NMR WATER RELAXATION, WATER ACTIVITY AND BACTERIAL SURVIVAL IN POROUS-MEDIA

Working from the simple assumption that bulk, surface and bound water in a heterogeneous system are characterised by different intrinsic nuc lear magnetic resonance (NMR) relaxation rates and water activities se veral nontrivial relationships are derived between NMR water relaxatio n times and water activity. These relationships are tested for randoml y packed beds of silica and Sephadex particles both in the fast and sl ow diffusive exchange regimes. It is shown that Archies law for electr ical conductivity is obeyed by these porous beds and this observation is used to establish further relationships between NMR relaxation rate s and electrical conductivity. By comparing the rates of recovery of v iable cells of Salmonella typhimurium with the NMR, sorption isotherm and conductivity data it is shown that the bacterial cells are monitor ing the local water activity in the fluid immediately surrounding them and not the experimental, global water activity coefficient. The impl ications of this observation for food preservation are considered.