IAIN C.M. DEA,     Leatherhead Food Research Association, Leatherhead, Surrey, United Kingdom

Publisher Summary

Chemical structures of polysaccharides are of prime importance in determining their properties. Thus, cellulose, a (l → 4)-β-D-glucan, is water insoluble and highly crystalline in relation to other polysaccharides. Amylose, a (1 → 4)-α-D-glucan, is sparingly soluble in water, crystallizes less well than cellulose, and can form rigid thermoreversible gels at low concentration. Dextran, a (1 → 6)-α-D-glucan with a small degree of branching, is extremely water soluble and non-gel-forming. Polysaccharides show such large differences in solubility and in solution and gel properties because of their great variation in chemical structures and because the chemical structures determine the shapes the molecules adopt both in aqueous systems and in the condensed, solid state. Polysaccharides are widely used industrially to gel the aqueous phase. This chapter illustrates the three different types of chain–chain interactions in polysaccharide systems. It describes a range of molecular mechanisms of network formation for polysaccharide gels and yield stress solutions.

INTRODUCTION

Chemical structures of polysaccharides are of prime importance in determining their properties. This can be appreciated by comparing the properties of some common homoglucans. Thus, cellulose, a (1 → 4)-β-D-glucan, is water insoluble and highly crystalline in relation to other polysaccharides. Amylose, a (1 → 4)-α-D-glucan, is sparingly soluble in water, crystallizes less well than cellulose, and can form rigid thermoreversible gels at low concentration. Dextran, a (1 → 6)-α-D-glucan (primarily) with a small degree of branching, is extremely water soluble and non-gel-forming.

Polysaccharides show such large differences in solubility and in solution and gel properties because of their great variation in chemical (primary) structures and because the chemical structures determine the shapes the molecules adopt both in aqueous systems and in the condensed, solid state. A knowledge of polysaccharide molecular shape (tertiary structure) and of the potential polysaccharides have for intermolecular interactions is essential for understanding rheological properties and controlling rheology.

In industrial applications, polysaccharides are used to control water and the rheology of aqueous phases in three different ways. First, they are used as simple viscosifiers, usually giving shear thinning solutions. Here, the polysaccharide molecules exist as fluctuating disordered chains (random coils). Their viscosity behavior is nonspecific, in that when molecular weight is normalized, a general pattern describing concentration dependence and shear dependence can be seen.1 Industrially important examples of polysaccharide thickeners of this type include λ-carrageenan, sodium alginate, dextran, and carboxymethylcellulose. For such polysaccharides, double logarithmic plots of zero-shear specific viscosity (ηsp) against concentration (c) show a pronounced increase in gradient above a specific critical concentration that is different for different polysaccharides.

The product of polymer concentration and intrinsic viscosity (c[η]) gives a dimensionless parameter that is a measure of the degree of occupancy of space by the polysaccharide in solution. When the concentration dependence of zero-shear viscosity is replotted using this parameter, a striking general behavior is seen (Fig. 1). The break in this plot is at the transition from dilute solution behavior (low degrees of coil overlap) to concentrated solution behavior (total interpenetration of random coil molecules). Departures from this generalized relationship between viscosity and concentration have been observed for locust bean gum and guar gum. They show an earlier onset of concentrated solution behavior and substantially greater concentration dependence thereafter. This behavior is attributed to specific intermolecular associations that are of longer duration than are the nonspecific physical entanglements between fluctuating disordered chains. Despite this complication, polysaccharide thickeners differ from each other quantitatively rather than qualitatively.

Polysaccharides are also widely used industrially to gel the aqueous phase. Whereas thickened polysaccharide solutions depend on the properties of disordered polymer chains interacting via entanglements, the origin of rigid gel structure is due to permanent chain–chain interactions. The relevant interactions are hydrogen bonding, dipole and ionic interactions, and interactions with solvent. Individually, these interactions are so weak that conformational stability is achieved only when a large number occur simultaneously, that is, when they act cooperatively to give an ordered polymer conformation. Such cooperative stabilization of ordered conformations seldom occurs for a single chain, but rather requires the alignment and interaction of segments of two or more convalently regular chains. These regions are usually terminated by changes in sequence that prevent the continuation of ordered association; otherwise insolubility (such as in the case of cellulose and mannans) occurs. Because of these interruptions, a single polysaccharide chain can take part in several regions of ordered conformation (junction zones), each involving different partners, to form a three-dimensional network, or gel structure. The relative proportion of ordered polysaccharide chains and disordered polysaccharide chains between junction zones is important in determining final gel properties. Gelling polysaccharides of this type include seaweed polysaccharides (for example, agarose, carrageenans, alginates), plant polysaccharides (for example, pectin, amylose), and microbial polysaccharides (for example, curdlan, gellan).

The association of polysaccharide chains to form gels may be promoted in a number of ways. Since chain-chain association is in competition with chain–solvent association, reduction of water activity may cause an increase in interchain binding. Water activity may be reduced by addition of a low-molecular-weight, hydrophilic molecule that binds water in competition with the polymer. Examples are use of high concentrations of sucrose in the gelation of high-methoxyl pectin and tamarind seed polysaccharide. Association is also facilitated by decreasing interchain repulsions. Thus, interactions of acidic polysaccharides, such as pectins, may be promoted by lowering the pH to suppress ionization and/or increasing the ionic strength to further lower electrostatic repulsions between the chains. Interchain association is also promoted by freeze–thaw cycles. On freezing a polysaccharide solution, ice formation progressively raises the effective polymer concentration with consequent promotion of association. The presence of low-molecular-weight solutes will minimize this effect because of restriction of ice formation. Many interchain junctions formed in this way redissolve on thawing. However, where the barrier to spontaneous association in solution is kinetic rather than thermodynamic, junctions may persist. A practical exploitation of this effect is in the purification of agar.2

Certain polysaccharides give rheological properties intermediate between those of thickened solutions and rigid gels. Dispersions of these polysaccharides have some properties of thickened solutions, in that they...