Water and Freezing in Food

As an overwhelmingly major component, the role of water in food is reviewed comprehensively from a physicochemical view point. In aqueous solutions, the deviation of water activity, a, from the ideal solution was proved to arise mainly from solute hydration. In macromolecule solutions, interactions among macromolecules were affected by a and also the balance between the hydration and cosolute-binding of macromolecules. The ice fraction, which affects various properties of frozen food, was described well by a two-parameter equation with the initial freezing temperature and the unfrozen water content. In crystallization, ice crystal growth is extensively driven by heat transfer, causing unstable dendritic growth that is limited by water diffusion. Methods for freeze concentration were classified into four categories, and equilibrium suspension crystallization and progressive freeze-concentration were compared as possible alternatives in practice. Some chemical reactions are accelerated in the frozen state. This mechanism was explained by the concentration effect with freezing. In the freezing of cells, the freeze-induced dehydration rates of plant cells were much lower than those of microorganisms and animal cells, which seemed to explain the lower freeze-tolerance of plant cells and tissues.