The glass transition in the freezing process

When ice separates out from a food system during freezing, the state of the unfrozen matrix between the ice crystals is changed, due to both the lowering of temperature and the increasing solute concentration. The state of the unfrozen matrix markedly affects the stability of the frozen product. Under appropriate conditions, the matrix may enter the glassy state. This paper will discuss the role of the glassy state in the stability of frozen products. It will describe methods for identifying the temperature at which the glassy state is entered, and the composition of the glassy phase, and discuss the differences in the results obtained from the application of different methods. The use of temperatures associated with the transformations of the unfrozen phase as references for discussion of product stability will be discussed. Data will be presented which illustrate the utility of employing the Tg, temperature reference. The mechanisms of freezing will also be considered, since these may affect the final composition of the unfrozen matrix.