Rheological properties of polysaccharides

Rheology details the behaviour of liquid-like materials under the influence of mechanical stresses and covers viscous and viscoelastic behaviour. 

Most pure liquids and solutions of small molecules show a simple proportionality between sheer stress and sheer strain rate. Such liquids are known as Newtonian, after their discoverer. Solutions of polysaccharides usually show Newtonian behaviour only below a certain critical concentration, c, at which the chains start to interact. Above this concentration, except at very low sheer strain rates, viscosity decreases rapidly with sheer strain rate until at very high values of y it becomes constant. This is not a small effect with xanthan gum (see Section 4.6.10.3.1), the fall in viscosity is lO -fold. The dependence of viscosity on sheer strain rate is often given by 

The reason for shear thinning is straightforward above the critical concentration c, the polysaccharide chains start to interact with each other, but when placed under shear, the linear molecules align with the flow, reducing resistance. 

Shear thinning (and shear thickening) are instantaneous, in contrast to thixotropic behaviour, which occurs when a response occurs well after a perturbation.  

Traditionally, viscosities (in the Newtonian range) were measured by timing the flow of a given volume of liquid (F) through a capillary of length / and radius r under a pressure p, when the viscosity is given by Poiseuille s law  

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Starches. Starch (qv) granules must be cooked before they wiU release their water-soluble molecules. It is common to speak of solutions of polysaccharides, but in general, they do not form tme solutions because of their molecular sizes and intermolecular interactions rather they form molecular dispersions. The general rheological properties of polysaccharides like the starch polysaccharides are described below under the discussion of polysaccharides as water-soluble gums. Starch use permeates the entire economy because it (com starch in particular) is abundantly available and inexpensive. Another key factor to its widespread use is the fact that it occurs in the form of granules. 

In this system, the high molecular weight polysaccharide polymer, is used to extend the rheological properties of bentonite. 

Grant, J., Cho, J., Allen, C. (2006). Self-assembly and physicochemical and rheological properties of a polysaccharide-surfactant system formed front the cationic biopolymer chitosan and nonionic sorbitan esters. Langmuir, 22,4327- 4335. 

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

The improvements of the rheological properties of microbial exopolysaccharides through mutagenesis and mutant selection has been demonstrated for Escherichia coli and Enterobacter Klebsiella aerogenes) The chemical composition of each group of polysaccharides produced by the mutants did not show any significant differences from those originating from the parent strain. 

Furuta, H., Maeda, H., 1999. Rheological properties of water-soluble soybean polysaccharides extacted under weak acidic conditions. Food Hydrocolloids 13, 267-274. 

Karakasyan C, Lack S, Brunei F, Maingault P, Hourdet D (2008) Synthesis and rheological properties of responsive thickeners based on polysaccharide architectures. Biomacromolecules 9 2419-2429... 

The fourth part of the book covers some relevant aspects of food rheology. The gelation mechanism of -Carrageenan and its interaction with starch and non-starch polysaccharides is reviewed in chapter 10 by Tecante and Nunez-Santiago. Later, in chapter 11, by Komlenic et al., it is demonstrated that rheological properties of dough strongly depend on acidification. 

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