Bp. Hills et al., NMR WATER RELAXATION, WATER ACTIVITY AND BACTERIAL SURVIVAL IN POROUS-MEDIA, Journal of the Science of Food and Agriculture, 71(2), 1996, pp. 185-194
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.