MELTING AND GLASS TRANSITIONS OF LOW-MOLECULAR-WEIGHT CARBOHYDRATES

Glass-transition temperatures at various water contents (T(g)) and in maximally freeze-concentrated solutions (T(g)), fusion temperatures (melting points, T(f)), and heats of fusion (DELTAH(f)) were determined for pentoses, hexoses, disaccharides, and alditols, using differential scanning calorimetry. The T(g) and T(g)' values of the anhydrous compounds increased in the order pentoses < hexoses < disaccharides. The T(g) values of the alditols were lower than those of the corresponding sugars. Compounds with high T(f) / T(g) ratios had high DELTAH(f) values and showed rapid crystallisation. The T(g) and T(g)' values allowed the prediction of T(g) as a function of the water content and of the concentration of the maximally freeze-concentrated matrices (C(g)'). The C(g)' concentration was approximately 80% (w/w) for each carbohydrate. The results can be used to evaluate the physical stability of various carbohydrates at various water contents and in freeze-concentrated solutions.