Dehydration mechanism and crystallisation behaviour of lactose

The dehydration mechanism of alpha-lactose monohydrate was investigated by several techniques and interpreted on the basis of structural data. Whatever the dehydration conditions (heating or use of hygroscopic organic solvents), the departure of water molecules occurs cooperatively in channels parallel to the c axis of the initial structure. Subsequently, the reorganization leads to the closest packing (hygroscopic metastable form, Lalpha(11)) under heating or to the stable anhydrous form (Lalpha(S)), probably via a nucleation and growth process in ethanol. The use of acetone as dehydrating solvent on single crystals of alpha-lactose monohydrate led to the unexpected formation of single crystals of the anomeric beta-lactose at room temperature, from which the crystal stricture of beta-lactose could be accurately redetermined. Recrystallization experiments of anhydrous lactose allowed to prepare N-methylpyrrolidinone and DMSO solvates of alpha-lactose.