Peter A Williams is Professor of Polymer and Colloid Chemistry and Director of the Centre for Water Soluble Polymers at the North East Wales Institute. Has published over 170 scientific papers and edited over 30 books. He is Editor-in-Chief of the international journal Food Hydrocolloids. His research is in the area of physicochemical characterisation, solution properties and interfacial behaviour of both natural and synthetic polymers. Recent work has been involved with the determination of molecular mass distribution using flow field flow fractionation coupled to light scattering, rheological behaviour of polymer solutions and gels, associative and segregative interaction of polysaccharides, development of polysaccharide-protein complexes as novel emulsifiers.

Gums and Stabilisers for the Food Industry 12

By P. A. Williams, G. O. Phillips

The Royal Society of Chemistry

Copyright © 2004 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-891-5

Contents

Applications of Hydrocolloids,
Rheological Properties of Hydrocolloids,
Mixed Hydrocolloid Systems,
Chemical, Biochemical and Physicochemical Characterisation of Hydrocolloids,
Role of Hydrocolloids on the Stability of Emulsions,
Hydrocolloids in Low Moisture Systems,
Hydrocolloids as Dietary Fibre: From Structure to Functionality,
Subject Index, 597,

CHAPTER 1

DESIGNING STRUCTURE INTO FOOD

Anne-Marie Hermansson, Maud Langton and Camilla Olsson

SIK, The Swedish Institute for Food and Biotechnology PO Box 5401, SE 402 29 Goteborg, Sweden

1 INTRODUCTION

The microstructure determines the characteristics of many semi-solid and solid food products. One of the most important challenges of food technology is to gain knowledge on how structures are being formed and how we can use this knowledge to design food structure. Cooking can be regarded as an art. This reflects the way the food can be perceived but it also reflects that it is almost impossible to replace a master chef. Until rather recently food production was a craftsmanship pursued by craftsmen who knew the process and the products and their judgments had a decisive effect on the quality of the products. To a large extent they evaluated the structure formation by changes in the appearance, smell and consistency of the food during production. They knew of the process involved, its structure and the dynamics, indirectly. Today we need a proper understanding of structure engineering in order to design the structure of food, both with regard to formulation, process and handling prior to consumption.

Structure engineering helps us to tailor-make food structures with the desired sensory perception. This is schematically illustrated in Figure 1. We know fairly well how a process or a formulation affects the structure of the end product, but much more knowledge is needed to follow the evolution of a structure throughout the whole process. Even more challenging is to link process design with structure design. For many food products there is a need for microtechnology rather then nanotechnology, since many foods are colloidal systems on the micron scale. The dual direction of the arrow between the structure and the sensory control, their mutual dependence, is equally important. In order to design structure into food we need more knowledge about the sensory perception of structures over several length scales.

2 THE SIGNIFICANCE OF FOOD STRUCTURE OVER SEVERAL LENGTH SCALES

2.1 Cheese

Many types of cheese product such as Greve, Emmentaler, Jarlsberg and Maasdamer are characterised by the presence of large spherical holes, round-eyed cheeses. Sometimes slits or cracks form at the expense of hole formation, as illustrated in Figure 2. Production of CO2 is commonly held responsible for hole formation and most considerations with regard to defects relate to gas production. We can detect the cracks by eyesight, but the microscope reveals considerable changes in the structure over several length scales. Thus, the cracks can be an indirect measure of structure-related cheese quality. Differences have been found in the microstructure of Swedish Greve cheese between cheese with and without cracks regarding the curd structure, the fat distribution, the protein structure and the interfacial spaces between the fat phase and the protein network. This work was done in collaboration with Lars Moberger at Aria Foods to obtain information of importance to minimize crack formation in cheese.

On the macroscopic level shown in Figure 3, we can see that the curd grain boundaries are less distinct and have partly disappeared in the cheese with cracks. This is a sign of a weakened structure. The fat phase is stained with osmium. Normally the boundaries do not contain fat but the macrograph indicates the presence of fat in the boundary areas between the curd particles. Thus clear structural changes are observed on the mm scale.

The presence of darkly stained, free fat in the boundary areas is clearly seen at a higher magnification (Figure 4a). This is an indication of a more drastic effect of the cheese microstructure than just fracture and crack formation. Figure 4b illustrate the big difference in the state of the fat phase from thin sections at an even higher magnification, where the protein phase has been stained and the fat is white. Both cheeses are produced at the same production site, but the cheese with the cracks has much bigger fat domains consisting of flocculated and/or coalescenced fat. This difference is pronounced and will most likely be detected as a quality difference by the consumer.

One contributing factor for the fat instability and the weakening of the structure could be the growth of microorganisms in the interfacial space between the fat droplets and the continuous protein phase. Due to internal syneresis, there is often an interfacial aqueous phase between the fat and the protein phase. This space allows microbial growth that result in even more space and a weakening of the structure and instability of the fat phase. Fracture and crack formation are also facilitated by increased void size. Figure 6 shows a freeze etched replica at high magnification, where a thin interfacial space can be seen as well as a CLSM image with microorganisms growing at the interface.

The results show that a defect such as cracks that can be observed with the naked eye may indicate a large number of structural changes in the microstructures over several length scales.

2.2 Yoghurt

Yoghurt is one of the most well-designed food products with regard to structure. The structure can be manipulated by the process, the raw material and the composition with regard to the additives used, resulting in yoghurts of different consistency, creaminess, taste etc. Yoghurt is another example where the product structure is determined over several length scales. Image analysis can be used to relate structure parameters from the nano to the micro-scale to the sensory perception of yoghurts. On the micro-scale stirred yoghurt is characterized by a loose network of big aggregated clusters that can be up to 100 um in diameter, see figure 6. The size of the clusters is process-related and the characteristics of the cluster structure have a bearing on the consistency of the yoghurt. The matrix of clusters is composed of a protein network based on casein micelles. Depending on factors such as the homogenisation pressure, the fat can be distributed in the pores of the protein network or being integrated into the protein network in various ways.

Additives such as whey protein have an impact on the size of the micelles and the interaction between the micelles. The TEM micrographs in Figure 6 show spherical micelles at two levels of whey protein addition. The whey protein can be seen as the grey diffuse structure gluing the micelles together.

The effect of changing the ratios of casein and whey protein on the nano scale of the yoghurt structure can be further clarified by image analysis. Figure 7 shows the results of an experimental design, in which two levels of whey protein and caseinate were used. We can see that the size of the micelles is decreased at the higher level of whey protein addition and that whey protein has an effect on the cluster size at the lower level of caseinate addition.

The results illustrate the potential of microscopy combined with image analysis for the design of food structure. The importance of a process parameter or a variation in the composition can be determined over several length scales. In both cheese and yoghurt examples it was necessary to use several techniques in order to cover structural differences at different length scales. In the yoghurts a low magnification is needed to quantify the size of clusters, around 100µm, whereas a very high magnification is required to quantify the size of micelles typically around 300-400 nm. Thus more than one magnification must be used if all structure related properties are to be revealed and understood. A good experimental design also makes it possible to detect interaction effects in complex systems. In the example above, the addition of whey protein increased the cluster size at the lower level of caseinate addition, but there is no significant effect of whey protein at the higher caseinate concentration. Such effects are important to consider in product development. It is also possible to relate structure parameters to the sensory perception of a product. But then, we need also to take into account the dynamic process of structure breakdown during eating.

(Continues…)Excerpted from Gums and Stabilisers for the Food Industry 12 by P. A. Williams, G. O. Phillips. Copyright © 2004 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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