Viscoelastic food systems

In the rheological structure of most food systems there is a viscous element present, and the deformation curves are often highly influenced by the rate of the imposed strain. This is due to the fact that the material relaxes (or flows) while tested under compression and the resultant deformation of this flow is dependent on the nature of the viscous element (Szczesniak, 1963 Peleg and Bagley, 1983). In the viscoelastic food systems, where during processing it is caused to oscillate sinusoidally, the strain curve may or may not be a sine wave. In cases when a periodic oscillatory strain is applied on a food system like fluid material, oscillating stress can be observed. The ideal elastic solid produces a shear stress wave in phase with... 

Figure 8-33 Mechanical Model Proposed by Morrow and Mohsenin. (A) Viscoelastic foods, (B) the strain time, (C) stress time, characteristics of the system.

The creep test is a simple and inexpensive test for viscoelastic foods which provides valuable information on the rheological parameters. Davis (1973) pointed out that indeed too much information can be obtained from the creep test. For example, an eight-parameter rheological model defined by eleven parameters was required for shortening and lard. One drawback with creep studies using concentric cylinder systems is that the materials structure is disturbed when the sample is being loaded. 

Time-dependent rheological properties reflect the nature of a system s structure and can be due to viscoelasticity, structural changes, or both (Cheng and Evans, 1965 Harris, 1972). Structure breakdown can result in a decrease in the viscosity of a substance. It occurs in emulsions, suspensions, and sols. The characterization of the time-dependent flow properties of food systems is important for process design and control, for product devel-... 

There are some Upid food systems that, during measurement, exhibit so-called retarded elasticity. Such deformation is a function of time as well as stress (Figure 4.10). In this case, there is no permanent deformation. The area (A) in Figure 4.10 results from the upward part of the curve with increasing values of stress, and the downward part of the curve when the stress is reduced the corresponding strains are greater. When the stress reaches zero, the strain has a finite value (Ej), which from this point will slowly return to zero. There are also lipids that present the combined properties of elastic and viscous parameters, and the former may be partially retarded elasticity. Under the influence of a small stress, viscoelastic lipids may flow slowly and nonreversibly. Under larger stresses, the elastic part becomes apparent. 

The viscoelastic samples to be tested by this method may be in different forms. The simplest to work with is a soft or liquid-like viscoelastic material such as mayonnaise or other food emulsions. These are easy samples to work with terms of sample loading. More solid-like samples such as cheese or food gels are more difficult to load onto the instrument in a consistent matter. The degree of compression of soft samples should ideally be controlled using a normal force measure or force rebalance system. Slippage is also a concern and roughened plates or even adhesives may be needed if slip is an issue. As this protocol is a general one, it is assumed that the sample is already loaded on the rheometer and has achieved equilibrium in terms of temperature and viscoelastic stmcture (time-dependent behavior). 

Doublier, J.-L., and Llamas, G. (1991). Flow and viscoelastic properties of mixed xanthan gum + galactomannan systems. In Food Polymers, Gels and Colloids, Dickinson, E. (Ed.), pp. 349-356. Royal Chem. Soc., London. 

Kohyama, K., Iida, H., and Nishinari, K. (1993). A mixed system composed of different molecular weights konjac glucomannan and kappa carrageenan large deformation and dynamic viscoelastic study. Food Hydrocoll. 7 213—226. 

Semisolid foods belong, in general, to a group than can be characterized by viscoelastic parameters. Viscoelasticity is due to the delayed motion and retarded response of a system to a shear resulting from a joint viscous and elastic nature. Jellies and deserts fall into this category. Jams are also included however, some additional measure of elasticity of the product is required. 

However, most real systems do not comply with the presumptions made in the derivation of Eq. (17.4). Generally, more than one type of interaction force will act, and the structural elements often vary in type or size, which implies a spectrum of interaction forces we will see examples of this in the following sections. The contributions to the modulus of the various forces involved are not additive, primarily because the bonds vary in direction. Moreover, such materials are generally not fully elastic, which implies that the modulus will be complex [see Eq. (5.12)] and depend on deformation rate virtually all soft-solid foods show viscoelastic behavior of some type. Finally, some systems are quite inhomogeneous, which further complicates the relations. 

Some consequences of the interaction of proteins with interfaces have already been mentioned for example, the high viscoelasticity imparted to interfaces and the precipitation of soluble protein caused by shaking solutions. As a result of their ubiquitous presence at interfaces, a wide range of apparently unrelated phenomena are affected by protein adsorption. The role that proteins play at interfaces in biomedical systems is extensively discussed in other contributions to this book. Here it is intended to focus more on areas related to food and agriculture. Some of the relevant topics are ... 

Adsorbed protein molecules interact at the interfaces to form viscoelastic films. The viscoelastic properties of protein films adsorbed at fluid interfaces in food emulsions and foams are important in relation to the stability of such systems with respect to film rupture and coalescence. Interfacial rheology techniques are very sensitive methods to measure the viscoelastic properties of proteins, thereby evaluating the protein-protein or protein-surfactant interactions at the interfaces. There was an excellent review about the principal and methods of interfacial rheology [17]. 

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