Rheology of Fluid and Semisolid Foods

By definition, rheology is the study of deformation and flow of matter. The science of rheology grew considerably due to research work done on synthetic polymers and their solutions in different solvents that in turn was necessary due to the many uses of the polymers (“plastics”) in day-to-day and industrial applications. Nevertheless, because of the biological nature of foods, food rheology offers many unique opportunities of study and there exists a large body of food rheology literature. Many foods are composed mainly of biopolymers and aqueous solutions containing dissolved sugars and ions. The former are large molecules, often called macromolecules, such as proteins, polysaccharides, and lipids from a wide range of plant and animal sources. In addition, water is an important component in many foods and plays a major role in the creation of edible structures and their storage stability (Rao, 2003). Processed foods may be viewed as edible structures that are created as a result of the responses of proteins, polysaccharides, and lipids in aqueous media to different processing methods, such as thermal processing, homogenization, and other physical treatments. Most, if not all, of those responses are physical in nature. The measured rheological responses are those at the macroscopic level. However, they are directly affected by the changes and properties at the microscopic level (Rao, 2006). Thus, it would be helpful to understand the role of structure of foods on their rheological behavior.

A flow model may be considered to be a mathematical equation that can describe rheological data, such as shear rate versus shear stress, in a basic shear diagram, and that provides a convenient and concise manner of describing the data. Occasionally, such as for the viscosity versus temperature data during starch gelatinization, more than one equation may be necessary to describe the rheological data. In addition to mathematical convenience, it is important to quantify how magnitudes of model parameters are affected by state variables, such as temperature, and the effect of structure/composition (e.g., concentration of solids) of foods and establish widely applicable relationships that may be called functional models.

Techniques for measuring rheological properties of polymeric materials have been well described previously by others (e.g., Whorlow, 1980; Macosko, 1994). The text by (1963) is still a valuable reference that explains in detail many facets of earlier attempts to measure rheological properties of polymeric materials as well as basic equations of viscometric flows. The unique nature of fluid foods prompted this author to review both the rheological properties of fluid foods and their measurement about 30 years ago (Rao, 1977a, 1977b). Subsequent efforts on rheology of foods include those of (1992, 2005) and (1996).

Gums and starches are used extensively as thickening and gelling agents in foods. Therefore, understanding their rheological characteristics is of considerable interest. Because many food gums in dispersions have random coil configuration and starch dispersions have granules, it would be better to study their rheological behavior separately.

In this chapter, the rheological properties of processed fluid and semisolid foods will be discussed. Where data are available, the role of the composition of the foods on their rheological behavior will be emphasized. In addition, literature values of data on several foods, many of which are discussed here and some that are not discussed, are given at the end of this chapter.

Rheological studies can provide much useful information on sol-gel and gel-sol transition, as well as on the characteristics of a gel. There are several definitions of what a gel is that are based on either phenomenological and/or molecular criteria. (1953) defined a gel to consist of polymeric molecules cross-linked to form a tangled interconnected network immersed in a liquid medium. (1949) defined it as a two-component system (e.g., gelling polymer and the solvent, water or aqueous solution in foods) formed by a solid finely dispersed or dissolved in a liquid phase, exhibiting solid-like behavior under deformation; in addition, both components extend continuously throughout the entire system, each phase being interconnected. At the molecular level, gelation is the formation of a continuous network of polymer molecules, in which the stress-resisting bulk properties (solid-like behavior) are imparted by a framework of polymer chains that extends throughout the gel phase. Further, gel setting involves formation of cross-links, while softening or liquefaction (often called melting) involves their destruction.

Sensory perception of foods is based on the integration of information about numerous aspects of a food, through a number of senses that reach the brain. Among these, the structural information plays an important role. The surface structure of a food product is first perceived by vision, and then the bulk structure is assessed by tactile and kinaesthetic senses combined with hearing while the food is chewed. In spite of the fact that texture is a perceived attribute, which is dynamically evaluated during consumption, many attempts have been done to gain insights into the texture of foods through rheological and structural studies. Several reviews have been published, by (1978), (1982), (1987), and (1988).

In this chapter, we consider application of rheology to handling and processing operations. However, it should be noted that there are many situations where rheology is applied. Earlier, sensory assessment and swallowing of foods were considered in Chapter 7. Table 8-1 contains some of the phenomena in which rheological behavior plays an important role and the typical shear rates encountered in them. The latter should also provide guidelines for obtaining the shear rate range over which rheological data should be obtained.