Impact of compression,physical aging,and freezing rate on the crystallization characteristics of an amorphous sugar matrix

Amorphous matrices made up of sugar molecules, are frequently used in food and pharmaceutical industries. A drawback to their use is that they are susceptible to collapse, as a result of water uptake and an increase in temperature and subsequently crystallize. Herein, the crystallization characteristics of amorphous sugar (sucrose and α-lactose) preparations were analyzed, with the purpose of obtaining knowledge that could lead to the prediction of how long the amorphous state is retained under various conditions. The impact of compression, physical aging and freezing rate on the induction period (t_ind) for crystallization were examined. Freeze-dried sugar samples were compressed at 74 or 443 MPa (5 min) and then rehumidified at specified RHs. Some freeze-dried sucrose samples were physically aged, and alternatively freeze-drying was conducted under different conditions. The isothermal crystallization of the prepared samples at different temperatures (T), the glass transition and the crystallization temperature (T_cry) were measured, using differential scanning calorimetry. The compression markedly decreased the t_ind, while significantly lowered the hygroscopicity. Physical aging and slower-freezing also shortened the t_ind. The t_ind was found to be correlated exclusively with (T_cry–T), regardless of rehumidification, compression, sugar type, physical aging and freezing rate in the freeze-drying process.