Water (moisture) is the predominant constituent in many foods. As a medium water supports chemical reactions, and it is a direct reactant in hydrolytic processes. Therefore, removal of water from food or binding it by increasing the concentration of common salt or sugar retards many reactions and inhibits the growth of microorganisms, thus improving the shelf lives of a number of foods. Through physical interaction with proteins, polysaccharides, lipids and salts, water contributes significantly to the texture of food.
The function of water is better understood when its structure and its state in a food system are clarified. Special aspects of binding of water by individual food constituents and meat are discussed in the indicated sections.
Capillaries close during drying in food, thus decreasing the free inner surface. To fill the remaining capillaries (i.e. rehydrate), of which the geometry differs strongly in various foods, a higher partial pressure of water vapor is required than in the case of moisture removal. Three regions of water binding are distinguished in sorption isotherm curves.
The attainment of a complete monomolecular coating of water on the inner surface of a food is marked by the BET-point.
The rates of many reactions are influenced by the extent of water binding in foods in which the water content is lower than the content of total solids.
The storage stability of food with an aw between 0.2 and 0.4 is the highest. This aw range obviates the need for preservatives against microbial spoilage and food quality is unaffected by non-enzymatic browning and lipid auto-oxidation since these reactions are essentially prevented.