NONISOTHERMAL AND ISOTHERMAL CRYSTALLIZATION OF SUCROSE FROM THE AMORPHOUS STATE

The crystallization of a model compound, sucrose, from the amorphous s olid state has been studied non-isothermally using differential scanni ng calorimetry to determine crystallization temperature, Tc, and isoth ermally at 30 degrees C by subjecting samples to 32.4% relative humidi ty and gravimetrically monitoring water vapor uptake and subsequent lo ss with time due to crystallization. From the measurement of glass tra nsition temperature, Tg, and melting temperature, Tm, for sucrose alon e and in the presence of absorbed water it was possible to predict Tc and thus to directly relate the plasticizing effects of water to its t endency to promote crystallization. Colyophilization of sucrose with l actose, trehalose, and raffinose, all having Tg values greater than th at of sucrose, increased Tc significantly, even at levels as low as 1- 10% w/w. In the isothermal studies the time required for crystallizati on to commence, due to the plasticizing effects of water, i.e., the in duction time, assumed to be mostly affected by rates of nucleation, wa s greatly increased by the presence of the additives at these low leve ls, with raffinose producing a greater effect than lactose and trehalo se. Similarly, these additives reduced the rate of water loss, i.e., t he rate of crystal growth, but now no significant differences were not ed between the three additives. The possible relationships of nucleati on and crystal growth and the effects of additives on molecular mobili ty are discussed.