CALORIMETRIC STUDY OF GLASS-TRANSITION IN GELATIN

Temperature dependence of the absolute values of heat capacity of crystalline and amorphous gelatins has been studied by DSC method in the wide temperature range (-50-130-degrees) for various, water concentrations. For all concentrations of bound water (5-30%) for amorphous gelatins the heat capacity jump is observed related with devitrification (softening). The value of this jump has been determined being equal to 0.50 +/- 0.05 J/g.K. An increase of the content of bound water results in decrease of T(g). The appearance of free water in the system changes essentially the character of devitrification (softening) process. Free water unlike the bound one is not a plastifier, but forms the ice rigid matrix preventing the glass transition. In crystalline gelatins with small values of melting heats (approximately 10 J/g) the glass transition process is practically the same as in amorphous ones with the same content of water. For melting heats above 30 J/g the softening results only in appearance of the small additional maximum on the linear dependence of the heat capacity on temperature without any jump.