Many frozen foods contain an unfrozen phase with a very low freezing p
oint, due to the process of freeze-concentration of the solutes as wat
er is removed in the form of ice. Hence, many reactions that are diffu
sion-controlled, such as enzymatic degradation or ice recrystallizatio
n, can occur within this unfrozen phase even at very low temperatures,
giving frozen foods a finite shelf-life. Recent research in the area
of low-temperature stability of foods has focused on the importance of
the viscosity of the frozen solution and the rates of diffusion of wa
ter and solutes within the unfrozen phase (Goff, 1992). At very high v
iscosities (> 10(12) Pa s), an amorphous solid state or glass may form
within this unfrozen phase, thus tremendously decreasing the rates of
molecular diffusion and rendering the frozen food stable for much lon
ger periods of time. Awareness of this glass formation emphasizes the
importance of appropriate freezing and storage temperatures to stabili
ty and may also allow formulation of foods with a higher glass transit
ion temperature or reduced molecular diffusion rates to improve shelf
life. The measurement and interpretation of this glass transition will
be discussed.