Non-isothermal and isothermal crystallization of sucrose from the amorphous state

Abstract

The crystallization of a model compound, sucrose, from the amorphous solid state has been studied non-isothermally using differential scanning calorimetry to determine crystallization temperature, Tc, and isothermally at 30 degrees C by subjecting samples to 32.4% relative humidity and gravimetrically monitoring water vapor uptake and subsequent loss with time due to crystallization. From the measurement of glass transition temperature, Tg, and melting temperature, Tm, for sucrose alone 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 tendency to promote crystallization. Colyophilization of sucrose with lactose, trehalose, and raffinose, all having Tg values greater than that of sucrose, increased Tc significantly, even at levels as low as 1-10% w/w. In the isothermal studies the time required for crystallization to commence, due to the plasticizing effects of water, i.e., the induction time, assumed to be mostly affected by rates of nucleation, was greatly increased by the presence of the additives at these low levels, with raffinose producing a greater effect than lactose and trehalose. Similarly, these additives reduced the rate of water loss, i.e., the rate of crystal growth, but now no significant differences were noted between the three additives. The possible relationships of nucleation and crystal growth and the effects of additives on molecular mobility are discussed.