Dickinson Food Polymers, Gels and Colloids

1. Auflage 1991
ISBN: 978-1-84569-833-1
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 588 Seiten

Manufactured foodstuffs typically exist in the form of complex, multi-phase, multi-component, colloidal systems. One way to try to make sense of their chemical and structural complexity is to study simple model systems in which the nature and properties of the polymer molecules and dispersed particles are relatively well known. This volume consists of a collection of papers delivered at a conference on food colloids, the main theme of which was the role of food macromolecules in determining the stability, structure, texture and rheology of food colloids, with particular reference to gelling behaviour and interactions between macromolecules and interfaces. A feature of the collection is the wide range of physico-chemical techniques now being used to address problems in this field.

Autoren/Hrsg.

Dickinson, E.

Leseproben

Aggregation Mechanisms in Food Colloids and the Role of Biopolymers

A. Lips, I.J. Campbell and E.G. Pelan,     UNILEVER RESEARCH, COLWORTH LABORATORY, COLWORTH HOUSE, SHARNBROOK, BEDFORD MK44 1LQ

Publisher Summary

Biopolymers play a central role in the stability of oil-in-water food emulsions. In recent years, there have been significant advances in statistical theories of steric interactions of polymers between surfaces. The lattice-based theory of Scheutjens and Fleer (S-F theory) provides a useful framework for discussing the relative interaction effects of food polymers. All the adsorbing food polymers studied have the capacity to induce bridging minima when adsorbed from good solvents. Of these, gum arabic shows the least flocculating power, which may be consistent with its relatively pronounced non-random block co-polymeric nature. Dextran is an excellent flocculant possibly because of its weak adsorption, enabling stronger interaction minima to be realized more in line with full rather than restricted equilibrium theory. The observed flocculation behavior of xanthan and guar is not inconsistent at a quantitative level with the predictions of theories of depletion flocculation for non-adsorbing polymers. The adsorbed layer thickness and the interaction between adsorbed layers of caseinate display a consistent temperature dependence expected within the framework of the Scheutjens-Fleer theory in the direction of water becoming a poorer solvent for caseinate with increasing temperature. This finding has relevance for understanding the storage and whip stability of food emulsions. This chapter considers the extent to which the above theories can serve as a framework for understanding, at a more informed and less qualitative level, the steric effects of food biopolymers between surfaces.

1 Introduction

Biopolymers play a central role in the stability of oil-in-water food emulsions. More commonly they act by positive adsorption at interfaces, which, as is now appreciated in some theoretical detail,1 can lead to both stability () and instability (). At a qualitative level, these phenomena are well documented for milk proteins adsorbed on model dispersions2 and food emulsions.3–5 Such studies have also addressed the relative effects of milk protein components and their competitive action6–9 including mixtures of proteins and emulsifiers.10 The evidence for negative adsorption effects ()1,11 in food emulsions is less definitive.12 However, by analogy with model studies on concentrated dispersions,13,14 depletion flocculation can be expected in concentrated food emulsions in the presence of non-adsorbing polysaccharide thickeners at solution concentrations close to or greater than the overlap value *.

In recent years there have been significant advances in statistical theories of steric interactions of polymers between surfaces. An important development has been the elaboration of self-consistent treatments for nonanchored homopolymers1,15 to complement earlier advances in the description of terminally anchored chains.16–19 The lattice-based theory of Scheutjens and Fleer2 (S–F theory) provides a comprehensive framework for representing adsorption and depletion effects for homopolymers between parallel plates for both (chains free to leave the gap) and (chains trapped but subject to local thermodynamic equilibrium). Under the approach the interaction between the plates is predicted to be always attractive. Such a situation is relevant for non-adsorbing or very weakly adsorbing polymers and possibly also for flexible surfaces, liquid emulsion films. The model of is held to be the more appropriate for typical kinetic conditions of approach of coated surfaces. In restricted equilibrium, the free energy will always be greater than that at full equilibrium. For large and intermediate plate separations, the models do not deviate substantially in their prediction of the attractive well due to bridging. At shorter range, however, the restricted equilibrium model always indicates a steric barrier. The parameters of lattice treatments are the number of segments of the polymer chain , the familiar Flory–Huggins solvency parameter , and an adsorption energy per segment S usually measured in units of For adsorbing polymers under conditions of restricted equilibrium, the free energy of the polymer interaction between approaching plates is a subtle interplay of attractive contributions from bridging and repulsive contributions from segmental overlap and loss of conformational entropy. In general, the attractive minimum shifts to larger separations with increasing surface coverage and occurs at a separation comparable to the radius of gyration G of the isolated polymer chain. The depth of the bridging minimum increases with surface coverage to a maximum at an intermediate coverage. For high adsorbed amounts in good solvents, the minimum can disappear altogether and only repulsion is then predicted. An interesting aspect of the theory is that the interaction is expected to have little dependence on the molecular weight of the polymer provided that the adsorbed amount is the same. However, since, at a given solution concentration, the surface coverage increases with molecular weight, shorter chains can yield deeper minima; complete steric stabilization is possible only with polymers of very high molecular weight.

Direct measurements of interaction forces between polymer covered mica surfaces20–22 lend qualitative support to the S–F theory for restricted equilibrium. At short range, strong repulsion is seen, and bridging minima are observed even in good solvents at distances comparable to G. The minimum disappears for high adsorbed amounts in good solvents. Colloid stability studies on dispersions with polymers adsorbed from good solvents generally support the theoretical conclusion that bridging can be a dominant mechanism at low to intermediate surface coverage and that therefore it is not necessary to invoke poor solvent conditions ( 2) to explain instability. Quantitative comparison between theory and experiment, however, is still difficult as the level of adsorption of polymer in the model interaction studies is not easily determined. Also, the steric effects can be strongly dependent on heterodispersity, and in practice it is difficult to obtain a polymer fraction of adequate monodispersity to test theories. Another problem is that typical experimental situations, for example in emulsification or whipping, could imply an approach of surfaces at greater rates than those of local equilibration of tightly confined polymer. Even the restricted equilibrium model may then be inappropriate.

With concentrated dispersions subject to depletion flocculation, the link between theory and experiment is more fully established. It is assumed then that S ˜ 0.1,11 Statistical mechanical predictions of the phase behaviour of concentrated dispersions,23 with interparticle potentials modelled on the volume depletion argument of Asakura and Oosawa24 and Vrij,25 are in reasonable agreement with experiment. However, quantitative discrepancies14 are apparent when particle and polymer are of comparable size.

Regarding more complicated macromolecular behaviour, a successful model26–28 has been elaborated for the adsorption of heterodisperse polymers. Advanced statistical treatments for polyelectrolyte adsorption29–31 are also now available. Self-consistent lattice treatments have recently been developed to represent the adsorption of copolymers32–33 as well as interactions between adsorbed layers of block copolymers.33 These studies include the complications from bulk association of the polymers. Non-random block copolymers with strongly adsorbing blocks and non-adsorbing protruding moieties are not expected to yield bridging minima in good solvents even under conditions of full equilibrium. The behaviour predicted for restricted equilibrium can approach that for terminally anchored chains. The adsorption of a random copolymer can be modelled in terms of an equivalent homopolymer with a suitably weighted average segment adsorption energy.

theories of steric stabilization are useful for simple baseline predictions. The earliest is that of Fischer34 which models the overlap of the steric layers attached to two spheres on the basis of changes in mixing free energy for an assumed constant density of polymer segments in the gap. This approach can be criticized in that the model implies terminally anchored as opposed to volume-restricted chains and, more seriously, in that, even in tightly confined situations, the segment density is non-uniform. Also the mixing term is an incomplete...