Rheological Properties In Foods In Food Chemistry

Food texture can be defined as the way in which the various constituents and structural elements are arranged and combined into a micro- and macrostructure and the external manifestations of this structure in terms of flow and deformation.

Most of our foods are complex physico-chemical structures and, as a result, the physical properties cover a wide range—from fluid, Newtonian materials to the most complex disperse systems with semisolid character. There is a direct relationship between the chemical composition of a food, its physical structure, and the resulting physical or mechanical properties; this relationship is presented in Figure 8-1. Food texture can be evaluated by mechanical tests (instrumental methods) or by sensory analysis. In the latter case, we use the human sense organs as analytical tools. A proper understanding of textural properties often requires study of the physical structure. This is most often accomplished by light and electron microscopy, as well as by several other physical methods. X-ray diffraction analysis provides information about crystalline structure, differential scanning calorimetry provides information about melting and solidification and other phase transitions, and particle size analysis and sedimentation methods provide information about particle size distribution and particle shape.

In the study of food texture, attention is given to two interdependent areas: the flow and deformation properties and the macro-and microstructure. The study of food texture is important for three reasons:

1. to evaluate the resistance of products against mechanical action, such as in mechanical harvesting of fruits and vegetables

2. to determine the flow properties of products during processing, handling, and storage

3. to establish the mechanical behavior of a food when consumed

There is sometimes a tendency to restrict texture to the third area. The other two are equally important, although the first area is generally considered to belong in the domain of agricultural engineering.

Because most foods are complex disperse systems, there are great difficulties in establishing objective criteria for texture measurement. It is also difficult in many cases to relate results obtained by instrumental techniques of measurement to the type of response obtained by sensory panel tests.

MICROSCOPY (LM-TEM-SEM) _ X-RAY DIFFRACTION DSC

MECHANICAL TESTS

SENSORY ANALYSIS

MICROSCOPY (LM-TEM-SEM) _ X-RAY DIFFRACTION DSC

CHEMICAL COMPOSITION

>

t

PHYSICAL STRUCTURE

>

f

PHYSICAL PROPERTIES (TEXTURE)

CHEMICAL ANALYSIS

Figure 8-1 Interrelationships in Texture Studies. Source: From P. Sherman, A Texture Profile of Foodstuffs Based upon Well-Defined Rheological Properties, J. FoodSci., Vol. 34, pp. 458-462, 1969.

The terms for the textural properties of foods have a long history. Many of the terms are accepted but are often poorly defined descriptive terms. Following are some examples of such terms:

• Consistency denotes those aspects of texture that relate to flow and deformation. It can be said to encompass all of the rheological properties of a product.

• Hardness has been defined as resistance to deformation.

• Firmness is essentially identical to hardness but is occasionally used to describe the property of a substance able to resist deformation under its own weight.

• Brittleness is the property of fracturing before significant flow has occurred.

• Stickiness is a surface property related to the adhesion between material and adjoining surface. When the two surfaces are of identical material, we use the term cohesion.

A variety of other words and expressions are used to describe textural characteristics, such as body, crisp, greasy, brittle, tender, juicy, mealy, flaky, crunchy, and so forth. Many of these terms have been discussed by Szczesniak (1963) and Sherman (1969); most have no objective physical meaning and cannot be expressed in units of measurement that are universally applicable. Kokini (1985) has attempted to relate some of these ill-defined terms to the physical properties involved in their evaluation. Through the years, many types of instruments have been developed for measuring certain aspects of food texture. Unfortunately, the instruments are often based on empirical procedures, and results cannot be compared with those obtained with other instruments. Recently, instruments have been developed that are more widely applicable and are based on sound physical and engineering principles.

TEXTURE PROFILE

Texture is an important aspect of food quality, sometimes even more important than flavor and color. Szczesniak and Kleyn (1963) conducted a consumer-awareness study of texture and found that texture significantly influences people's image of food. Texture was most important in bland foods and foods that are crunchy or crisp. The characteristics most often referred to were hardness, cohesiveness, and moisture content. Several attempts have been made to develop a classification system for textural characteristics. Szczesniak (1963) divided textural characteristics into three main classes, as follows:

1. mechanical characteristics

2. geometrical characteristics

3. other characteristics, related mainly to moisture and fat content

Mechanical characteristics include five basic parameters.

1. Hardness—the force necessary to attain a given deformation.

2. Cohesiveness—the strength of the internal bonds making up the body of the product.

3. Viscosity—the rate of flow per unit force.

4. Elasticity—the rate at which a deformed material reverts to its unde-formed condition after the deforming force is removed.

5. Adhesiveness—the work necessary to overcome the attractive forces between the surface of the food and the surface of other materials with which the food comes in contact (e.g., tongue, teeth, and palate).

In addition, there are in this class the three following secondary parameters:

1. Brittleness—the force with which the material fractures. This is related to hardness and cohesiveness. In brittle materials, cohesiveness is low, and hardness can be either low or high. Brittle materials often create sound effects when masticated (e.g., toast, carrots, celery).

2. Chewiness—the energy required to masticate a solid food product to a state ready for swallowing. It is related to hardness, cohesiveness, and elasticity.

3. Gumminess—the energy required to disintegrate a semisolid food to a state ready for swallowing. It is related to hardness and cohesiveness.

Geometrical characteristics include two general groups: those related to size and shape of the particles, and those related to shape and orientation. Names for geometrical characteristics include smooth, cellular, fibrous, and so on. The group of other characteristics in this system is related to moisture and fat content and includes qualities such as moist, oily, and greasy. A summary of this system is given in Table 8-1.

Based on the Szczesniak system of textural characteristics, Brandt et al. (1963) devel

Table 8-1 Classification of Textural Characteristics MECHANICAL CHARACTERISTICS

Secondary

Primary Parameters Parameters Popular Terms

Table 8-1 Classification of Textural Characteristics MECHANICAL CHARACTERISTICS

Secondary

Primary Parameters Parameters Popular Terms

Hardness

Soft Firm Hard

Cohesiveness

Brittleness

Crumbly Crunchy Brittle

Chewiness

Tender Chewy —> Tough

Gumminess

Short -» Mealy —> Pasty Gummy

Viscosity

Thin Viscous

Elasticity

Plastic —> Elastic

Adhesiveness

Sticky —> Tacky —> Gooey

GEOMETRICAL CHARACTERISTICS

Class

Examples

Particle size and shape

Gritty, Grainy, Coarse, etc.

Particle shape and orientation

Fibrous, Cellular, Crystalline, etc.

OTHER CHARACTERISTICS

Secondary

Primary Parameters

Parameters

Popular Terms

Moisture content Dry Moist -> Wet Watery

Fat content Oiliness Oily

Greasiness Greasy

Moisture content Dry Moist -> Wet Watery

Fat content Oiliness Oily

Greasiness Greasy

Source: From A.S. Szczesniak, Classification of Textural Characteristics, J. Food Sci., Vol. 28, pp. 385-389,1963.

oped a method for profiling texture so that a sensory evaluation could be given that would assess the entire texture of a food. The texture profile method was based on the earlier development of the flavor profile (Cairncross and Sjostrom 1950).

The Szczesniak system was critically examined by Sherman (1969), who proposed some modifications. In the improved system, no distinction is drawn among analytical, geometrical, and mechanical attributes. Instead, the only criterion is whether a charac teristic is a fundamental property or derived by a combination of two or more attributes in unknown proportions. The Sherman system contains three groups of characteristics (Figure 8-2). The primary category includes analytical characteristics from which all other attributes are derived. The basic rheological parameters, elasticity, viscosity, and adhesion form the secondary category; the remaining attributes form the tertiary category since they are a complex mixture of these secondary parameters. This system is initial perception

Initial perception on palate

Mastication (high shearing stress)

Residual masticatory impression

Rheological Instrumental Techniques
Figure 8-2 The Modified Texture Profile. Source: From P. Sherman, A Texture Profile of Foodstuffs Based upon Well-Defined Rheological Properties, J. FoodSci., Vol. 34, pp. 458-462, 1969.

interesting because it attempts to relate sensory responses with mechanical strain-time tests. Sensory panel responses associated with masticatory tertiary characteristics of the Sherman texture profile for solid, semisolid, and liquid foods are given in Figure 8-3.

OBJECTIVE MEASUREMENT OF

TEXTURE

The objective measurement of texture belongs in the area of rheology, which is the science of flow and deformation of matter. Determining the rheological properties of a food does not necessarily mean that the complete texture of the product is determined. However, knowledge of some of the rheological properties of a food may give important clues as to its acceptability and may be important in determining the nature and design of processing methods and equipment.

Food rheology is mainly concerned with forces and deformations. In addition, time is an important factor; many rheological phenomena are time-dependent. Temperature is another important variable. Many products show important changes in rheological behavior as a result of changes in temperature. In addition to flow and deformation of cohesive bodies, food rheology includes such phenomena as the breakup or rupture of solid materials and surface phenomena such as stickiness (adhesion).

Deformation may be of one or both of two types, irreversible deformation, called flow, and reversible deformation, called elasticity. The energy used in irreversible deformation is dissipated as heat, and the body is permanently deformed. The energy used in reversible deformation is recovered upon release of the deforming stress, when the body regains its original shape.

Force and Stress

When a force acts externally on a body, several different cases may be distinguished: tension, compression, and shear. Bending involves tension and compression, torque involves shear, and hydrostatic compression involves all three. All other cases may involve one of these three factors or a combination of them. In addition, the weight or inertia of a body may constitute a force leading to deformation. Generally, however, the externally applied forces are of much greater magnitude and the effect of weight is usually neglected. The forces acting on a body can be expressed in grams or in pounds. Stress is the intensity factor of force and is expressed as force per unit area; it is similar to pressure. There are several types of stress: compressive stress (with the stress components directed at right angles toward the plane on which they act); tensile stress (in which the stress components are directed away from the plane on which they act); and shearing stress (in which the stress components act tangentially to the plane on which they act). A uniaxial stress is usually designated by the symbol c, a shearing stress by x. Shear stress is expressed in dynes/cm2 when using the metric system of measurement; in the SI system it is expressed in N/m2 or pascal (P).

Deformation and Strain

When the dimensions of a body change, we speak of deformation. Deformation can be linear, as in a tensile test when a body of original length L is subjected to a tensile stress. The linear deformation AL can then be expressed as strain £ = AL/L. Strain can be

TERTIARY CHARACTERISTICS

Mechanical properties

(masticatory)

TERTIARY CHARACTERISTICS

Crisp, brittle, powdery Moist, dry, sticky Tough, tender

Pasty, crumbly, coherent Moist, dry, sticky, soggy Lumpy, smooth

Mechanical properties

(masticatory)

Crisp, brittle, powdery Moist, dry, sticky Tough, tender

Rubbery, spongy, tender, plastic Moist, dry, sticky, soggy Smooth, coarse

Pasty, crumbly, coherent Moist, dry, sticky, soggy Lumpy, smooth

Chocolate, cookies, frozen ice cream, frozen water ices, hard vegetables, hard fruit, corn flakes, potato crisps

Processed cheese, yogurt, cake batters, mashed potato, sausage meat, jam, high-fat content cream, synthetic cream

Meat, cheese, bread, cake, margarine, butter, gels, Jell-O, puddings

- Thin, watery, viscous

- Creamy, fatty, greasy

- Sticky

Thawed ice cream and water ices, mayonnaise, salad dressings, sauces, fruit drinks, soups

Figure 8-3 Panel Responses Associated with Masticatory Tertiary Characteristics of the Modified Texture Profile expressed as a ratio or percent; inches per inch or centimeters per centimeter. In addition to linear deformations, there are other types of deformation, such as in a hydrostatic test where there will be a volumetric strain A V/V.

For certain materials the deformation resulting from an applied force can be very large; this indicates the material is a liquid. In such cases, we deal with rate of deformation, or shear rate; cbfldt or y. This is the velocity difference per unit thickness of the liquid, y is expressed in units of s-1.

Viscosity

Consider a liquid contained between two parallel plates, each of area A cm2 (Figure 8-4). The plates are h cm apart and a force of P dynes is applied on the upper plate. This shearing stress causes it to move with respect to the lower plate with a velocity of v cm s_1. The shearing stress x acts throughout the liquid contained between the plates and can be defined as the shearing force P divided by the area A, or PI A dynes/cm2. The deformation can be expressed as the mean rate of shear y or velocity gradient and is equal to the velocity difference divided by the distance between the plates y = v/h, expressed in units of s-1.

The relationship between shearing stress and rate of shear can be used to define the flow properties of materials. In the simplest case, the shearing stress is directly proportional to the mean rate of shear x = T|y (Figure 8-5). The proportionality constant T) is called the viscosity coefficient, or dynamic viscosity, or simply the viscosity of the liquid. The metric unit of viscosity is the dyne.s cm-2, or Poise (P). The commonly used unit is 100 times smaller and called centiPoise (cP). In the SI system, T| is expressed in N.s/m2. or

Pa.s. Therefore, 1 Pa.s = 10 P = 1000 cP. Some instruments measure kinematic viscosity, which is equal to dynamic viscosity x density and is expressed in units of Stokes. The viscosity of water at room temperature is about 1 cP. Mohsenin (1970) has listed the viscosities of some foods; these, as well as their SI equivalents, are given in Table 8-2.

Materials that exhibit a direct proportionality between shearing stress and rate of shear are called Newtonian materials. These include water and aqueous solutions, simple organic liquids, and dilute suspensions and emulsions. Most foods are non-Newtonian in character, and their shearing stress-rate-of-shear curves are either not straight or do not go through the origin, or both. This introduces a considerable difficulty, because their flow behavior cannot be expressed by a single value, as is the case for Newtonian liquids.

The ratio of shearing stress and rate of shear in such materials is not a constant value, so the value is designated apparent viscosity. To be useful, a reported value for apparent viscosity of a non-Newtonian material should be given together with the value of rate of shear or shearing stress used in the determination. The relationship of shearing stress and rate of shear of non-Newtonian materials such as the dilatant and pseudoplastic bodies of Figure 8-5 can be represented by a power law as follows:

Figure 8-4 Flow Between Parallel Plates

Figure 8-4 Flow Between Parallel Plates materials, it is less than 1. In its logarithmic form, log x = log A + n log y

A plot of log x versus log y will yield a straight line with a slope of n.

For non-Newtonian materials that have a yield stress, the Casson or Hershel-Bulkley models can be used. The Casson model is represented by the equation,

where x0 = yield stress.

This model has been found useful for several food products, especially chocolate (Kleinert 1976).

The Hershel-Bulkley model describes material with a yield stress and a linear relationship between log shear stress and log shear rate:

Table 8-2 Viscosity Coefficients of Some Foods

Viscosity

Table 8-2 Viscosity Coefficients of Some Foods

Viscosity

Product

Temperature (°C)

(CP)

(Pa.s)

Water

0

1.79

0.00179

Water

20

1.00

0.00100

Skim milk

25

1.37

0.00137

Milk, whole

0

4.28

0.00428

Milk, whole

20

2.12

0.00212

Cream (20% fat)

4

6.20

0.00620

Cream (30% fat)

4

13.78

0.01378

Soybean oil

30

40.6

0.0406

Sucrose solution (60%)

21

60.2

0.0602

Olive oil

30

84.0

0.0840

Cottonseed oil

16

91.0

0.0910

Molasses

21

6600.0

6.600

Source: Reprinted with permission from N.N. Mohsenin, Physical Properties of Plant and Animal Materials, Vol. 1, Structure, Physical Characteristics and Mechanical Properties, © 1970, Gordon and Breach Science Publisher.

Source: Reprinted with permission from N.N. Mohsenin, Physical Properties of Plant and Animal Materials, Vol. 1, Structure, Physical Characteristics and Mechanical Properties, © 1970, Gordon and Breach Science Publisher.

Figure 8-5 Shearing Stress-Rate of Shear Diagrams. (A) Newtonian liquid, viscous flow, (B) dilatant flow, (C) pseudoplastic flow, (D) plastic flow.

where A and n are constants. A is the consistency index or apparent viscosity and n is the flow behavior index. The exponent is n = 1 for Newtonian liquids; for dilatant materials, it is greater than 1; and for pseudoplastic

The value of n indicates how close the linear plot of shear stress and shear rate is to being a straight line.

Principles of Measurement

For Newtonian fluids, it is sufficient to measure the ratio of shearing stress and rate of shear from which the viscosity can be calculated. This can be done in a viscometer, which can be one of various types, including capillary, rotational, falling ball, and so on. For non-Newtonian materials, such as the dilatant, pseudoplastic, and plastic bodies shown in Figure 8-5, the problem is more difficult. With non-Newtonian materials, several methods of measurement involve the ratio of shear stress and rate of shear, the relationship of stress to time under constant strain (relaxation), and the relationship of strain to time under constant stress (creep). In relaxation measurements, a material is subjected to a sudden deformation e„, which is held constant. In many materials, the stress will decay with time according to the curve of Figure 8-6. The point at which the stress has decayed to ale, or 36.7 percent of the original value of a(), is called the relaxation time. When the strain is removed at time T, the stress returns to zero. In a creep experiment, a material is subjected to the instantaneous application of a constant load or stress and the strain measured as a function of time. The resulting creep curve has the shape indicated in Figure 8-7. At time zero, the applied load results in a strain e0, which increases with time. When the load is removed at time T, the strain immediately decreases, as indicated by the vertical straight portion of the curve at T; the strain continues to decrease thereafter with time. In many materials, the value of 8 never reaches zero, and we know, therefore, a permanent deformation zp has

Measure Time Constant Stress Relaxation
Figure 8-6 Relaxation Curve (Relationship of Stress to Time under Constant Strain)

resulted. The ratio of strain to applied stress in a creep experiment is a function of time and is called the creep compliance (J). Creep experiments are sometimes plotted as graphs relating J to time.

DIFFERENT TYPES OF BODIES

The Elastic Body

For certain solid bodies, the relationship between stress and strain is represented by a straight line through the origin (Figure 8-8)

Figure 8-7 Creep Curve (Relationship of Strain to Time under Constant Stress)

up to the so-called limit of elasticity, according to the law of Hooke, a = Ee.. The proportionality factor E for uniaxial stress is called modulus of elasticity, or Young's modulus. For a shear stress, the modulus is G, or Coulomb modulus. Note that a modulus is the ratio of stress to strain, E = o/e. The behavior of a Hookean body is further exemplified by the stress-time and strain-time curves of Figure 8-9. When a Hookean body is subjected to a constant strain e0, the stress a will remain constant with time and will return to zero when the strain is removed at time T. The strain e will follow the same pattern when a constant stress is applied and released at time T.

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