Activity of water in frozen systems

Some fundamental thermodynamic aspects of water activity in general, and of water activity in a frozen medium especially, are reviewed briefly. Two types of calculations for determination of water activity in a frozen state will be mainly discussed. It is evident from these equations, that the water activity in a frozen state, that is, a three phase system, solid-liquid-gas, depends only on the temperature. This means that the dissolved substances have no influence on the water activity, but for the limitation that the water crystallizes out as pure ice. This was also assumed in our paper. In part two of this work, the influence of water activity in frozen systems on diffusion phenomena, enzymatic reactions and growth of microorganisms, is discussed. The aim, here, is to try to relate the said phenomena to common physical parameters.