Calorimetric study of the glass transition in gelatin

Abstract

The temperature variation of the absolute heat capacity values of crystalline and amorphous gelatin at various water concentrations was studied by the DSC method in a broad temperature range (from -50 to 130°C). At all concentrations of bound water (5–30%), with amorphous gelatins a heat capacity jump is observed, caused by devitrification (softening). The value of this jump was determined to amount to 0.50±0.05 J/g K. With increasing content of bound water, the glass temperature decreases. The appearance of free water in the system substantially affects the process of devitrification (softening). In contrast to bound water, free water does not act as plasticizer, but on the contrary, it forms a rigid ice matrix which prevents the appearance of a glass transition. It was determined how the glass-transition process is manifested in the course of heating of crystalline gelatins, of various heats of melting. In gelatins with small heats of melting, ≤ J/g, the glass-transition process practically does not differ from the process in amorphous gelatins of the same water content. In gelatins with greater heats of melting (≈ 30 j/g), softening only leads to the appearance of an additional small maximum on the linear heat-capacity-temperature plot, a heat-capacity jump is not observed.