Measurement And Interpretation Of The Glass Transition In Frozen Foods
Published 1997 · Materials Science
The glass transition associated with the freeze-concentrated unfrozen phase (UFP) in frozen foods has received considerable research attention in the last several years, primarily as a result of the efforts of (1988a), 1990; Slade and Levine, 1995) to relate polymer kinetics to food physicochemical processes. The glass transition is of interest for at least three reasons. First, quality losses (e.g., enzymatic reactions and recrystallization) in frozen foods may be greatly reduced or inhibited when the UFP is in the glassy state. Second, the kinetics of reactions associated with quality losses at temperatures close to the glass transition temperature (T g) can be described by the Williams-Landel-Ferry (WLF) model, in which reaction rates are a function of the temperature difference (ΔT) between the storage temperature (T s) and T g Thus, WLF kinetics may be more applicable than Arrhenius kinetics to studying the rates of detrimental reactions, allowing one to more accurately predict the shelf life of stored frozen foods (Peleg, 1992). Third, knowledge of the influence of environmental and indigenous factors (i.e., temperature, freezing rate, and composition) on the glass transition may make possible the formulation, processing, or storage of frozen foods in a manner that will enhance shelf life. The basic aspects of food freezing have been discussed in the previous chapter, and in detail elsewhere (Goff, 1992; Sahagian and Goff, 1996). This chapter will focus on the physical chemistry and relevant definitions associated with the glass transition and the glassy state, the formation of the glassy state in the UFP (based on the discussion in Sahagian and Goff, 1996), measurement of T g using several techniques, and the interpretation of the measurements.