Water mobility in glassy and rubbery solids as determined by oxygen-17 nuclear-magnetic resonance: Impact on chemical stability

Although chemical reactivity in solid food systems has been studied as a function of water activity and glass transition, the possible effects of water mobility on chemical reactions have not been investigated. The effect of the glass transition on water mobility at constant temperature and water content was determined. The relationship between the experimentally determined water mobility in polyvinylpyrrolidone (PVP) systems and chemical reactivity data from the same PVP,systems was evaluated. Water mobility, as determined via O-17-NMR, was not affected by the glass transition; PVP systems (it constant water activities and water contents, but different physical states (glassy and rubbery), had the same water mobility. An evaluation of four chemical reactions showed no relation between water mobility and kinetic data. The effect of water on chemical reactions is multidimensional and cannot be reduced to a single physicochemical parameter. (C) 2002 Elsevier Science Ltd. All rights reserved.