Glass transitions in frozen systems as influenced by molecular weight of food components.

ABSTRACT

Freezing is a frequently used way to expand the storage life of foods with high water content. Under suitable cooling rates, frozen systems attain a condition of maximum freeze concentration, which is characterized by the glass transition temperature (Tg '), end point of freezing or onset of melting (Tm '), and concentration of solids (Xs ') in the maximum-freeze-concentrated matrix. The value of Tg ', Tm ', and Xs ' depends on the chemical composition of frozen system. Below Tg ', the rates of deteriorative reactions are significantly reduced. In this article, the data for Tg ', Tm ', and Xs ' of different frozen systems including sugars, starches, proteins, and food are collected and compiled. The trends in Tg ' and Tm ' data of food are investigated using molecular weight (MW) of food components. The Tg ' and Tm ' of most starches (increased by 2.46% to 87.3% and 10.8% to 85.0%) and some protein-rich foods (increased by 5.00% to 53.4% and 25.0% to 52.9%) were higher than the maximum values of sugar-rich foods. Both Tg ' and Tm ' values increased with increasing MW of solids in frozen food, reaching an asymptotic value. Moreover, there were exponential relationships between Tg ' or Tm ' values and MW for sugar and starch-rich foods taken together. Some studies found that frozen storage below Tg ' maintains the higher quality of food that was achieved by fast freezing. However, other studies found that there was no significant difference in the quality of frozen foods between storage temperature below and above Tg '. Therefore, storage below Tg ' is not the only factor for predicting the stability of frozen foods.