Effects of water on diffusion in food systems.

A number of operations in food processing, and the stability of stored foods, are affected by diffusional properties of food systems, which include the foods themselves, their immediate environment within a package, and any barriers (packaging or coating) used with the foods. Water content and "water activity" affect these diffusional properties dramatically, by plasticizing food and/or packaging polymers and affecting glass transition temperatures of components, and in some cases, water may serve as an internal transport medium. The present paper reviews the current state of knowledge of effects of water on diffusion in food systems, and in particular, changes in diffusivity above glass transition temperatures. The complexity of resultant effects on food stability in dry and frozen foods is discussed. Examples of potential effects of glass transitions on stability of frozen foods are provided by computer simulation of shelf life using specified assumptions.