Structuring Water by Gelation

The Swedish Institute for Food and Bioteehnology, PO Box 5401, 40229 Gothenburg, Sweden, amh sik.se 

Gels are of central importance for most semisolid food products. A gel can contain more than 99% water and still retain the characteristics of a solid. The network structure will determine whether the water will be firmly held or whether the gel will behave more like a sponge, where water is easily squeezed out. The gel structure will also have a major impaet on the texture as well as diffusion of water and soluble compounds. Many food matrixes are based on colloidal gels such as yoghurts, cheeses, many desserts, sausages etc (see also Chapters 19 and 20). In whole foods, there is often a combination of colloidal structures and fragments of biological tissues or gel structures in combination with particles, emulsion and foam structures. This level of complexity of composite food structures will not be dealt with here. 

Since several mechanisms often come into play in a gelation process, the relative kinetics will determine the final structure and related properties. Nowadays we have tools that enable us to follow the development of the microstmcture directly under the microscope giving insights of great interest to food engineering (see also Chapter 

Here we will discuss not only structure formation but also structural breakdown. 

The way a gel structure deforms and breaks under stress is crucial for properties such as flow and fracture behaviour, sensory perception of structure and release of water and flavours. 

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Nylon 12 has been studied by solution casting from a phenol-ethanol mixture and shown to isomerize from the y to the a form. The permeation character istics of nylon 12 monbranes have also been evaluated and shown to be related to structural features associated with uptake of water by the casting solution, and the composition of the water-methanol mixed solvent used as the gelation medium. 

The structures of sol-gel-derived inorganic polymers evolve continually as products of successive hydrolysis, condensation and restructuring (reverse of Equations 1-3) reactions. Therefore, to understand structural evolution in detail, we must understand the physical and chemical mechanisms which control the sequence and pattern of these reactions during gelation, drying, and consolidation. Although it is known that gel structure is affected by many factors including catalytic conditions, solvent composition and water to alkoxide ratio (13-141, we will show that many of the observed trends can be explained on the basis of the stability of the M-O-M condensation product in its synthesis environment. 

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