2-6

D. REID, University of California-Davis, Davis, CA

The polymer dynamics model has, in recent years, been shown to provide significant insights into the mechanisms that provide extended storage life to frozen foods. According to the model, the influence of the unfrozen phase can be understood by examining the mobilities of its constituent molecules. As a frozen system is cooled, the unfrozen matrix exhibits a series of property changes which may be attributed to alterations in molecular mobilities. It is assumed that immobile, glassy states are formed at low temperatures, where no further ice crystallization occurs. Significant temperatures, identified by a range of techniques that probe mobility-related properties, have been labeled Tg, Tg', Tm' etc. Tg' and Tm' may be alternative descriptions of the same event. Levine and Slade used Tg' to refer to the temperature of the glass transition of the maximally freeze-concentrated glass. Roos and Karel labeled this same point Tm', the initiation of melting, and identified lower temperature transitions as the glass transitions. However described, this temperature (Tg' or Tm') has been shown to be of considerable significance to the stability of frozen foods. It may be understood as defining the temperature at which the solute mobility in the matrix, as the temperature is increased, becomes sufficient to allow water molecules to dilute the matrix at a detectable rate. Both glass transitions and this mobility threshold are seen in different temperature ranges depending upon the detection technique employed. This presentation will discuss the range of techniques available to locate these temperatures, the types of result obtained, and the limitations of each method. A simple standard method, suited to general use, will be proposed which can be used to identify a particular Tg'/Tm', which might be termed the "mobility temperature", and which can be used to characterize frozen foods.