PROBING FOOD STRUCTURE

V. J. Morris

Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, UK

1 INTRODUCTION

When I joined the Institute of Food Research in 1979 I was given the brief to develop a molecular description of the functionality of food materials. At that time I knew very little about food research and food hydrocolloids. The first Wrexham meeting provided me with an excellent introduction to the subject and a chance to meet the leading researchers in this area. Since then the meetings have allowed me to keep up-to-date with developments in this area. Thus it is an honour for me to review some of the research that I have been involved in over the last 26 years which has, I hope, helped to improve the understanding of the behaviour of food materials. I would like to illustrate the power and also the limitations of physical methods for studying mixtures of hydrocolloids, and to indicate why there was a need to develop methods of molecular microscopy of food materials. Finally, I would like to show how the use of such a microscopic technique, atomic force microscopy, has shed new light on the structure of gels, complex interactions between biopolymers at interfaces, and is providing new insights into the structure and behaviour of starch. The ability to visualise individual molecules can reveal new structural information and simple investigations can open up new areas of research. Such studies will be illustrated through images of beet pectin which account for its behaviour as an emulsifier and simple studies on whey proteins that seem to indicate unexpected interactions between milk proteins.

2 METHODS AND RESULTS

Food materials are complex mixtures which are heterogeneous at the molecular level. Given the wide range of biopolymers used by the food industry it would seem at first that there would be almost an infinite numbers of different types of mixtures that could be produced. However, in some cases it is possible to simplify the description of their behaviour by dividing them into classes of materials which show similar structures or function. This type of approach can best be illustrated through studies on the gelation of polysaccharide mixtures.

2.1 Binary Polysaccharide Gels

Mixtures of two polysaccharides can form four broad classes of gels: swollen networks (Figure 1a), interpenetrating networks (Figure 1b), phase-separated gels (Figure 1c) and coupled networks (Figure 1d). The phase-separated networks are the commonest type of structure and these have been the most useful for formulating new types of food structures. Swollen networks are most likely formed from mixtures of neutral and charged polysaccharides where the entropy of mixing term for the mobile counterions inhibits phase separation. These structures differ in whether one or both of the networks gels. The most intriguing types of gel are the coupled networks which gel under conditions for which the individual components alone will not gel. These types of gels are formed between various types of mixtures, but the most interesting are those formed between xanthan, or xanthan-like polysaccharides, and certain galactomannans or glucomannans. For these mixtures it was possible to show through x-ray fibre diffraction studies" that a new structure is formed between the two different polysaccharides, in order to link the chains together into a network. For the gels formed with the glucomannan Konjac mannan the mixtures show 6-fold helical structures with the same pitch as the 5-fold helical structure of xanthan or acetan. Given that denaturing the xanthan helix favours gelation, and the stereochemical compatibility of the backbone structures of the glucomannans and the backbones of acetan or xanthan, it is not unreasonable to suggest that the linkage is due to a mixed double helix containing both polysaccharide chains. Acceptable left-handed 6-fold mixed helical structures have been published for acetan-Konjac mannan mixtures. This result is an example of how a physical method can yield very significant information on the structure of a complex mixture allowing the functional behaviour to be explained. However, to obtain this information it was necessary to dry the gel to a hydrated film and to align the ordered structures within the network. The data tells about the junction zones within the gel but it tells us little about the long-range structure of the gel.

To investigate the detailed structure of gel networks it is necessary to use microscopic methods capable of achieving molecular resolution. Electron microscopy has the required resolution but the complex sample preparation methods required to remove and replace water in order to image the gel structure, plus the relatively poor contrast in the images, make this technique difficult to apply for routine studies. Furthermore, the images obtained are often on sections making it difficult to visualise the 3-D network within the gel. The development of probe microscopes offered an alternative method with the prospect of molecular information under more realistic conditions. Over the last 18 years the use of atomic force microscopy (AFM) has been developed for studying biological systems and, in particular, food materials. The use of AFM has allowed new types of food structure to be investigated for the first time and has led to new solutions to previously intractable problems in food science. Examples of such results include new models for the gel structure of polysaccharides, new models for the competitive displacement of proteins from interfaces by surfactants, new insights into the structure and functionality of starch, the discovery of previously unknown branching structures of polysaccharides and new molecular mechanisms of action for glucoamylose.

2.2 Gellan Gels

The conventional models of polysaccharide gels shown in most textbooks picture the gels as ordered junction zones connected by essentially random-coil chains (Figure 2). This is essentially a modified rubber-like structure for the gel with the point cross-links replaced by extended junction zones. Atomic force microscopy provides a method for visualising the long-range structure within gels by studying the association of polysaccharides as gel precursors, films and bulk gels. Gellan gum provides a good model system for probing gelation. Gellan forms thermoreversible gels on cooling and heating and the aggregation of the polysaccharide chains occurs through helix formation and then an additional step involving cation binding. The cation binding stage can be eliminated by forming the tetra-methyl ammonium (TMA) salt form. This forms weak thermoreversible gels which show no thermal hysteresis. By depositing dilute solutions of TMA gellan onto freshly cleaved mica it is possible to induce aggregation on drying in air. The gel precursors formed are elongated branched structures or fibrils. These fibrils are longer than individual molecules and their height suggests that the gellan is in the helical form. Mismatching of...