Water sorption and time-dependent crystallization behaviour of freeze-dried lactose–salt mixtures

Water sorption properties of freeze-dried lactose, lactose/CaCl2, lactose/NaCl, lactose/MgCl2, and lactose/KCl mixtures in their molar ratio of (9:1) were investigated. Brunauer–Emmett–Teller (BET) and Guggenheim–Anderson–de Boer (GAB) models were used to model water sorption properties. Water is known to function as a plasticizer, depressing the glass transition and facilitating crystallization. Crystallization in the present study resulted in loss of sorbed water from lactose. The crystallization of pure lactose and lactose/salt mixtures was observed at RVP⩾44.0% within 24 h. At RVP⩾54.4% water contents were higher in lactose/CaCl2 and lactose/MgCl2 mixtures than in pure lactose, lactose/NaCl, and lactose/KCl.Water content in pure lactose after crystallization was ⩽5.0%, suggesting that lactose crystallized as a mixture of α-lactose monohydrate and various anhydrous forms of α/β-lactose crystals. Anhydrous lactose/CaCl2 and lactose/MgCl2 had higher glass transition temperatures than lactose, but other salts (NaCl and KCl) with lactose gave lower glass transition than amorphous lactose. It seems that bivalent salts in mixtures with lactose gave a higher Tg than smaller monovalent ions. Salts delayed lactose crystallization. The effect on lactose crystallization was highest with calcium chloride (CaCl2) and lowest with potassium chloride (KCl). It seems that different salts interacted with lactose to different extents. For water sorption, GAB model gave a better fit than BET model. Water sorption and time-dependent crystallization properties of lactose/salt mixtures should be considered in manufacturing and storage of dairy-based dehydrated materials.