Structural glass transitions and thermophysical processes in amorphous carbohydrates and their supersaturated solutions

Abstract

The nature of the thermophysical changes occurring during the heating of vitrified, water-soluble materials is re-examined, with emphasis on a comparison between freeze-concentrated and freeze-dried sucrose solutions.

Two relaxation processes, commonly observed in differential scanning calorimetry (DSC) heating scans of frozen solutions and of anhydrous amorphous sugars are assigned to a structural glass transition, T_g(reversible) and a real-time softening process, T_s(irreversible), respectively. Results from DSC experiments on quench-frozen, slow-frozen (maximally freeze-concentrated) and freeze-dried sucrose solutions and on fructose quenched from the melt are compared, and the observed behaviour is accounted for on the basis of the above assignments. The interpretations differ in some respects from those reported in the past by others and ourselves. They constitute what we now believe to be an improved understanding of the relaxation processes in the systems under study, particularly because of the closely analogous behaviour displayed by one-component (sugar), homogeneous, and two-component (sugar–water), heterogeneous systems. An optimum freeze-drying protocol is developed, based on a knowledge of the composition dependence of T_g and T_s.