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 nuclear magnetic resonance (NMR) relaxation rates and water activities several nontrivial relationships are derived between NMR water relaxation times and water activity. These relationships are tested for randomly packed beds of silica and Sephadex particles both in the fast and slow diffusive exchange regimes. It is shown that Archies law for electrical conductivity is obeyed by these porous beds and this observation is used to establish further relationships between NMR relaxation rates and electrical conductivity. By comparing the rates of recovery of viable cells of Salmonella typhimurium with the NMR, sorption isotherm and conductivity data it is shown that the bacterial cells are monitoring the local water activity in the fluid immediately surrounding them and not the experimental, global water activity coefficient. The implications of this observation for food preservation are considered.