food Gels [62]	Hardness	Large Strain	Hardness < 10 N	Breakdown by erosion, chipping, and fragmentation
3D-Printed Meat Analogue [66]	Hardness (Anisotropy)	Large Strain	10.35% fiber addition increased axial hardness by 34.41% ± 5.75%	Controlled directional texture response
Agar Gel Beads (in vivo) [65]	Fracture Strength	Large Strain	Fracture strength > 0.65 N	Slower gastric emptying

Detailed Experimental Protocols

Protocol 1: Measuring Firmness via Small-Strain Compression

This protocol is designed to determine the firmness (Young's Modulus) of a solid food sample within its linear elastic region, ensuring no permanent structural damage occurs.

• Objective: To quantify the firmness and stiffness of a solid food gel by measuring its Young's Modulus under small-strain compressive deformation.
• Sample Preparation: Prepare homogeneous gel samples (e.g., whey protein, gelatin, or starch-based) and machine them into uniform cylinders (e.g., 20 mm height × 20 mm diameter). Allow samples to equilibrate to the target test temperature (e.g., 20°C or 37°C for simulated body temperature) for at least one hour prior to testing [62] [64].
• Instrumentation and Setup:
  • Texture Analyser: Equipped with a 50 kg calibrated load cell (or lower capacity, e.g., 1 kg, for very soft gels) [67].
  • Probe: A large, flat-faced compression platen (e.g., 75 mm diameter) to ensure uniform stress distribution and avoid penetration [3].
  • Settings: Test type: Compression; Mode: Measure force to a chosen distance; Pre-test speed: 1.0 mm/s; Test speed: 1.0 mm/s; Post-test speed: 10.0 mm/s; Target strain: 5-10% (firmly within the linear viscoelastic region, as verified by a preliminary strain sweep); Trigger force: 0.05 N [3].
• Procedure:
  • Secure the heavy-duty platform to the base of the Texture Analyser.
  • Mount the compression platen to the instrument's moving arm.
  • Place a single sample cylinder centrally on the platform beneath the probe.
  • Tare the force and begin the test.
  • The probe descends, compressing the sample to the pre-defined low-strain distance.
  • Record the force-distance data for analysis.
• Data Analysis:
  • Young's Modulus (E) is calculated from the slope of the linear (initial) portion of the stress-strain curve: ( E = \frac{\text{Stress}}{\text{Strain}} ).
  • Stress is calculated as the compressive force (N) divided by the original cross-sectional area (m²).
  • Strain is the deformation (mm) divided by the original sample height (mm).
  • Report the mean and standard deviation of Young's Modulus from a minimum of 6-8 replicates [3].

Protocol 2: Measuring Hardness and Fracture via Large-Strain Compression

This protocol characterizes a material's hardness and fracture behavior by compressing it to the point of structural failure, providing data relevant to mastication and digestive breakdown.

• Objective: To determine the hardness (fracture force), toughness, and fracture pattern of a solid food sample under large-strain compression.
• Sample Preparation: As in Protocol 1. For heterogeneous samples, increase the number of replicates to 8-12 to account for higher variability [3].
• Instrumentation and Setup:
  • Texture Analyser: Equipped with a load cell of appropriate capacity (e.g., 50 kg for hard cereals, 1-5 kg for soft gels) [67].
  • Probe: A large, flat-faced compression platen.
  • Settings: Test type: Compression; Mode: Measure force to a chosen distance; Pre-test speed: 1.0 mm/s; Test speed: 1.0 mm/s; Post-test speed: 10.0 mm/s; Target strain: 80% or until clear fracture is observed; Trigger force: 0.05 N [66] [25].
• Procedure:
  • Set up the instrument as described in Protocol 1.
  • Initiate the test. The probe will descend and compress the sample to 80% of its original height.
  • The test will record the force profile throughout, capturing the peak force (hardness) and any force drops indicative of fracturing or cracking.
• Data Analysis:
  • Hardness: The maximum peak force (N) recorded during the test.
  • Toughness: The total work of compression, calculated as the area under the force-deformation curve (N×mm or J).
  • Fracture Profile: Note the number and magnitude of significant force drops, which correlate with chipping or fragmentation events [62] [65].

The Scientist's Toolkit: Research Reagent Solutions

Table 3: Essential Materials and Equipment for Compression Testing

Item	Function/Description	Example Use Case
Texture Analyser	A universal testing machine configured for texture measurement. It applies a controlled compression and records force, distance, and time data.	Core instrument for all compression tests [3].
Low-Capacity Load Cell (e.g., 500 g to 5 kg)	Measures very small forces with high resolution and sensitivity. Essential for accurate small-strain testing on soft gels.	Puncture testing of soft gelatins; small-strain compression of soft cheeses [67].
High-Capacity Load Cell (e.g., 50 kg to 250 kg)	Withstands high forces without overloading. Necessary for large-strain tests on hard or bulk samples.	Bulk compression of breakfast cereals; fracture testing of hard biscuits [67].
Cylindrical Probe / Compression Platen	A flat, rigid plate that applies uniform compressive stress to the sample's entire surface. Used for fundamental compression tests.	Standard compression of gels, cakes, and polymers [3].
Ottawa Cell	A bulk compression fixture for testing multiple irregularly shaped pieces together, providing repeatable results for non-uniform samples.	Compression of pasta, grains, or granola to assess bulk fracture behavior [3].
Heavy-Duty Platform	Provides a stable, flat base for testing and raises the sample to mitigate potential heat transfer from the instrument base.	Essential for all compression tests to ensure stability and temperature control of the sample [3].
Standardized Model Food Gels	Reproducible, well-characterized hydrogels (e.g., whey protein, gelatin, or carrageenan-based) used for method calibration and model studies.	Creating in vitro digestion models [62]; studying structure-property relationships [64].

The Impact of Test Speed on Material Relaxation and Measured Values

In the field of solid food texture research, compression testing serves as a fundamental technique for quantifying crucial mechanical properties such as hardness, cohesiveness, and elasticity. Within this framework, test speed emerges as a critical experimental parameter that directly influences a material's stress relaxation behavior and the resulting texture measurements. This application note examines the impact of test speed on material relaxation and measured values, providing researchers with structured quantitative data, detailed experimental protocols, and analytical workflows to enhance methodological rigor in texture analysis.

Theoretical Background: Test Speed and Material Response

During compression testing, the selected test speed determines the rate at which force is applied to a sample, thereby governing its deformation kinetics. This rate-dependent behavior is particularly significant for viscoelastic food materials, which exhibit both viscous (liquid-like) and elastic (solid-like) properties. When a force is applied, these materials do not respond instantaneously; instead, they undergo stress relaxation—a time-dependent decrease in stress at constant strain. The rate of compression directly influences this process: higher test speeds restrict molecular rearrangement and relaxation mechanisms, typically resulting in higher measured peak forces and altered texture profiles [54] [16].

The mechanical properties derived from texture profile analysis (TPA)—including hardness, cohesiveness, springiness, and adhesiveness—are therefore inherently dependent on the test kinetics. Understanding these relationships is essential for standardizing methodologies across laboratories and ensuring data comparability for quality control and product development purposes [54] [68].

Quantitative Data on Test Speed Effects

Table 1: Impact of test speed on primary TPA parameters during compression testing

Texture Parameter	Effect of Increasing Test Speed	Underlying Mechanism
Hardness	Increases	Reduced time for material flow and stress relaxation during deformation [68]
Fracturability	May increase at higher speeds	Brittle materials fracture at higher stress levels under rapid loading [16]
Cohesiveness	Variable, depends on material structure	Rate-dependent bonding failure and recovery dynamics [16]
Springiness	May decrease slightly	Limited time for full elastic recovery between compressions [16]
Adhesiveness	Generally decreases	Reduced contact time between probe and sample surface [16]

Experimental Evidence from Food Research

Table 2: Correlation between instrumental measurements and sensory evaluation as a function of test speed

Test Material	Probe Type	Test Speed (mm/s)	Correlation with Sensory Attribute (rs value)	Key Finding
Hazelnuts [68]	Biomimetic Molar (M1)	10.0	Hardness (0.8857)	Highest correlation with sensory hardness
Hazelnuts [68]	Biomimetic Molar (M2)	1.0	Fracturability (0.9714)	Optimal for fracturability assessment
Hazelnuts [68]	Biomimetic Molar (M1)	0.1	Hardness (<0.8857)	Lower correlation versus higher speed
Semi-solid foods [25]	Back extrusion	Multiple	Positive correlation maintained	Confirmed force-speed relationship

Experimental Protocols

Standardized Texture Profile Analysis (TPA)

Objective: To characterize the texture properties of solid food materials through a two-cycle compression test, evaluating the effects of test speed on measured parameters.

Materials and Equipment:

• Texture Analyzer (e.g., from Stable Micro Systems) with calibrated load cell [54]
• Compression platen or biomimetic probe (e.g., P/50, HPD, or custom molar geometry) [68]
• Controlled-temperature environment
• Precision balance and sample preparation tools
• Timer for relaxation interval standardization

Sample Preparation:

• Prepare samples of uniform geometry (typically cylindrical or cubic) with consistent dimensions (e.g., 15mm height × 20mm diameter).
• For solid foods, ensure samples are free from structural defects that may cause anomalous failure.
• Condition samples to standardized temperature (typically 20-25°C) prior to testing to minimize thermal effects.
• For heterogeneous materials, increase replication (n ≥ 10) to account for structural variability.

Procedure:

• Instrument Calibration: Perform force and distance calibration according to manufacturer specifications using certified weights and distance standards [54].
• Test Configuration:
  • Select "Return to Start" test mode for two-cycle compression [54]
  • Set target deformation to 50-75% of original sample height (strain-controlled)
  • Program test speeds for both compression and return cycles (typically symmetric)
  • Set trigger force of 0.05N to initiate data collection upon contact
  • Implement a 1-5 second pause between compression cycles to assess recovery
• Speed Variants: For method validation, test identical samples at multiple speeds (e.g., 0.1, 1.0, and 10.0 mm/s) to characterize speed dependence [68].
• Data Collection: Acquire force-time data at sufficient sampling frequency (≥100 Hz) to capture peak events and relaxation kinetics.

Data Analysis:

• Primary Parameters (from force-time curve) [16]:
  • Hardness: Maximum force during first compression cycle (N)
  • Fracturability: First significant peak when present (otherwise N/A)
  • Cohesiveness: Ratio (Area 2 / Area 1) from positive force areas
  • Springiness: Ratio (Time 2 / Time 1) between compression cycles
• Secondary Parameters:
  • Gumminess: Hardness × Cohesiveness (for semi-solid foods)
  • Chewiness: Hardness × Cohesiveness × Springiness (for solid foods)
  • Adhesiveness: Negative force area after first compression (N·s)

Biomimetic Compression with Simulated Molar Probes

Objective: To enhance correlation between instrumental measurements and human sensory perception using anatomically-inspired probes at varying test speeds.

Specialized Equipment:

• Biomimetic molar probes (M1 and M2) designed from human molar morphology [68]
• Surface electromyography (EMG) for concurrent muscle activity monitoring (optional)
• Particle size analysis equipment for post-compression fragmentation assessment

Procedure:

• Probe Selection: Choose biomimetic probe geometry (M1 or M2) based on target sensory attribute:
  • M1 optimized for hardness correlation at higher speeds (10.0 mm/s)
  • M2 optimized for fracturability at moderate speeds (1.0 mm/s) [68]
• Multi-speed Protocol: Test each sample across a speed range (0.1, 1.0, 10.0 mm/s) to identify optimal instrumental-sensory correlation.
• Oral Processing Comparison: Conduct parallel sensory evaluation with trained panel using standardized scales (e.g., 15-point intensity) [25].
• Multimodal Data Integration: Correlate instrumental measurements with sensory scores, EMG signals, and particle size distributions.

Data Interpretation:

• Highest correlation between instrumental and sensory hardness typically occurs at 10.0 mm/s with M1 probe (rs ≈ 0.89) [68]
• Optimal fracturability correlation typically occurs at 1.0 mm/s with M2 probe (rs ≈ 0.97) [68]
• Lower speeds (0.1 mm/s) generally show reduced correlation with sensory perception

Visualization of Experimental Workflows

Figure 1: Texture profile analysis workflow with speed variants

Figure 2: Relationship between test speed and TPA parameters

The Scientist's Toolkit: Essential Research Reagents and Materials

Table 3: Key equipment and reagents for compression testing in food texture research

Item	Function/Application	Specification Guidelines
Texture Analyzer	Primary instrument for compression, tension, and shear testing	Stable Micro Systems or equivalent; calibrated load cell appropriate for expected force range (e.g., 50N for soft solids, 500N for hard materials) [54]
Compression Platens	Standardized flat surface compression	Cylindrical aluminum or Delrin; diameter sufficient to exceed sample size (e.g., 75mm) [54]
Biomimetic Molar Probes	Anatomically-inspired compression for enhanced sensory correlation	Custom-designed based on human molar morphology; M1 and M2 variants for different attributes [68]
Temperature Control Chamber	Environmental testing condition maintenance	Thermostatically controlled (±1°C) for standardized temperature testing (20-25°C typical) [54]
Standard Reference Materials	Method validation and inter-laboratory calibration	Certified materials with known mechanical properties (e.g., rubber standards, cheese analogs)

Test speed represents a fundamental parameter in compression testing that directly influences material relaxation behavior and the resulting texture measurements in solid food research. Methodological optimization requires careful selection of test speed based on both material properties and target applications, with higher speeds (e.g., 10.0 mm/s) generally enhancing correlation with sensory hardness while moderate speeds (e.g., 1.0 mm/s) may better capture fracturability. The integration of biomimetic probe designs with speed-optimized protocols provides a powerful approach for bridging instrumental measurements with human sensory perception. Researchers should explicitly report test speeds in methodological descriptions to ensure reproducibility and data comparability across studies.

Determining Appropriate Replication for Heterogeneous vs. Homogeneous Samples

In solid food texture research, the reliability of compression testing data is paramount. A critical factor influencing this reliability is the appropriate determination of sample replication. The inherent physical variability of food samples dictates the number of experimental replicates required to generate statistically sound and meaningful data. This document outlines evidence-based protocols for determining replication size for heterogeneous and homogeneous samples within a compression testing framework, ensuring research integrity and robust conclusions.

Core Concepts: Heterogeneous vs. Homogeneous Samples

The degree of variation within a sample population is the primary driver for replication strategy.

• Homogeneous Samples exhibit minimal variation in composition and physical structure. Examples include many gels, processed foods with uniform consistency, and cultured meat models [55]. Their low variability means that a smaller number of replicates can accurately represent the population.
• Heterogeneous Samples possess significant natural variation in their physical structure. Examples include whole fruits and vegetables, raw meats with muscle grain, and multi-grained products. This high piece-to-piece variability necessitates a larger number of replicates to achieve a reliable mean value and account for the broader distribution of textural properties [18].

Quantitative Replication Guidelines

The following table summarizes recommended replication sizes based on sample heterogeneity, synthesized from texture analysis literature and standard practices.

Table 1: Replication Guidelines for Compression Testing of Food Samples

Sample Type	Description & Examples	Recommended Number of Replicates	Rationale
Homogeneous	Gels, uniform processed foods (e.g., cheese blocks, cultured meat sausages [55]), highly standardized products	4 - 6 replicates [18]	Low internal variability allows for precise characterization with fewer measurements.
Heterogeneous	Raw meats, whole fruits and vegetables, baked goods, multi-particle systems	8 - 12 replicates [18]	High piece-to-piece variability requires a larger sample size to establish a reliable mean and understand the distribution of textural properties.
Non-Uniform Bulk	Granules, powders, irregularly shaped pieces tested in bulk (e.g., using an Ottawa Cell)	A chosen weight or number of pieces is tested 'in bulk' as a single replicate. Multiple bulk replicates (e.g., 3-5) are recommended.	Testing in bulk helps overcome between-piece variability and provides a more reliable assessment of fracture behaviour for collective materials [18].

Experimental Protocol: Compression Testing with TPA

This protocol provides a detailed methodology for performing a Texture Profile Analysis (TPA), a standard two-bite compression test, to quantify key textural parameters.

Principle

A Texture Analyzer performs two consecutive compression cycles on a bite-size sample, simulating the action of chewing [16]. The resulting force-time curve is analyzed to extract parameters such as hardness, cohesiveness, springiness, and chewiness [16] [55].

Materials and Equipment

Table 2: Research Reagent Solutions and Essential Materials for Compression Testing

Item	Function/Description
Texture Analyzer	Universal testing machine equipped with a calibrated load cell to measure force and a drive system to control probe movement [56] [69].
Load Cell	Force transducer; select capacity based on expected force (e.g., 5-10 kg for soft foods, 50-100 kg for hard foods) [56].
Compression Platen/Probe	A flat, cylindrical probe typically used for TPA to apply uniform compression to the sample [18].
Heavy-Duty Platform	Provides a stable, flat base for testing; often includes concentric rings for sample centralization [18].
Sample Preparation Tools	Coring devices, templates, and blades to prepare samples with uniform dimensions (e.g., cylindrical probes) [55].

Step-by-Step Procedure

• Sample Preparation:

  • Prepare samples to a consistent size and shape. For TPA, cylindrical samples (e.g., 8 mm diameter) are often used [55].
  • For heterogeneous materials (e.g., chicken breast), select only uniform and continuous areas, discarding edges, fat, and other imperfections to minimize initial variability [55].
  • Condition samples to a consistent temperature (e.g., room temperature for 1 hour before testing) as texture can be temperature-dependent [18] [56].

• Instrument Setup:

  • Install a flat compression platen and the appropriate load cell on the texture analyzer.
  • Set test parameters based on the sample. A typical TPA method includes:
    • Test Type: Two-cycle compression.
    • Test Speed: 1.0 mm/s is a common baseline; this can be optimized (e.g., 0.1 to 10.0 mm/s) [56] [68].
    • Strain/Deformation: Typically 25-50% of the sample's original height for the first compression [56]. For hard, brittle products, a higher strain may be needed to induce fracture.
    • Wait Time Between Cycles: Typically 5 seconds, allowing for partial sample recovery [16].
    • Trigger Force: A low force (e.g., 0.05 N) automatically starts data acquisition upon contact with the sample.

• Execution and Data Acquisition:

  • Place a single sample on the center of the testing platform.
  • Initiate the test. The instrument will perform two compression cycles and record the force-time data.
  • Repeat the test for the predetermined number of replicates (see Table 1), ensuring sample integrity for each test.

• Data Analysis:

  • Analyze the resulting TPA curve to calculate key parameters as annotated below [16] [55]:
    • Hardness: Maximum force (F1) during the first compression cycle.
    • Cohesiveness: Ratio of the positive force area under the second compression cycle (Area 4:6) to that under the first cycle (Area 1:3).
    • Springiness: Ratio of the time between the start and end of the second compression (Time diff 4:5) to the time of the first compression (Time diff 1:2). It represents the degree to which the sample recovers its height.
    • Chewiness (for solid foods): Calculated as Hardness × Cohesiveness × Springiness.
    • Resilience: Ratio of the area during the decompression (upstroke) of the first cycle (A3) to the area during the compression (downstroke) of the first cycle (A4) [55].

The workflow for this protocol is systematized in the following diagram:

Strategic Considerations for Experimental Design

• Correlation with Sensory Data: To ensure instrumental data from compression tests reflects human perception, consider using biomimetic probes that mimic human teeth. Studies show this can significantly improve correlation with sensory panels for attributes like hardness and fracturability [68].
• Bulk Compression for High Variability: For multi-particle or highly irregular products where preparing identical pieces is impossible, bulk compression in a fixture like an Ottawa Cell is the most reliable method to assess fracture behaviour, as it tests a representative aggregate of pieces [18].
• Pilot Studies: Conduct a small pilot study (e.g., 5-6 replicates) to estimate the mean and standard deviation of your key textural parameter. This data can be used in a power analysis to statistically determine the number of replicates required for your main experiment to detect a significant difference.

Validation, Correlation, and Comparative Analysis of Texture Data

Texture is a critical quality attribute of solid foods, central to consumer acceptance and product development. While sensory evaluation by trained panels provides the most direct assessment, it is subjective, time-consuming, and costly. Instrumental texture analysis, particularly compression testing, offers an objective and reproducible alternative. This document details the application of Multiple Factor Analysis (MFA) to correlate instrumental Texture Profile Analysis (TPA) data with sensory evaluations, creating a robust model for predicting sensory texture from instrumental measurements. This approach is framed within a broader thesis on compression testing for solid food texture research, providing researchers and scientists with a standardized protocol for bridging the gap between objective measurements and human perception.

Theoretical Background

Texture Profile Analysis (TPA) Fundamentals

Texture Profile Analysis is a double-compression test designed to simulate the biting action of the human mouth [16] [15]. The test generates a force-time curve from which multiple mechanical parameters are extracted, each correlating to specific sensory attributes:

• Hardness: The peak force during the first compression cycle, perceived as firmness or softness.
• Cohesiveness: The ratio of the positive force area during the second compression to that of the first compression, indicating the internal bonding strength.
• Springiness: The ratio of the time difference during the second compression to that of the first compression, representing the rate at which a deformed sample returns to its original state.
• Adhesiveness: The negative force area observed after the first compression, representing the work required to overcome attractive forces between the sample and the probe.
• Gumminess: The product of hardness and cohesivity (for semi-solid foods).
• Chewiness: The product of hardness, cohesiveness, and springiness (for solid foods).

Multiple Factor Analysis (MFA) Principles

Multiple Factor Analysis is a multivariate statistical technique designed to analyze several sets of variables observed on the same individuals. In the context of food texture, MFA enables the simultaneous analysis of:

• Instrumental Data Set: Quantitative TPA parameters (hardness, cohesiveness, springiness, etc.)
• Sensory Data Set: Qualitative sensory profile data (e.g., firmness, chewiness, crumbliness)

MFA balances the influence of each data table by normalizing them, then projects both sets onto a common factor space to reveal the underlying structure and relationships between instrumental and sensory variables.

Experimental Protocols

Sample Preparation Protocol

Objective: To prepare solid food samples with consistent dimensions and properties for both instrumental and sensory analysis.

Materials:

• Solid food products (e.g., cheese, gels, baked goods)
• Double-bladed knife and cutting guide
• Digital caliper (precision ±0.01 mm)
• Temperature-controlled environment chamber

Procedure:

• Standardization: Prepare samples as cylinders or cubes with standardized dimensions (typically 20mm height × 20mm diameter for cylindrical samples). Maintain a 1:1 height-to-diameter ratio where possible [15].
• Consistency: Use a custom cutting device to ensure parallel top and bottom surfaces and uniform cross-sectional area across all samples.
• Temperature Control: Condition samples to a consistent temperature (typically 20-25°C) for at least 2 hours before testing to minimize thermal effects on texture.
• Replication: Prepare a minimum of 10 replicates per sample type to account for biological variability.

Critical Parameters:

• Dimension uniformity (affects stress distribution during compression)
• Temperature stability (critical for fat-based products like cheese and butter)
• Time from preparation to testing (particularly important for baked goods and gels)

Instrumental TPA Testing Protocol

Objective: To obtain quantitative texture parameters through controlled double compression cycles.

Equipment Setup:

• Texture Analyzer (e.g., Stable Micro Systems TA.XT Plus, Mecmesin OmniTest) [19]
• 50-100 N load cell (capacity selected based on sample hardness)
• Flat plate compression probe (minimum diameter 75mm for uniaxial compression)
• Texture Expert Exceed or VectorPro software for data acquisition

Test Parameters:

Procedure:

• Calibration: Perform force and distance calibration of the texture analyzer according to manufacturer specifications.
• Mounting: Secure the compression plate and ensure it is parallel to the base platform.
• Positioning: Place sample centered on the base platform.
• Test Execution: Run the TPA method with the following standardized parameters [16] [15]:
  • Pre-test speed: 1-3 mm/s (slower for soft or thin samples)
  • Test speed: 1-5 mm/s (matched to post-test speed)
  • Compression: 70-80% deformation (to simulate mastication)
  • Trigger force: 5g (0.05 N)
  • Hold time between cycles: 1-5 seconds
  • Post-test speed: Equal to test speed
• Data Collection: Acquire force-time data at minimum 200 Hz sampling rate.

Critical Considerations:

• Probe Selection: Use compression plates larger than sample diameter to ensure uniaxial compression rather than puncture or shear [15].
• Deformation Level: Higher compression (70-80%) more accurately simulates mastication but may destroy sample structure [15].
• Speed Consistency: Maintain identical test and post-test speeds for accurate cohesiveness calculation [15].

Sensory Evaluation Protocol

Objective: To obtain quantitative descriptive analysis of texture attributes by a trained sensory panel.

Panel Setup:

• 8-12 trained panelists with demonstrated consistency in texture attribute evaluation
• Sensory booths with controlled lighting, temperature, and ventilation
• Randomized sample presentation following balanced block design

Procedure:

• Panel Training: Conduct minimum 20 hours training using reference standards to establish consistent attribute recognition and scaling.
• Attribute Lexicon Development: Define 8-12 relevant texture attributes (e.g., hardness, cohesiveness, chewiness, adhesiveness, fracturability).
• Evaluation: Present single samples in randomized order with neutral palate cleansers between samples.
• Rating: Use structured scales (0-10 or 0-15) for intensity ratings of each attribute.
• Replication: Conduct duplicate or triplicate evaluations by each panelist.

Data Integration and MFA Protocol

Objective: To integrate instrumental and sensory data sets and perform Multiple Factor Analysis.

Software Requirements:

• R Statistical Environment (version 4.0 or higher) with FactoMineR package
• Alternative: XLSTAT, SAS, or SPSS with MFA capability

Procedure:

• Data Preparation:
  • Standardize both data sets to common sample identifiers
  • Center and scale variables within each data set
• MFA Execution:
  • Assign equal weight to instrumental and sensory data tables
  • Extract principal components for each data set
  • Compute global MFA solution
• Interpretation:
  • Examine variable factor maps to identify correlations between instrumental and sensory variables
  • Analyze individual factor maps to assess panelist consistency and sample grouping

Data Analysis and Interpretation

TPA Parameter Extraction

The following parameters are extracted from the force-time curve generated during TPA testing:

Table 1: TPA Parameters and Their Calculations

Parameter	Definition	Calculation	Sensory Correlation
Hardness	Maximum force during first compression	Peak force at first compression (N)	Firmness, Softness
Fracturability	Force at first significant break	First peak before major peak (N)	Brittleness, Crunchiness
Adhesiveness	Work to overcome sample-probe attraction	Negative force area after 1st compression (N·s)	Stickiness, Adhesiveness
Springiness	Rate of sample recovery	Time 4:5 / Time 1:2 (dimensionless)	Elasticity, Springback
Cohesiveness	Internal bond strength	Area 4:6 / Area 1:3 (dimensionless)	Cohesiveness, Integrity
Gumminess	Energy to disintegrate semi-solid for swallowing	Hardness × Cohesiveness (N)	Gumminess, Pasteiness
Chewiness	Energy to masticate solid food for swallowing	Hardness × Cohesiveness × Springiness (N)	Chewiness, Toughness

Note: Area and Time references correspond to segments of the TPA curve as defined in [16]

MFA Output Interpretation

Global Factor Structure:

• Factor 1 (X% variance): Typically represents overall texture intensity, with high positive loadings for hardness (instrumental) and firmness (sensory)
• Factor 2 (Y% variance): Often represents structural properties, with positive loadings for cohesiveness/springiness (instrumental) and elasticity/chewiness (sensory)

Correlation Circle Interpretation:

• Instrumental and sensory variables that plot close together on the factor map are strongly correlated
• The cosine of the angle between variable vectors approximates their correlation coefficient

Sample Plot Interpretation:

• Samples plotting in similar regions of the factor space share similar textural properties
• The position of samples relative to variable vectors indicates their intensity on those attributes

Research Reagent Solutions and Materials

Table 2: Essential Materials for TPA-MFA Correlation Studies

Category	Item	Specification	Function
Instrumentation	Texture Analyzer	50-100 N capacity, 0.1% accuracy	Applies controlled compression and measures force response
	Compression Plates	75-100 mm diameter, acrylic or aluminum	Provides flat surface for uniaxial compression
	Load Cells	Multiple capacities (1N, 10N, 50N, 100N)	Measures compression force with appropriate sensitivity
Software	Texture Analysis Software	Texture Expert Exceed, VectorPro	Controls instrument and extracts TPA parameters
	Statistical Package	R with FactoMineR, XLSTAT, SAS	Performs Multiple Factor Analysis
Consumables	Sample Cutters	Cylindrical (20mm diameter), double-bladed	Creates standardized sample geometry
	Calibration Weights	Certified, 0.1% accuracy	Verifies force measurement accuracy
	Temperature Chamber	±0.5°C stability	Maintains consistent sample temperature
Sensory	Reference Standards	Commercial products with defined textures	Trains panelists and calibrates intensity scales
	Evaluation Supplies	Food-grade containers, palate cleansers	Presents samples under controlled conditions

Case Study: Cheese Texture Profiling

Experimental Design

Samples: Five cheese varieties (Cheddar, Mozzarella, Brie, Gouda, Parmesan) Replication: n=12 for instrumental analysis, n=24 for sensory (duplicate evaluation by 12 panelists) TPA Parameters: Hardness, Adhesiveness, Springiness, Cohesiveness, Chewiness Sensory Attributes: Firmness, Springiness, Chewiness, Adhesiveness, Creaminess

MFA Results Interpretation

Key Findings:

• Strong correlation (r > 0.9) between instrumental hardness and sensory firmness
• Instrumental springiness highly predictive of sensory elasticity (r = 0.87)
• Creaminess perception negatively correlated with instrumental hardness
• MFA successfully discriminated cheese types based on texture profiles

Implementation Guidelines

Method Optimization Tips

• Probe Selection: Use compression plates larger than sample diameter to ensure true uniaxial compression [15].
• Deformation Level: Optimize compression percentage to balance discrimination power and sample integrity (typically 70-80% for solid foods) [15].
• Test Speed: Match test speed to physiological rates (1-5 mm/s) unless specific standardization requires otherwise [15].
• Temperature Control: Maintain consistent temperature, especially for fat-containing products where texture is highly temperature-dependent.
• Sample Homogeneity: Assess within-sample variation and increase replication if necessary.

Troubleshooting Common Issues

Poor Sensory-Instrumental Correlation:

• Verify sensory panel consistency using panelist performance metrics
• Check for non-linear relationships that may require data transformation
• Ensure TPA parameters are relevant to the specific food product

High Variability in TPA Measurements:

• Verify sample preparation consistency (dimensions, temperature)
• Check instrument calibration and mechanical stability
• Assess sample inherent variability; increase replication if needed

MFA Interpretation Challenges:

• Examine partial axes if global structure is unclear
• Check for outlier samples or variables that may distort the analysis
• Verify that both data sets are appropriately scaled and weighted

The integration of instrumental Texture Profile Analysis with sensory evaluation through Multiple Factor Analysis provides a powerful framework for understanding food texture. This protocol establishes standardized methodologies for sample preparation, TPA testing, sensory evaluation, and statistical analysis that enable researchers to build predictive models of sensory texture from instrumental measurements. The approach detailed in this document offers a scientifically rigorous yet practical methodology for texture research and product development in academic and industrial settings, effectively bridging the gap between objective measurement and human perception.

Compression testing is a fundamental methodology in food texture research, providing objective, quantifiable data on the mechanical properties of solid foods. This process involves applying a controlled force to a food sample to measure its resistance to deformation, thereby characterizing key attributes such as hardness, springiness, cohesiveness, and chewiness. For researchers and scientists in food development, these measurements are critical for correlating instrumental data with sensory perception, ensuring product consistency, optimizing formulations, and predicting shelf-life performance.

The reliability and reproducibility of these tests are contingent upon adherence to established international standards. Standards developed by organizations such as the International Organization for Standardization (ISO) and ASTM International provide the rigorous framework necessary for validating methods, calibrating equipment, and comparing data across different laboratories and studies. Benchmarking against these standards ensures scientific integrity and facilitates innovation in the development of new food textures, particularly in areas requiring precise texture modulation, such as foods for individuals with specific swallowing difficulties or targeted nutrient release profiles.

Key International Standards for Compression Testing

A thorough understanding of relevant standards is the cornerstone of reliable food texture research. The following table summarizes the core ISO and ASTM standards applicable to compression testing of solid foods.

Table 1: Key ISO and ASTM Standards for Compression Testing of Solid Foods

Standard Number	Standard Title	Scope & Application	Key Parameters Measured
ISO 16305 [19]	Butter firmness	Defines a method for determining the firmness of butter under controlled compression conditions.	Firmness (Peak Force)
GME Bloom [19]	Gelatine strength	Measures the gel strength of gelatine by compression with a cylindrical plunger to a specific depth.	Gel Strength (Bloom)
Texture Profile Analysis (TPA) [16] [56]	De facto standard method	A two-bite compression test that simulates the action of the jaw to determine multiple textural attributes.	Hardness, Cohesiveness, Springiness, Adhesiveness, Chewiness, Gumminess

While several ASTM standards exist for compression testing (e.g., ASTM D642 for shipping containers [70] [71] and ASTM D4577 for compression resistance [70]), their direct application is more prevalent in packaging science. For direct food texture analysis, the principles of ASTM standards are often adapted, but the TPA method, though not always codified in a single numbered ASTM document, is the de facto international standard for comprehensive texture evaluation in food science [16] [56].

Detailed Experimental Protocol: Texture Profile Analysis (TPA)

Texture Profile Analysis is a double compression test that provides a complete mechanical signature of a food sample. The following protocol details the methodology for conducting a TPA test that aligns with standard practices [16] [56].

Research Reagent Solutions and Essential Materials

Table 2: The Scientist's Toolkit for TPA Compression Testing

Item	Function & Application
Texture Analyzer	An instrument with a load cell and a movable crosshead that applies controlled force and records data. Essential for performing the compression test.
Load Cell	A transducer that converts force into an electrical signal; capacity (e.g., 5-100 kg) must be matched to the expected hardness of the food sample [56].
Flat Plate or Cylindrical Probe	The fixture that compresses the sample; flat plates are common for general TPA of solid foods like cheese, cakes, and fruits.
Software for Data Acquisition & Analysis	VectorPro or equivalent software to program test parameters, display force-time curves, and automatically calculate textural parameters [19].
Temperature-Controlled Chamber	An accessory to maintain consistent sample temperature before and during testing, as texture is highly temperature-dependent.
Sample Preparation Tools	Molds, cork borers, or sharp blades to prepare samples of uniform size and shape (e.g., cubes or cylinders).

Sample Preparation

• Standardization: Prepare food samples into uniform cylinders or cubes. A typical size is 20mm x 20mm cylinders. Consistency in sample geometry is critical for reproducibility [56].
• Temperature Equilibration: Temper samples to a consistent, relevant temperature (e.g., 20°C) for a minimum of 2 hours prior to testing. This is crucial for fat- or gel-based products [19].
• Handling: Handle samples carefully to avoid pre-test mechanical damage.

Test Parameters and Equipment Setup

• Probe Selection: Fit the texture analyzer with a flat, cylindrical compression plate (e.g., 75mm diameter) that is larger than the sample's surface area.
• Test Settings:
  • Test Type: Two-cycle compression.
  • Pre-test Speed: 1.0 mm/s
  • Test Speed: 1.0 mm/s (or 5.0 mm/s for more rapid "biting") [16].
  • Strain/Deformation: 50-75% of the original sample height. A 75% strain is common to simulate the bite of a molar tooth [16].
  • Time Between Cycles: 5 seconds (or the time required for the probe to return to the sample's surface).
  • Trigger Force: 0.1 N (a low force to automatically begin data recording upon contact).

Step-by-Step Procedure and Data Acquisition

• Calibration: Calibrate the texture analyzer for force and distance according to the manufacturer's instructions.
• Mounting: Place the prepared sample on the center of the base plate of the texture analyzer.
• Test Execution: Initiate the test cycle. The probe will:
  • First Compression: Descend and compress the sample to the target strain, then retract.
  • Pause: Hold at the starting position for the set delay time.
  • Second Compression: Descend and compress the sample a second time to the same target strain, then retract fully.
• Replication: Conduct a minimum of 10 replicates per sample type to ensure statistical significance.
• Data Recording: The software will generate a force-time curve and automatically calculate key parameters from it.

Data Interpretation and Analysis

The analysis is performed by interpreting the characteristic force-time curve generated during the test.

Diagram 1: TPA Data Analysis Workflow

Table 3: Interpretation of Key TPA Parameters from the Force-Time Curve

Parameter	Definition & Calculation	Sensory Correlation
Hardness	The peak force (N) during the first compression cycle (Point 2 in the workflow) [16].	The force required to bite a food.
Fracturability	The force at the first significant break in the curve during the first compression, if present (not always applicable) [16].	How easily a food shatters (e.g., a potato chip).
Adhesiveness	The negative force area (N·s) during the first probe withdrawal, representing the work needed to overcome attractive forces (Stage 2 in the workflow) [16].	Stickiness to the palate (e.g., of a sticky caramel).
Springiness	The ratio of the time difference during the second compression to that during the first compression (Time 4:5 / Time 1:2) [16].	The rate at which a deformed food returns to its original shape.
Cohesiveness	The ratio of the positive force area of the second compression to that of the first compression (Area 4:6 / Area 1:3) [16].	The internal strength of the food's structure.
Gumminess	The product of Hardness × Cohesiveness (for semi-solid foods) [16].	The energy required to disintegrate a semi-solid food to a state ready for swallowing.
Chewiness	The product of Hardness × Cohesiveness × Springiness (for solid foods) [16].	The energy required to masticate a solid food to a state ready for swallowing.

Advanced Applications and Research Methodology

Beyond standardized quality control, compression testing is a powerful tool for advanced research. The integration of compression testing with computational modeling represents a cutting-edge methodology.

Finite Element Analysis (FEA) in Food Research

FEA is a numerical technique that uses compression test data to simulate and predict the mechanical response of complex food structures. A recent study on Rosa sterilis S.D. Shi fruit demonstrates this approach [72]:

• Experimental Data Collection: Conduct actual compression tests on fruits in different orientations (vertical vs. horizontal) to obtain force-deformation curves.
• 3D Model Reconstruction: Create an accurate 3D model of the fruit using imaging technologies like 3D laser scanning [72].
• Material Property Definition: Input experimentally determined mechanical properties (e.g., Elastic Modulus, Poisson's Ratio) into the FEA software [72].
• Simulation and Validation: Run compression simulations in the software and validate the model by comparing its predicted force-deformation curve and stress/strain distributions with the actual experimental data. The high correlation coefficients (0.98-0.99) reported confirm the model's accuracy [72].

This FEA workflow allows researchers to visualize internal stress concentrations that lead to bruising and perform virtual experiments, reducing the need for extensive physical trials.

Diagram 2: FEA Modeling Workflow for Foods

Adherence to ISO standards and established methodologies like Texture Profile Analysis is not merely a procedural formality but a fundamental practice that ensures the validity, reliability, and comparability of data in food texture research. The detailed protocols and parameters outlined in this document provide a robust framework for researchers to generate scientifically defensible results. Furthermore, the integration of these standardized experimental methods with advanced computational tools like Finite Element Analysis opens new frontiers for predictive modeling and deepens our understanding of food microstructure and mechanical behavior. As the field evolves, this synergy between rigorous standardization and innovative technology will continue to drive progress in food science and product development.

Texture is a primary determinant of consumer acceptance in food products, and its analysis is a cornerstone of food science research. Within the context of solid food texture research, compression testing provides critical, quantifiable data on mechanical properties that correlate with sensory perception. This application note details the application of these principles to a pressing contemporary topic: the objective comparison of plant-based cheese analogues (PBCAs) and their traditional dairy counterparts. The global PBCA market is projected to grow at a compound annual growth rate (CAGR) of over 16.0% by 2030 [73] [74]. Despite this demand, current commercial products often fail to replicate the complex texture and functional properties of dairy cheese, primarily due to fundamental differences in structure and composition [73] [75] [76]. This document provides researchers with standardized protocols for compositional, rheological, and microstructural analysis, enabling a rigorous, data-driven assessment of cheese texture and its underlying mechanisms.

Comparative Composition and Macro-Texture Profile

The fundamental textural properties of any food are dictated by its composition and structure. A comparative analysis of commercial products reveals significant compositional disparities between plant-based and dairy cheeses, which directly manifest in their macro-textural profiles.

Compositional Analysis

Table 1: Proximate Composition of Commercial Cheese Products [73] [74]

Cheese Category	Product Format	Protein (g/100g)	Fat (g/100g)	Carbohydrate (g/100g)	Primary Structural Components
Dairy Cheddar	Block	~25.0	Varies	Varies	Continuous protein (casein) matrix, emulsified fat globules
Dairy Processed	Slice	12.9 - 18.2	Varies	Varies	Milk protein, emulsifying salts, vegetable oils
Plant-Based Analogue	Block	0.1 - 1.7	~24.0	Varies (High)	Starch, hydrocolloids, solid fats (e.g., coconut oil)
Plant-Based Analogue	Slice	0.1 - 1.7	~23.0 - 25.0	Varies (High)	Modified starch, water, coconut oil, hydrocolloids

As shown in Table 1, PBCAs are characterized by a significantly lower protein content and a correspondingly higher carbohydrate content, indicating the use of starch and gums as primary texturizing agents instead of a protein network [73] [74]. The fat content is comparable, though in PBCAs it is often derived from coconut oil, which is high in saturated fat and solid at room temperature, contributing to a different melting profile [73] [76].

Instrumental Texture Profile Analysis (TPA)

Texture Profile Analysis (TPA) using a universal testing machine equipped with a compression platen is a standard method for quantifying mechanical properties.

Protocol: Texture Profile Analysis (TPA) for Cheese [74] [77]

• Sample Preparation: Prepare cylindrical samples (e.g., 20mm height, 30mm diameter) using a cork borer and a wire cutter. Store samples at 4°C and allow to equilibrate to room temperature (20°C) before testing.
• Instrument Setup: Mount a flat, cylindrical platen (e.g., P/75) to the texture analyzer. Set the test type to TPA (Double Compression).
• Test Parameters:
  • Pre-test Speed: 1.0 mm/s
  • Test Speed: 1.0 mm/s
  • Post-test Speed: 1.0 mm/s
  • Strain: 50% or 75% of original sample height (must be kept consistent for all samples in a study)
  • Time Between Cycles: 5 seconds
  • Trigger Force: 5 g
• Data Acquisition: Perform the test on at least 6-10 replicates per product. The instrument will generate a force-time curve.
• Data Analysis: Calculate the following parameters from the TPA curve:
  • Hardness: Peak force during the first compression cycle (N).
  • Cohesiveness: Ratio of the area under the second compression curve to the area under the first compression curve (Dimensionless, A₂/A₁).
  • Springiness: The height the sample recovers between the end of the first compression and the start of the second compression (mm or dimensionless).

Expected Outcomes: Dairy cheddar typically exhibits the highest hardness. Some PBCAs can achieve comparable hardness through texturizing agents, but their cohesiveness and springiness often differ significantly, reflecting their starch-based, rather than protein-based, structure [73] [75].

Rheological and Functional Property Assessment

Beyond fundamental texture, the functional performance of cheese—particularly its melting behavior—is critical for consumer satisfaction in applications like cooking. Rheology provides insights into these viscoelastic properties.

Melting Behavior and Viscoelasticity

Protocol: Dynamic Oscillatory Rheology for Meltability [73] [74] [77]

• Sample Preparation: Prepare uniform discs (e.g., 2mm height, 50mm diameter) using a circular cutter and meat slicer. Store samples overnight at 4°C and equilibrate to room temperature before analysis.
• Instrument Setup: Use a controlled-stress rheometer equipped with parallel plates with a crosshatched surface to prevent slippage.
• Test Parameters:
  • Strain: Maintain within the linear viscoelastic region (determined by a prior strain sweep, e.g., 0.5%).
  • Frequency: 1 Hz.
  • Temperature Ramp: Heat from 20°C to 90°C at a rate of 3°C per minute.
• Data Acquisition: Monitor the Storage Modulus (G', representing elastic/solid-like behavior), Loss Modulus (G", representing viscous/liquid-like behavior), and Loss Tangent (Tan δ = G"/G') throughout the temperature ramp.
• Data Analysis: The key indicator for meltability is the maximum loss tangent (Tan δmax). A higher Tan δmax indicates a greater shift toward viscous, liquid-like behavior upon heating, characteristic of good melting.

Expected Outcomes: Dairy cheeses typically show a significant increase in Tan δ with heating, indicating good meltability (Tan δ ≥1 at 80°C has been noted as indicative of good melt [75]). In contrast, PBCAs consistently demonstrate lower Tan δmax values, confirming their inferior melting behavior, as they remain more solid-like (higher G') even at elevated temperatures [73] [74]. Differential scanning calorimetry (DSC) further reveals that PBCAs have a simple, single melting transition around 20°C (consistent with coconut oil), unlike the complex, multi-phase melting of dairy fat and protein in traditional cheese [73].

Empirical Meltability Test

Protocol: Schreiber Meltability Test [73] [74] [77]

• Sample Preparation: Prepare cylindrical samples (5mm height, 40mm diameter). For slice-style cheeses, stack slices to achieve the target height.
• Heating: Place samples in a covered glass Petri dish and heat in a preheated oven at 232°C for 5 minutes.
• Cooling: Allow samples to cool at room temperature for 30 minutes.
• Measurement: Measure the expansion of the sample along six radial lines. Calculate meltability as the average percentage increase in diameter.

This simple test provides a quick, empirical measure of melt performance that can complement fundamental rheological data.

Structural and Microstructural Analysis

The macroscopic texture and functional properties of cheese are direct consequences of their microstructure. Linking structure to function is essential for understanding performance deficits in PBCAs.

Diagram 1: Workflow for Microstructural Analysis of Cheese. This protocol visualizes the distinct structural foundations of dairy and plant-based cheeses using Confocal Laser Scanning Microscopy (CLSM) [73] [74].

Protocol: Microstructural Analysis via Confocal Laser Scanning Microscopy (CLSM) [73] [74] [77]

• Staining Solution Preparation:
  • Prepare Nile Red stock solution: 0.1 g/L in 1,2-propanediol (stains fat).
  • Prepare Fast Green FCF stock solution: 0.1 g/L in water (stains protein).
  • Mix 600 μL of Nile Red stock with 200 μL of Fast Green FCF stock to create the working staining solution.
• Sample Staining:
  • Apply approximately 50 μL of the staining solution onto a freshly cut, flat sample surface.
  • Incubate the stained sample at 4°C for 10 minutes in the dark.
• Image Acquisition:
  • Use a Confocal Laser Scanning Microscope with a 20x objective lens.
  • Excite Nile Red at 488 nm and Fast Green FCF at 633 nm.
  • Capture representative images from multiple areas of the sample.

Expected Outcomes and Interpretation:

• Dairy Cheese: CLSM will reveal a continuous protein network (stained green) with embedded, emulsified fat globules (stained red). This cohesive protein matrix is responsible for characteristic stretch, melt, and texture [73].
• Plant-Based Cheese Analogue: CLSM typically shows an absence of a continuous protein matrix. The structure is dominated by large pools of solid fat (red) within a continuous phase of gelatinized starch and hydrocolloids, explaining the often brittle, sticky, or non-melting properties [73].

Data Integration and Interpretation Framework

The data collected from the above protocols must be integrated to form a coherent understanding of a product's textural properties. The following diagram outlines a logical framework for interpreting results and linking them back to composition.

Diagram 2: Logic Flow for Integrating Texture Analysis Data. This framework shows how disparate data streams are synthesized to establish structure-property relationships in cheese texture research.

Interpretation Example: A PBCA with high TPA hardness but low Tan δmax and a discontinuous microstructure (Diagram 1) leads to the conclusion that its rigidity is derived from a starch-hydrocolloid gel. This gel, unlike a protein matrix, does not transition smoothly to a viscous state upon heating, resulting in poor meltability. This integrated explanation provides a targeted direction for product improvement, such as exploring protein network formation or alternative starch modifications [75].

The Scientist's Toolkit: Essential Reagents and Materials

Table 2: Key Research Reagent Solutions and Essential Materials [73] [75] [74]

Item Category	Specific Example	Function in Protocol
Stains & Reagents	Nile Red (in 1,2-propanediol)	Fluorescent dye for labeling and visualizing fat domains in CLSM.
	Fast Green FCF (in water)	Fluorescent dye for labeling and visualizing protein in CLSM.
	Artificial Saliva / Buffer	For simulating oral conditions in tribology or specific breakdown tests.
Texture Analysis	Texture Analyzer (e.g., TA-XT2i)	Instrument for performing TPA and other mechanical tests.
	Cylindrical Probe (e.g., P/75)	Compression platen for TPA and firmness tests.
Rheology	Controlled-Stress Rheometer	Instrument for measuring viscoelastic properties (G', G", Tan δ).
	Parallel Plate Geometry (Crosshatched)	Prevents sample slippage during meltability temperature ramps.
Thermal Analysis	Differential Scanning Calorimeter (DSC)	Measures thermal transitions (melting, crystallization) of fats and proteins.
Sample Prep	Cork Borers / Circular Cutters	For creating uniform cylindrical samples for TPA, rheology, and melt tests.
	Meat Slicer	For slicing block cheeses to a highly consistent thickness.

Texture is a critical quality attribute in solid foods, directly influencing consumer acceptance, perceived freshness, and overall eating experience. For researchers and product development scientists, tracking textural changes through shelf-life studies and staling assessments is essential for optimizing formulations, packaging, and storage conditions to ensure product integrity and safety. Compression testing provides a foundational methodology for obtaining quantitative, objective data on the mechanical properties of food, correlating well with sensory perception and yielding reproducible results critical for scientific and industrial applications. This application note details the integration of compression testing, specifically Texture Profile Analysis (TPA), into structured protocols for monitoring and predicting textural stability.

Fundamentals of Texture Profile Analysis (TPA)

Texture Profile Analysis (TPA) is a double-compression test designed to simulate the biting action of the human mouth. By analyzing the resulting force-time curve, researchers can deconstruct texture into a set of quantitative parameters that provide a comprehensive mechanical profile of a food sample [15].

The following diagram illustrates the standard TPA curve and the derivation of its primary parameters from a typical two-bite test:

• Hardness: The peak force during the first compression cycle, representing the force required to achieve a given deformation [15].
• Fracturability: The force at the first significant break in the curve, if present. Not observed in all materials [15].
• Cohesiveness: The ratio of the positive force area during the second compression to that of the first compression. It represents the strength of the internal bonds within the product [15].
• Springiness: The rate at which a deformed sample returns to its original condition after the deforming force is removed. It is calculated as the ratio of the time difference during the second compression to that during the first compression [15].
• Adhesiveness: The negative force area representing the work necessary to pull the compressing probe away from the sample [15].
• Gumminess and Chewiness: Secondary parameters calculated as Hardness × Cohesiveness, and Hardness × Cohesiveness × Springiness, respectively. Gumminess applies to semi-solid foods, while Chewiness applies to solid foods [15].

Critical Methodological Considerations for TPA

To ensure data integrity and reproducibility, specific test conditions must be meticulously controlled [15]:

• Sample Preparation: Sample dimensions (height and cross-sectional area) must be consistent to allow for valid comparisons between products. Variations in size can lead to significant differences in measured hardness.
• Probe Selection: A compression platen larger than the sample ensures forces are derived from uniaxial compression. A smaller probe introduces shear forces, altering the resulting parameters.
• Degree of Deformation: The extent of compression should be sufficient to mimic the destructive process of mastication. For many gelled systems, deformations of 70-80% are required to cause structural breakdown. Low deformation levels (20-50%) may not induce fracture but can still provide comparative data for hardness, springiness, and cohesiveness.
• Test Speed: The speed of the probe's descent and ascent must be consistent, as the rate of force application affects the material's response. To accurately calculate cohesiveness, the test speed and post-test speed should be identical.
• Time Between Compressions: The dwell time between the two compression cycles significantly impacts parameters like springiness and cohesiveness, especially in viscoelastic materials.

Experimental Protocols

Protocol 1: Shelf-Life Study of a Solid Food Product

This protocol outlines a comprehensive approach to determining the shelf-life of a solid food (e.g., cake, cheese, or plant-based meat analog) by tracking textural changes under controlled storage conditions.

1. Objective: To monitor the temporal changes in the textural properties of a solid food product stored under ambient conditions throughout its anticipated shelf-life.

2. Materials and Equipment:

• Texture Analyzer (e.g., TA.XT+ or Mecmesin OmniTest) equipped with a 50 kg load cell [78] [19]
• Large Diameter Flat Plate Compression Probe (e.g., 75 mm diameter)
• Standardized StableMicro Systems or Mecmesin software
• Precision Balance
• Temperature- and Humidity-Controlled Storage Chamber [78]
• Sample Preparation Tools

3. Methodology:

• Step 1: Sample Preparation. Prepare a single, large batch of the product following a standardized recipe and process. Slice or cut the product into uniform cylinders (e.g., 20 mm height x 20 mm diameter). Record the exact dimensions and weight of each sample.
• Step 2: Initial Time Point (T=0). Analyze a minimum of 10 replicates immediately after production and cooling to ambient temperature.
• Step 3: Storage and Sampling. Place the remaining samples in a stability chamber set to 25°C and 60% relative humidity [78]. Remove and test a minimum of 10 replicates at predetermined intervals (e.g., 1, 2, 4, 8, 12, 16, 20, and 24 weeks).
• Step 4: Compression Test Setup.
  • Test Type: TPA
  • Pre-test Speed: 1.0 mm/s
  • Test Speed: 1.0 mm/s [15]
  • Post-test Speed: 1.0 mm/s [15]
  • Target Strain: 50% (or 70% for fracture-prone products)
  • Trigger Force: 5 g
  • Time Between Compressions: 3 seconds
• Step 5: Data Collection. For each sample, record Hardness, Springiness, Cohesiveness, and Chewiness.

4. Data Analysis:

• Plot the mean value for each TPA parameter against storage time.
• Fit appropriate models (e.g., zero-order or first-order kinetic models) to the data to predict the rate of textural change.
• Define the end of shelf-life as the time point at which a key textural parameter (e.g., Hardness) changes beyond a predefined acceptable threshold (e.g., a 25% increase from T=0).

Protocol 2: Accelerated Shelf-Life and Staling Study

This protocol uses elevated stress conditions to rapidly predict the long-term textural stability and staling kinetics of bakery products.

1. Objective: To accelerate the staling process of a bakery product (e.g., bread) to rapidly estimate its textural shelf-life under normal storage conditions.

2. Materials and Equipment: (As in Protocol 1, with the following addition)

• Stability Chamber capable of 40°C and 75% Relative Humidity [78]

3. Methodology:

• Step 1: Sample Preparation. Follow Step 1 from Protocol 1.
• Step 2: Storage. Divide samples into two groups:
  • Group 1 (Ambient Control): Store at 25°C / 60% RH.
  • Group 2 (Accelerated): Store at 40°C / 75% RH [78].
• Step 3: Sampling. Test samples from both groups simultaneously at frequent intervals (e.g., Days 0, 1, 2, 3, 5, and 7 for the accelerated group; and Weeks 0, 1, 2, 4, and 8 for the ambient group).
• Step 4: Compression Test. Use the same TPA settings as in Protocol 1, with a target strain of 50%.

4. Data Analysis:

• Calculate the rate of hardness increase for both storage conditions.
• Determine the acceleration factor (AF) by comparing the rate constants (kambient vs. kaccelerated), where AF = kaccelerated / kambient.
• Use this AF to extrapolate the ambient shelf-life from the accelerated data and validate with the actual ambient data.

The workflow for designing and executing a shelf-life study incorporating these protocols is systematic, as shown below:

Data Presentation and Analysis

Quantitative data from TPA and shelf-life studies should be systematically organized to facilitate interpretation and comparison. The following table summarizes key textural parameters and their relevance in shelf-life assessment.

Table 1: Key TPA Parameters and Their Significance in Shelf-Life Studies

Parameter	Definition	Significance in Shelf-Life	Typical Change During Aging
Hardness	Peak force during first compression cycle.	Indicates firming or softening. Critical for consumer perception of freshness.	Increases (e.g., bread staling, cheese hardening).
Springiness	Ability to recover shape after deformation.	Reflects loss of elastic components (e.g., moisture, fat).	Decreases.
Cohesiveness	Strength of internal bonds.	Indicates structural integrity and breakdown.	Decreases.
Chewiness	Hardness × Cohesiveness × Springiness.	Overall work required to masticate a solid food.	Typically increases in bakery products due to staling.
Adhesiveness	Work to overcome attractive forces between food and other surfaces.	Can indicate moisture loss or ingredient interactions.	Varies by product.

Recent research provides concrete examples of TPA data. A 2025 study on pureed meats for dysphagia patients demonstrated that the addition of a 1% food-shaping agent significantly increased hardness (from a range of ( 1.5 \times 10^3 ) to ( 3.2 \times 10^3 ) N/m² to a range of ( 2.8 \times 10^3 ) to ( 4.1 \times 10^3 ) N/m²) and adhesiveness (p < 0.001), while cohesiveness remained unchanged [4]. This objective data was crucial for standardizing texture for safety.

Another 2025 study on plant-based deli meats used mechanical testing to reveal that plant-based products were more than twice as stiff as their animal counterparts (e.g., plant-based turkey: 378 kPa vs. animal turkey: 134 kPa). This instrumental stiffness showed a strong positive correlation (Spearman's ρ=0.857, p=0.011) with the sensory perception of "brittleness," validating the use of instrumental metrics for product development [79].

Table 2: Exemplar TPA Data from a Theoretical Cake Shelf-Life Study (Stored at 25°C)

Storage Time (Weeks)	Hardness (N)	Springiness (Ratio)	Cohesiveness (Ratio)	Chewiness (N)
0	15.2 ± 1.5	0.92 ± 0.03	0.68 ± 0.04	9.5 ± 1.2
2	18.5 ± 1.8	0.90 ± 0.04	0.65 ± 0.03	10.8 ± 1.5
4	24.1 ± 2.1	0.87 ± 0.05	0.60 ± 0.05	12.6 ± 1.8
8	32.7 ± 2.5	0.82 ± 0.06	0.55 ± 0.04	14.8 ± 2.1
12	41.5 ± 3.0	0.78 ± 0.07	0.51 ± 0.05	16.5 ± 2.4

The Scientist's Toolkit

Successful execution of texture-based shelf-life studies requires specific reagents and equipment. The following table lists key solutions and their functions.

Table 3: Research Reagent Solutions for Texture Analysis Studies

Item	Function/Application	Example Use-Case
Food-Shaping Agents	Polysaccharide-based powders (e.g., dextrin, xanthan gum) to modify cohesion, water-binding, and texture stability.	Standardizing texture of pureed meals for dysphagia patients to meet IDDSI Level 4 criteria [4].
Texture Analyzer	Instrument to perform TPA and other mechanical tests via controlled compression/extension.	TA.XT+ (Stable Micro Systems) or OmniTest (Mecmesin) systems for quantifying hardness, springiness, etc. [78] [19].
Flat Plate Compression Probe	A probe larger than the sample to apply uniaxial compression for TPA.	Measuring firmness of cakes, cheeses, and gels [15].
Stability Chambers	Environmental chambers to control temperature and humidity during shelf-life studies.	Maintaining standard conditions (e.g., Ambient: 25°C/60% RH; Accelerated: 40°C/75% RH) [78].
Hunter Colorimeter	Instrument for objective color measurement (L, a, b*).	Tracking color changes in parallel with texture degradation during storage [78].

Compression testing, particularly Texture Profile Analysis, provides an objective, robust, and standardized methodology for quantifying critical textural changes in solid foods during storage. The protocols outlined herein for both real-time and accelerated shelf-life studies offer researchers a clear framework for determining product stability, optimizing ingredients, and validating packaging solutions. By integrating these instrumental measurements with sensory evaluation and consumer insights, scientists can effectively bridge the gap between laboratory data and real-world product experience, ultimately driving innovation and ensuring quality in food product development.

Texture is a pivotal parameter in the world of alternative proteins, often as important as flavor in determining consumer preference [80] [19]. As the sector strives to replicate the sensory attributes of traditional animal proteins, quantifying and understanding textural properties becomes paramount for product acceptance and market success [5] [80]. Compression testing, particularly Texture Profile Analysis (TPA), provides researchers with objective, quantitative methodologies to characterize the mechanical properties of solid food matrices, enabling data-driven formulation decisions [16] [15].

The double-compression cycle of TPA simulates the biting action of the human mouth, providing parameters that correlate well with sensory evaluation [16] [15]. This technical note outlines standardized protocols and application case studies for utilizing compression testing in alternative protein development, with specific focus on plant-based meat analogs. By employing these methodologies, researchers can systematically optimize formulations, validate processing parameters, and ensure batch-to-batch consistency in this rapidly evolving sector [80].

Theoretical Framework: Fundamentals of Texture Profile Analysis

Texture Profile Analysis (TPA) is a widely used destructive force/deformation method that involves compressing a bite-size piece of food twice in a reciprocating motion to simulate the action of the jaw [16] [15] [81]. This technique provides multiple quantitative parameters from a single test, generating a force-time curve that reveals fundamental structural characteristics of the sample material [16].

Key TPA Parameters and Their Sensory Correlates

The following parameters are derived from the TPA curve and correlate with specific sensory experiences:

Table 1: Fundamental TPA Parameters and Their Significance

Parameter	Definition	Sensory Correlation	Calculation
Hardness	Peak force during first compression cycle	Firmness, resistance to biting	Force at first peak [16] [15]
Springiness	Degree to which sample returns to original height after deformation	Elastic recovery, bounce-back	Time diff 4:5/Time diff 1:2 [16] [15]
Cohesiveness	Extent of sample integrity under compression	Internal bonding strength, structural integrity	Area 4:6/Area 1:3 [16] [15]
Adhesiveness	Work necessary to overcome attractive forces between food and other surfaces	Stickiness, mouth coating	Negative force area [16] [15]
Chewiness	Energy required to masticate solid food to swallowing consistency	Chewing effort	Hardness × Cohesiveness × Springiness [16] [15]
Gumminess	Energy required to disintegrate semi-solid food to swallowing consistency	Thickness perception	Hardness × Cohesiveness [16] [15]

Material-Specific Texture Profiles

Different material compositions produce distinctive TPA curve characteristics, enabling researchers to classify samples based on their mechanical properties [16] [15]:

• Hard and brittle materials: Exhibit steep initial rise and high first peak with much smaller second compression area, indicating low cohesiveness and elasticity [16].
• Soft and elastic materials: Display gradual rise to first peak with similarity between first and second compression areas, reflecting high cohesiveness and elasticity [16].
• Gummy or sticky materials: Show pronounced negative area (adhesiveness) with less distinct peaks due to material deformation and adhesion to the probe [16].
• Firm but cohesive materials: Demonstrate similar first and second peaks with minimal negative area, indicating good internal bonding and recovery [16].

Experimental Protocols: Standardized Methodologies for Alternative Protein Analysis

Protocol 1: Fundamental TPA for Plant-Based Meat Analogs

This protocol adapts the standardized TPA method for characterizing plant-based burger patties, sausages, and whole-muscle analogs, enabling direct comparison with animal meat benchmarks.

Materials and Equipment

Table 2: Essential Research Reagent Solutions and Equipment

Item	Specification	Function/Application
Texture Analyzer	Stable Micro Systems TA.XT Plus or equivalent	Controlled compression/deformation testing [80] [49]
Compression Platen	75-100mm diameter (P/75, P/100)	Uniform compression of self-supporting samples [80]
Warner-Bratzler Blade	HDP/BS	Measurement of bite force, firmness, toughness in sausage-type products [80]
Multiple Puncture Probe	A/MPP	Penetration testing of non-uniform products (nuggets, patties) for averaging effect [80]
Kramer Shear Cell	5-bladed (A/KS5)	Bulk shearing measurement for non-uniform shapes and variable textures [80]
Sample Preparation Mold	Cylindrical, 20mm diameter × 20mm height	Standardized sample geometry for comparable TPA results [15]
Temperature Control Chamber	5-40°C range	Maintain consistent sample temperature during testing [49]

Sample Preparation

• Formulation: Prepare plant-based protein matrix using standard ingredients (soy, pea, wheat gluten, etc.) with binding agents (methylcellulose, starches, gums) as per experimental design [80].
• Processing: Utilize appropriate processing method (high-moisture extrusion, shear cell technology, or simple mixing) to create structured protein matrix [80].
• Shaping: Form cylindrical samples (20mm diameter × 20mm height) using precision mold to ensure uniform geometry [15]. For anisotropic, whole-muscle analogs, ensure consistent fiber orientation across samples.
• Cooking: Apply standardized thermal processing (grilling, boiling, or steaming) to internal temperature of 75°C, monitoring with thermocouple [5].
• Equilibration: Cool to room temperature (20°C) and maintain at testing temperature for minimum of 30 minutes prior to analysis [49].

Instrumental Parameters

Table 3: Standard TPA Instrument Settings for Meat Analogs

Parameter	Setting	Rationale
Pre-test Speed	2 mm/s	Prevents overshooting trigger force; ensures accurate initial contact [15]
Test Speed	1 mm/s	Simulates typical biting speed [15]
Post-test Speed	1 mm/s	Must match test speed for accurate cohesiveness calculation [15]
Compression	70-80% strain	Ensures sample breakdown mimicking mastication [15]
Trigger Force	0.05 N	Ensures probe contact without premature triggering [15]
Time Between Compressions	3-5 seconds	Allows partial sample recovery; simulates subsequent bite [15]
Data Acquisition Rate	200-500 Hz	Captures sufficient data points for accurate peak detection [16]

Data Analysis and Interpretation

• Curve Analysis: Identify key points on force-time curve including first peak (hardness), negative area (adhesiveness), and second peak [16].
• Parameter Calculation: Calculate TPA parameters using software algorithms or manual calculation according to standard formulas [16] [15].
• Statistical Treatment: Perform ANOVA with post-hoc testing (Tukey's HSD) for multiple formulation comparisons (n ≥ 8 replicates per formulation).
• Sensory Correlation: Establish correlation coefficients between instrumental parameters and sensory panel evaluations for key attributes (hardness, chewiness, springiness).

Protocol 2: Back-Extrusion Testing for Pureed and Soft Alternative Protein Products

This protocol is optimized for characterizing texture-modified alternative protein products, including purees, spreads, and pâtés, particularly relevant for specialized nutritional applications [49].

Materials and Equipment

• Texture analyzer with 5-50 kg load cell capacity [49]
• Back-extrusion cylindrical probe (35 mm diameter) [49]
• Methacrylate cells (50 mm inner diameter × 60 mm height) [49]
• Temperature control system (5°C, 20°C, 40°C) [49]

Testing Parameters

• Test distance: 30 mm (60% strain level) [49]
• Test speed: 5 mm/s [49]
• Pre-test and post-test speed: 10 mm/s [49]
• Trigger force: 0.049 N [49]
• Temperature conditions: 5°C, 20°C, and 40°C to simulate consumption scenarios [49]

Case Study: Plant-Based Meat Stiffness Validation

A recent study characterized the texture of five plant-based and three animal meats using texture profile analysis and rheology, reporting ten mechanical features associated with each product's elasticity, viscosity, and loss of integrity [5].

Comparative Stiffness Analysis

Table 4: Stiffness and Viscoelastic Properties of Alternative and Animal Meat Products

Product Type	Stiffness (kPa)	Storage Modulus, G' (kPa)	Loss Modulus, G'' (kPa)
Plant-based turkey	418.9 ± 41.7	50.4 ± 4.1	25.3 ± 3.0
Animal turkey	194.2 ± 22.5	22.8 ± 2.1	10.1 ± 1.2
Animal sausage	153.7 ± 18.9	18.3 ± 1.8	8.7 ± 0.9
Animal hotdog	132.4 ± 15.3	15.9 ± 1.5	7.2 ± 0.8
Tofu	56.7 ± 14.1	5.7 ± 0.5	1.3 ± 0.1

Key Findings and Implications

The research demonstrated that plant-based turkey exhibited significantly higher stiffness (418.9 kPa) compared to all animal meat products tested, while tofu showed considerably lower stiffness (56.7 kPa) [5]. All three animal products (turkey, sausage, and hotdog) consistently ranked between these two extremes, with stiffness values ranging from 132.4 to 194.2 kPa [5]. Of the ten mechanical features evaluated, stiffness, storage modulus, and loss modulus proved to be the most meaningful and consistent parameters, while other TPA parameters showed limitations in interpretability and inconsistent definitions across studies [5].

The findings confirm that modern food fabrication techniques can successfully create plant-based meats that replicate the full viscoelastic texture spectrum of processed animal meat, providing a quantitative benchmark for future formulation optimization [5].

Methodological Considerations for Robust Texture Analysis

Critical Factors Influencing TPA Results

Several methodological factors significantly impact TPA results and must be carefully controlled for reproducible data [15]:

• Sample dimensions: TPA parameters are highly dependent on sample geometry; consistent dimensions are essential for valid comparisons [15].
• Probe selection: Compression probes larger than sample size measure largely uniaxial compression forces, while smaller probes introduce puncture and shear components [15].
• Extent of deformation: Compression to 70-80% deformation ensures sample breakdown mimicking mastication, unlike lower deformations (20-50%) that may not rupture structure [15].
• Test speed: Slower rates allow greater sample relaxation; speed should reflect actual eating conditions for the specific food type [15].
• Time between compressions: This delay determines recovery period and significantly affects springiness, cohesiveness, and chewiness parameters [15].

Parameter Relevance and Interpretation

Not all TPA parameters are applicable to every product, and researchers should exercise judgment in parameter selection and interpretation [15]. For instance, springiness values for products like chocolate may not be repeatable or meaningful, just as adhesiveness may not be relevant for bread [15]. It is recommended to identify the important textural parameters for a specific product before testing rather than collecting all possible TPA parameters without consideration of their relevance [15].

Compression testing through Texture Profile Analysis provides alternative protein researchers with robust, quantitative methodologies for validating novel formulations against traditional animal-based benchmarks. The standardized protocols outlined in this application note enable systematic characterization of key textural attributes, facilitating data-driven formulation decisions and processing optimization.

As the alternative protein sector continues to evolve, compression testing will play an increasingly important role in quantifying the textural properties of emerging protein sources, including algae, fungi, insect, and cell-based proteins [80]. The integration of advanced analytical techniques with fundamental compression testing will further enhance our understanding of structure-function relationships in alternative protein matrices, accelerating the development of products that successfully replicate the sensory experience of animal-based foods.

Alternative Protein Texture Validation Workflow

Establishing Reference Values and Pass/Fail Limits for Quality Assurance

In solid food texture research, compression testing provides objective, quantitative data on fundamental mechanical properties such as firmness, hardness, and elasticity [19]. Establishing statistically justified reference values and pass/fail limits is critical for ensuring product consistency, meeting regulatory standards, and maintaining consumer satisfaction [19] [82]. This protocol details the methodology for determining these essential quality parameters within a research and development framework.

Defining Key Texture Parameters

Theoretical Foundation: Hardness vs. Firmness

Instrumental measurements of texture must distinguish between two frequently confused parameters:

• Hardness is properly defined as the stress or force required to break a food structure and should be measured via large deformation compression, typically to the point of fracture [2].
• Firmness describes a moderate level of hardness, often associated with a material's resistance to small deformation (typically around 0.1 strain) and is generally non-destructive [2].

The table below summarizes the critical distinctions:

Table 1: Differentiation between Hardness and Firmness Testing

Parameter	Definition	Typical Strain	Nature of Test	Common Applications
Hardness	Force/Stress required to break a food structure [2]	High (e.g., 0.75)	Destructive	Biscuits, hard candies, brittle gels
Firmness	Resistance to a small, compressive deformation [2]	Low (e.g., 0.1)	Non-destructive	Fruit, cheese, soft cakes, bread

Standards and Units of Measurement

Ambiguity exists in reporting, with some studies using force (N) and others using stress (Pa) [2]. The contact geometry (e.g., plate vs. die loading) influences whether a true stress calculation is possible. Adherence to documented international standards, such as ISO 16305 for butter firmness or GME Bloom for gelatine strength, ensures consistency and comparability [19].

Experimental Protocol for Establishing Reference Values

Equipment and Reagent Solutions

The following toolkit is essential for executing standardized compression tests.

Table 2: Research Reagent Solutions and Essential Materials

Item	Function/Description	Key Considerations
Texture Analyser	Main instrument applying controlled compression and measuring force/distance [3]	Requires calibrated load cell appropriate for expected force range (e.g., 1 N to several kN) [19] [3]
Compression Platens/Probes	Apply force to the sample; choice depends on sample geometry and test goal [3]	Cylindrical probes for general use; flat platens for uniform materials; bulk fixtures (e.g., Ottawa Cell) for multi-particle samples [3]
Temperature Control Chamber	Maintains sample at a specified temperature during testing	Critical for temperature-sensitive materials like fats and gels [3]
Standard Reference Materials	Used for instrument calibration and method validation	Materials with known, stable mechanical properties (e.g., calibrated springs, standard gels)
Sample Preparation Tools	For cutting and shaping samples to consistent dimensions	Cutters, cork borers, and precision knives ensure sample uniformity [19]

Step-by-Step Test Methodology

This workflow outlines the process from sample preparation to data analysis for establishing reference values.

Figure 1: Experimental workflow for establishing reference values via compression testing.

• Define Objective and Select Standard Method: Determine the key attribute (e.g., hardness, firmness) and identify a relevant international or internal standard (e.g., ASTM, ISO) to follow [19] [83].
• Sample Preparation:
  • Prepare a minimum of 8-12 replicates for heterogeneous foods to account for natural variability; 4-6 replicates may suffice for homogeneous gels [3].
  • Control and document sample size, shape, and temperature precisely, as these significantly impact results [19] [2].
• Equipment Configuration:
  • Select the appropriate probe (e.g., cylindrical probe for gels, platen for bakery items) and load cell capacity [3].
  • Set test parameters based on the standard method, including pre-test speed, test speed, target strain or distance, and post-test speed [19] [3].
• Execute Test and Data Collection:
  • Perform compression tests according to the defined protocol.
  • The instrument software (e.g., VectorPro) will typically record a force-time or force-distance curve [19].
• Data Analysis and Reference Value Establishment:
  • From the resulting curve, identify the target parameter (e.g., peak force for hardness, slope for firmness) [3].
  • Calculate the mean and standard deviation for the parameter across all replicates.
  • The reference value is the mean value representing the optimal product quality.

Protocol for Setting Pass/Fail Limits

Statistical Approach to Limit Setting

Pass/fail limits are derived from the reference value and the natural variability of the production process. The following methodology is recommended for robust quality control.

Figure 2: Logical process for defining and implementing pass/fail limits.

• Data Collection: Expand data collection beyond the initial reference set to include samples from multiple production batches over time. This provides a more robust estimate of process variability.
• Calculate Control Limits: Using historical data, calculate the standard deviation (σ) of the key texture parameter. Common statistical control limits are set at:
  • Warning Limits: Typically set at ±2σ from the reference mean.
  • Pass/Fail (Control) Limits: Typically set at ±3σ from the reference mean. A result outside these limits indicates the process is likely out of control [19].
• Define Action Limits Based on Sensory Correlation: Where possible, correlate instrumental data with sensory panel results. The pass/fail limits must ensure that products falling outside the range are perceptibly different or unacceptable to consumers [84]. For instance, if a compression force above 15 N is consistently described as "too hard" by a trained panel, the upper fail limit should be set at or below this value.
• Validate and Implement: Program the pass/fail limits into the texture analysis software for automated quality control. Operators can then run tests and receive immediate pass/fail determinations [19].

Example: Firmness Limits for a Model Food Gel

The following table illustrates hypothetical reference and limit values for a food gel's firmness, measured as peak force under compression.

Table 3: Example Reference Values and Pass/Fail Limits for a Model Food Gel

Parameter	Value (N)	Description	QA Action
Reference Value (Mean)	10.0	Target firmness, optimal quality	Target for production
Standard Deviation (σ)	0.5	Measure of process variability	-
Upper Warning Limit	11.0	+2σ	Monitor process closely
Lower Warning Limit	9.0	-2σ	Monitor process closely
Upper Fail Limit	11.5	+3σ	Reject batch; adjust process
Lower Fail Limit	8.5	-3σ	Reject batch; adjust process

Advanced Considerations and Method Validation

The Challenge of Perceived Texture

Instrumental measurements, while precise, may not always perfectly align with human perception. Research, such as that in sweet cherries, shows that standard compression tests do not always correlate well with perceived texture, prompting investigations into alternative methods like hyperspectral imaging [84]. Therefore, instrumental pass/fail limits should be validated against human sensory evaluation to ensure they reflect the consumer experience.

Factors Affecting Repeatability

To maintain the integrity of established limits, several factors must be rigorously controlled [19]:

• Instrument Calibration: Regular calibration of the texture analyser and load cell is non-negotiable.
• Environmental Control: Sample temperature and testing environment must be standardized.
• Sample Uniformity: Consistent sample preparation is critical for reproducible results.
• Operator Training: All personnel must follow the documented protocol without deviation.

Conclusion

Compression testing is an indispensable, versatile tool for quantifying the mechanical properties of solid foods, providing objective data that is critical for research, quality assurance, and product development. A deep understanding of foundational principles ensures accurate attribute definition, while robust methodological execution guarantees reliable and reproducible data. Success hinges on optimizing protocols to overcome material-specific challenges and acknowledging that instrumental measurements, while precise, are method-dependent. The ultimate value of compression data is realized through rigorous validation against sensory perception and established standards, creating a powerful feedback loop for product innovation. Future directions will see these methodologies increasingly applied to the development of next-generation foods, such as tailored senior-friendly products and sophisticated plant-based and cultured meat analogues, where replicating specific textural experiences is paramount to clinical, nutritional, and commercial success.

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