Sorption Phenomena in Foods: Theoretical and Practical Aspects

The control of moisture content in the processing of foods by various techniques is an ancient method of preservation, but only in recent years has the physical-chemical basis of the methods been studied and understood. When food is preserved the first and foremost principle is the destruction of control of deleterious or pathogenic microorganisms. With respect to water, the methods of preservation that operate on this principle are based on the fact that the water is made unavailable for microbial growth. These methods include: salting and sugaring of foods whereby the water present is tied up in some way so as to be made unavailable; freezing by which process the liquid water is converted into a solid state and thus is removed from being available to the organisms; and drying by which process the water is removed directly by vaporization from the food material. In most of the foods after the processing, microbiological growth is thus prevented during storage, however, chemical deterioration does occur to affect quality. As has been shown by Salwin (1959), Scott (1957) and reviewed by Labuza (1971) the relationship between the loss of quality of the food and the moisture content of the food is best represented by the term water activity or A w.