Moisture sorption by dairy powders studied by low-field NMR
Todor Vasiljevic, Antoinette Toebes, Thom Huppertz
Abstract
The influence of moisture sorption on spin–spin (T2) relaxation times for skim milk powder (SMP), milk protein concentrates (MPC50 and MPC80) and amorphous lactose was studied by low-field nuclear magnetic resonance (NMR). For amorphous lactose and MPC80, only one main peak was observed in the T2 distribution, whereas for SMP and MCP50, two main peaks, relating to the water associated with the amorphous lactose and protein, were observed at aw <0.4; at aw >0.4 lactose crystallised and no peak relating to water associated with lactose was visible in the T2 spectrum (water of crystallisation for crystalline lactose has shorter relaxation times, which cannot be detected by low-field NMR). T2 peak position and peak area for the protein fraction increased with increasing aw. Shifts in the T2 peak position showed the transition from mono- and bilayer water at low aw to more rapidly exchanging forms of water at higher aw.