MEASURING AND INTERPRETING THE GLASS-TRANSITION IN FROZEN FOODS AND MODEL SYSTEMS

Many frozen foods contain an unfrozen phase with a very low freezing point, due to the process of freeze-concentration of the solutes as water is removed in the form of ice. Hence, many reactions that are diffusion-controlled, such as enzymatic degradation or ice recrystallization, 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 water and solutes within the unfrozen phase (Goff, 1992). At very high viscosities (> 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 longer periods of time. Awareness of this glass formation emphasizes the importance of appropriate freezing and storage temperatures to stability and may also allow formulation of foods with a higher glass transition temperature or reduced molecular diffusion rates to improve shelf life. The measurement and interpretation of this glass transition will be discussed.