Rheology of Fluid and Semisolid Foods: Principles and Applications (eBook)

M. A. "Andy" Rao is a Professor Emeritus of Food Engineering in Cornell University’s Department of Food Science and Technology, Geneva, New York. For his contributions to the engineering and scientific community, and food processing industry, he received the Scott Blair Award for Excellence in Rheology of AACC (American Association of Cereal Chemists), the Distinguished Food Engineering Award of ASAE (American Society of Agricultural Engineers), and was elected a Fellow of IFT (Institute of Food Technologists) and AFST (Association of Food Scientists and Technologists-India). Dr. Rao helped develop the "Vane Method" measurement of yield stress for a variety of food products -- a standard measurement tool used by the food industry today. Also he has actively supported and promoted food engineering through cooperative professional teaching and research activities in Brazil, India, Mexico, New Zealand, Portugal, and Thailand.

Introduction: Food Rheology and Structure.- Flow and Functional Models for Rheological Properties of Fluid Foods.- Measurement of Flow and Viscoelastic Properties.- Rheology of Food Gum and Starch Dispersions.- Rheological Behavior of Processed Fluid and Semisolid Foods.- Rheological Behavior of Food Gels.- Role of Rheological Behavior in Sensory Assessment of Foods and Swallowing.- Application of Rheology to Fluid Food Handling and Processing.

"CHAPTER 6 Rheological Behavior of Food Gels (p. 339-340)

J. A. Lopes da Silva andM. A. Rao

Rheologi cal 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 . Flory (1953) defined a gel to consist of polymeric molecules cross-linked to form a tangled interconnected network immersed in a liquid medium. Hermans (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 ofcross-links , while softening or liquefaction (often called melting) involves their destruction. In many food products , gelation of polysaccharides is critical to the formation of desired texture. Most biopolymers form physical gels, structured by weak interactions (hydrogen , electrostatic, hydrophobic), and thus at a gross level belong to Flory s third mechan ism (Flory, 1974): "Polymer networks formed through physical aggregation, predominantly disordered, but with regions of local order."

They are often thermore versible and almost invariably occur in the presence of excess of solvent, usually water or aqueous electrolyte . Therefore , they are called solvated networks. In most biopolymer gels, the polymer chains form extended junction zones by means of side-by-side associations of a physical nature, in contrast to the typical single covalent bonds found in chemically cross-linked networks . Consequently, in physical gelation the formation and breakdown of the junction zones are usually reversible, the crosslink functionality is very high, and the junction zones have a finite lifetime .

The formation of these kinds of transient networks is determ ined by the chemical composition of both the polymer and the solvent which constitute the gelling system, and by temperature and time. In this chapter, the formation of gels and their softening (melting) are reviewed. Further, some of the contents of chapter have been discussed by the authors in a recent review of the subject (Lopes da Silva et aI., 1998). While most food gels are formed by first dissolving a gelling polymer in water and can be studied by means of traditional treatments, starch gels are composites of starch granules in a matrix of gelled amylose and they are not thermoreversible. Therefore, their rheological behavior is treated separately in this chapter. Additionally, information on rheological behavior of dispersions of starches alone and protein-starch mixtures can be found in Chapter 4."