Lactose Crystallization in Skim Milk Powder Observed by Hydrodynamic Equilibria, Scanning Electron Microscopy and ²H Nuclear Magnetic Resonance

Lactose crystallization in medium heat nonfat dry milk as a function of water activity (0.01 to 0.94 a_w) and time (2 wk) at constant temperature (20°C) was investigated by changes in moisture during the equilibration period and scanning electron microscopy. Discontinuous sorption behavior observed at a_w 0.54 and above was ascribed to lactose undergoing a transition from the amorphous to the crystalline state. Scanning electron micrographs taken throughout the equilibration period revealed increasing lactose crystallization with time at and above a_w 0.54. Deuterium(²H) Nuclear Magnetic Resonance (NMR) spectroscopy was used to monitor the water mobility during the amorphous to crystalline state transition. Changes in water mobility were found to be directly related to the observed moisture changes and the transition of lactose from the amorphous to crystalline state.