Glass transition and crystallization behaviour of freeze-dried lactose–salt mixtures

Abstract

Glass transition temperatures were determined for dehydrated lactose/salt mixtures with various water contents and water activities, and state diagrams were established. Crystallization behaviour was studied for pure amorphous lactose stored at various relative water vapour pressures (RVP). Furthermore, glass transitions temperatures and time-dependent lactose crystallization of freeze-dried lactose and lactose/CaCl 2, lactose/NaCl, lactose/MgCl 2 and lactose/KCl mixtures in molar ratios of 9:1 were determined. Glass transition temperatures ( T g) of lactose powder as determined by differential scanning calorimetry (DSC) was lower than that of lactose/CaCl 2 (9:1) , and lactose/MgCl 2 (9:1), but it was slightly higher than the T g of lactose/NaCl (9:1), and lactose/KCl (9:1). Lactose/KCl had the lowest glass transition temperature, but it had about the same crystallization temperature as lactose/NaCl, and lactose/MgCl 2. The glass transition temperatures decreased as water contents increased. The critical water contents and water activities at 23 °C were predicted using data on glass transition temperature and water sorption. Pure lactose had a different critical water activity and water content from lactose/salt mixtures. The critical values of lactose/CaCl 2 (9:1) were the highest. Loss of sorbed water, indicating lactose crystallization, was observed in lactose and lactose/salt mixtures stored above the critical RVP.