Freshness and Shelf Life of Foods - American Chemical Society

Chapter 19

Rheological Methods for Assessment of Food Freshness and Stability Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: October 10, 2002 | doi: 10.1021/bk-2003-0836.ch019

V. D. Truong and C. R. Daubert Department of Food Science, North Carolina State University, Raleigh, N C 27695-7624

Food rheology is considered the material science for food systems. Rheological approaches to shelf-life and freshness may range from a simple squeezing technique to advanced oscillatory methodologies probing material microstructure. As rheology relates to consumer perceptions of quality and freshness, a more common sensory term, texture, is employed. Changes in texture and stability during storage have been recognized as important factors influencing consumer acceptability of many food products. Texture is a complex attribute of food quality and can only be measured directly by sensory evaluation. However, many instrumental methods are used to measure mechanical properties of foods that are, up to a certain extent, related to sensory characteristics. Large strain methods such as puncture, penetration, bending, tension, shear, compression, and texture profile analysis are commonly used to evaluate freshness and textural changes of foods with respect to storage conditions. For fluid foods, various rheological techniques evaluate yield stress and shear viscosity, providing information about pourability, thickness, and dispersion or emulsion stability over time. Small strain methods, specifically dynamic oscillatory shear and mechanical analyses, are useful in probing microstructure, viscoelastic properties, and phase transitions in food materials. This chapter reviews rheological methods applicable to assessment of freshness and textural stability of foods.

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© 2003 American Chemical Society

In Freshness and Shelf Life of Foods; Cadwallader, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Introduction Changing texture and stability during storage is an important factor influencing consumer acceptability of many food products. However, the effect of storage on food texture, with few exceptions (bread and muscle meat), has not been studied extensively because texture changes are generally considered quality attributes rather than potential health hazards. Moreover, the importance of texture in consumer perception has only recently gained general recognition (1). Texture has been defined as a "group of physical characteristics that arise from the structural elements of the food, are sensed primarily by the feeling of touch, are related to the deformation, disintegration and flow of the food under a force, and are measured objectively by functions of mass, time, and length" (2). This definition indicates texture: 1) is a multidimensional property comprising a number of sensory characteristics, and 2) has roots in food structure and the manner by which this structure responds to applied conditions. In practice, the term texture is used primarily for solid and semi-solid foods, while viscosity and consistency describe the flow behavior of pourable foods. Being a complex sensory property, food texture can be measured directly by sensory evaluation. During the past few decades, many instrumental methods have been used to measure mechanical characteristics of solid- and liquid- type foods that are, up to a certain extent, related to sensory attributes. More details on textural measurements are available from several excellent sources (2-5). Food rheology, the material science for food, investigates the mechanical response of food systems during flow or deformation. Rheological analyses benefit scientists and engineers by providing a quantitative assessment of ingredient interactions, processing conditions, and shelf-life effects on food texture, consistency, and integrity. Essentially all rheological tests attempt to relate a magnitude of force with a corresponding shape change in a material. This simplistic view of rheology can be further extended to categorize measurements as either fundamental or empirical in nature.

Fundamental Fundamental procedures are derived from basic, physical relationships where stress is a force term and strain is the deformation parameter. Rheological properties, like viscosity and moduli, are important material parameters relating stress with strain. A stress is the force applied across a given area: force per area; and the relative deformation or strain describes the degree of shape change a material undergoes. The unique aspect of fundamental rheology is that these constitutive relationships between stress and strain are independent of sample volume or physical dimensions. Accordingly, these


250 basic properties transcend instrumentation selection and should be obtained regardless of the measuring system.


Empirical Empirical measurements, on the other hand, simply record a measurement of force and the corresponding sample deflection. Unfortunately, many extenuating circumstances tend to confound empirical measurements, being strongly dependent upon instrumentation, sample volume and geometry. Nevertheless, empirical techniques have a secure place in the food industry, providing a relatively quick indication of quality and texture for process control and assurance practices.

Large Strain Methods As the name indicates, large strain techniques subject samples to large deformation, resulting in structural breakdown and/or fracture. Depending on the test conditions and material characteristics, various degrees of correlation between the results of large strain tests and sensory data have been reported (6-8). The general principle of large strain tests is to promote contact between a probe and food sample, while measuring a force and/ or deformation during the process. Most methods are based on force measurements and can be performed with a single point device or universal testing machines. These techniques may be classified according to the type of action and/or probe involved.

Penetration and Puncture Tests Penetration and puncture are considered empirical tests, wherein both normal and shear forces may be involved (Fig. 1) during the procedure. Penetration and puncture techniques are widely used in texture measurement of many foods including fruits, vegetables, butter, hydrocolloid gels (2, 4, 9), cheese (10), chips (11), and sea foods (12). In addtion, this simple procedure is used to evaluate different parts of a food on the overall product texture, such as the crust and inner portion of a french fry (13) or the exocarp, mesocarp, and endocarp of a tomato (14). During penetration testing, the depth or time to reach a certain distance is measured under a constant force, while puncture tests measure the force resulting during probe penetration (2). Probes with



251

various shapes, such as a flat-ended cylinder (Bloom Gelometer), cone, sphere (penetrometer for firmness, consistency and spreadability of fat-based products) or needle (Magness-Taylor fruit firmness tester, multiple pea attachment of T A . X T 2 Texture Analyzer) are made to penetrate into the test sample (Fig. 1). The acquired force-deformation data (Fig. 2) are used to calculate indices of hardness, firmness, and toughness. These indices have been documented to decrease with storage of fruits, vegetables (15), fresh fish (16), and increase with ageing of other foods such as bread and carbohydratebased products (17). Attempts have been made to relate the penetration forces with fundamental parameter such as Young's modulus (18) and absolute shear modulus using finite element analysis (19, 20).

Shear and Cutting Tests

Shear tests can be performed with single (Warner-Bratzler shear device) or multiple blades (Kramer shear cell) that slice through samples (Fig. 3). The maximum force and energy required to shear are taken as an index of firmness, tenderness, or toughness of fibrous foods (2, 21-23). As shown in Fig. 4, the shear force of minimally processed sweetpotato strips decreased with storage time (25). For texture measurement of raw fish, the shear test was recommended because it was more sensitive than puncture using cylindrical or spherical probes (26). Although these tests are called "shear," forces due to friction, compression, and tension are also encountered. The magnitude of these forces depends on the structural properties and rupture mechanism of the tested materials and should be considered when interpreting results (27, 28). Therefore, shear devices can only provide empirical comparisons under specified conditions. Detailed discussion and interpretation of the force-deformation patterns of the shear tests are available in the literature (2,28). Cutting with a sharp blade or wire has also been used to measure the cutting forces of fruits, vegetables, meat, and dairy products (29-31). The wire cutting test involves fracture, deformation, and friction (Fig. 5). These parameters have been used to derive relationships among fundamental properties (32),

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Figure 1- Geometry of a penetration test.

Fracture (Hardness)

Deformation (mm) Figure 2- A typical force-deformation curve.



253

Figure 3- Cross-sectional view of (a) Warner-Bratzler shearing device and (b) Kramer shear cell.



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Figure 4- Change in force values with storage time of minimally processed sweetpotato strips tested in

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Figure 5-Wire cutting: (a) wire in a sample block; (b) plastic deformation and frictional forces (adaptedfrom32).


256 friction, σ is a yield stress, and d is the wire diameter. For steady state cutting, the constant force per unit width, F / B , is proportional to d with a slope of ( l + μ ) a and an intercept of G . The G values obtained from the wire-cutting test on cheeses were very comparable with those of the singleedge notch bending test (SENB, see below), and the slopes, ( l + μ ) a , were highly correlated with μ and a of compression and friction tests (32). The cutting test is easy to perform and generates fundamental variables. Alvarez et al. (33) applied the cutting test with a single-edge razor blade to monitor the textural changes of apples during storage. They reported that cutting energy decreased linearly with storage time and appeared to be a more precise measure than fracture toughness of the SENB test. γ

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Bending/Snapping This class of large strain tests measures the force required to bend or snap a food with a well defined shape, usually a bar, cylinder, or sheet (34, 13). The bending technique detected changes to tortilla texture during storage (35). The fundamental, bending parameters, namely fracture strain, fracture stress, and flexural modulus can be derived from the standard beam-bending equations adapted from material science (34, 36), (4) 3PL

(5) 2

where a = fracture stress (N m" ), e = fracture strain, F = force at fracture (N), L = span distance (m), W = width of specimen (m), Β = thickness of specimen (m), Y = deformation of the beam at fracture (m). f

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A major disadvantage of the bending test is that the upper surface will be in compression, and the lower curved surface will be in tension, resulting in the non-uniform strain and stress state in different parts of the specimen. These problems can be overcome in the notched bending tests such as SENB. The S E N B , a type of three-point bending test, is widely used in studying the fracture of metals and plastics (37). In this test, the sample is notched with a razor blade to give a sharp starter crack with a dimension a, loaded on a three-point bending fixture (Fig. 6), and deformed at a given crosshead speed to generate the force-deformation data. Assuming a linear elastic behavior, the fracture toughness, G , can be computed from c

U ΒΨφ


(6)

257 where U = energy to fracture (area under the load-distance curve), Β = sample thickness, W = sample width and φ (a/W) = a calibration factor. The crack propagated in the SENB test is somehow similar to the crack propagation leading to fracture during biting of a piece of food with the incisors in the mouth. This test was applied in studying the crispness of fruits and vegetables (33, 38), and the influence of ageing on fracture properties of Cheddar cheeses (39).


Compression, tension, and vane tests Compression is similar to puncture testing except that the area of the probe is equal or larger than the area of the sample surface. Several compression devices are widely used in assessing texture of baked goods (Baker Compressimeter), cheeses (Ball Compressor), and fruits and vegetables (Firmness Meter). Uniaxial compression tests have been documented to assess the storage effects on the texture of baked goods (40,41), cheeses (39), fruits and vegetables (2, 42, 43). The American Association of Cereal Chemists approved the compression method for testing firmness of bread and other baked goods (44). Baker and Ponte (45) reported consistent increasing patterns of firmness as function of bread staling time. Earlier work of Bashford and Hartung (46) established a relationship between the force at 25% compression and bread freshness. Guinot and Mathlouthi (40) applied the compression test in studying sponge cake firmness as a function of additive and storage conditions. Uniaxial compression methods generate several mechanical parameters (Fig. 7) namely stress, strain, moduli, and work at fracture to describe the textural properties of many food materials. For compressible materials, e.g. bread and other products with a spongy structure, the cross-sectional area of compression remains releatively unchanged, and the engineering stress ( σ ) and strain (ε ) can be used (47): Ε

Ε

σ

Ρ

= — A 0

_ ΔΗ E

" H

(8)

0

where F = force, A = initial specimen area, H = initial specimen height, ΔΗ = absolute deformation. 0

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Figure 7-Schematic view of stress-strain relationships in a compression-decompression cycle (adaptedfrom48).


259 In addition, the area under the compression-decompression curves (Fig. 7) relates to the recoverable work that describes the degree of elasticity of the materials (48). For incompressible materials or large strain conditions, adjustments were made to stress and strain calculations, correcting for changing area during specimen compression (49, 50): _F(H -AH) Q

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The problems associated with the compression test especially in studying the fracture properties of foods in relation to their sensory characteristics during mastication are: 1) fracture may not occur for highly deformable materials, and 2) compressive forces may dictate the failure mode and cause specimen slumping due to water excretion. Therefore, other techniques have been recommended to improve upon the limitations of compression testing, such as vane (51, 52), torsion (49), and tensile testing (47, 50, 53, 54). Plotting stress and strain at failure from these test modes generates "texture maps" (Fig. 8) describing characteristics of food products as affected by formulations, processing or storage conditions.

Texture Profile Analysis Instrumental texture profile analysis (TPA), an imitative method using a Universal Testing Machine (55), has been widely adapted to characterize textural properties of many food products. In a T P A test, a sample of specific dimensions is uniaxially compressed two times in a reciprocating motion, and the compressive force is recorded as a function of the degree of compression (distance). Several instrumental texture profile parameters can be derived from a T P A curve (Fig. 9): the force at the first significant break is called "fracturability (not all foods exhibit this peak); the maximum force at the end of the first compression cycle equates to "hardness;" the work done to compress the sample during the first and second cycles is the area (Ai and A ) under the respective curves, and the ratio A / A i is called cohesiveness. Other textural terms have been defined from the T P A data including, springiness, adhesiveness, gumminess, and chewiness (56). Basically, T P A is a compression test, therefore, the force-deformation relationship of the initial 2

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Freshness and Shelf Life of Foods - American Chemical Society

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