59D-30 |
Water mobility in glassy and rubbery solids as determined by oxygen-17 nuclear magnetic resonance: Impact on chemical stability |
L. N. BELL1, H. M. Bell2, and T. E. Glass2. (1) Dept. of Nutrition & Food Science, Auburn Univ., 328 Spidle Hall, Auburn, AL 36849, (2) Dept. of Chemistry, Virginia Polytechnic Institute & State Univ., Blacksburg, VA 24061 Water influences solid-state chemical reactions in various ways. Although chemical stability has been evaluated from the perspective of water activity and the glass transition, the possible effects of water mobility on chemical reactions in solids have not been investigated. The objectives of this study were to determine if water mobility was affected by the glass transition and to evaluate if water mobility influenced food chemical stability. Solutions of polyvinylpyrrolidone (PVP-K30, molecular weight 40,000 and PVP-LMW, molecular weight <3,500) were lyophilized as pellets. Dry PVP (0.50 g) was placed into NMR tubes containing a stearic acid plug. Triplicate samples were equilibrated at 25oC to water activities 0.33, 0.54, and 0.76 over saturated salt solutions containing 6-20% oxygen-17 (O-17) enriched water. Tubes were positioned such that the entire sample resided within the active region of the NMR. O-17 spectra were obtained at 22oC and consisted of a single Lorentzian peak. Transverse relaxation times (T2) were determined and evaluated for possible correlation to previously published kinetic data for reactions occurring in similar PVP systems. Using samples at constant moisture and temperature but having different glass transition temperatures, water mobility was shown not to be affected by the glass transition. At water activity 0.54 and 16% moisture, glassy PVP-K30 and rubbery PVP-LMW had T2 values of 0.0471 and 0.0467 msec, respectively. An evaluation of four chemical reactions (aspartame degradation, thiamin degradation, Maillard reaction, invertase stability) showed no relation between water mobility and kinetic data. At water activity 0.54, T2 values were similar in PVP-K30 and PVP-LMW while invertase half-lives were 139 and 24 days, respectively. Water mobility cannot be used as an indicator or predictor of chemical reactivity in solids. The effect of water on food chemical stability is multidimensional and cannot be reduced to a single physicochemical parameter.
Session 59D, Food Chemistry: Proteins and Physicochemical Properties
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