NMR water mobility in xanthan and locust bean gum mixtures: possible explanation of microbial response

Molecular and structural mobility of xanthan and locust bean gum mixtures (with and without mannitol) were studied in relation to microbial stability. Molecular mobility was measured by solid state 1H and 2H NMR and by 2H high resolution NMR while differential scanning calorimetry and dynamic mechanical analysis were used to investigate structural mobility. The NMR mobile signal was found to increase significantly with increasing moisture content and, at above 30% moisture content, the solid component disappeared with observed structural collapse. Cell survival decreased with moisture content increasing from 6 to 15% corresponding with an increasing mobile proton and deuterium signals (i.e. increased mobility). Presence of mannitol protected the cells from death at increasing moisture content and a relationship with a lower mobile 1H signal (as compared with the control) seems to be present. A strong mannitol–water interaction, leading to a decreased mobility, is suggested to play a role. No evidence of a glassy to rubbery transition was observed from DSC and DMA analyses, suggesting that structural relaxation did not play a significant role in NMR solid-liquid transition, i.e. NMR molecular mobility.