67D-5 |
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R. D. LUDESCHER and A. Zunic. Dept. of Food Science, Rutgers, The State Univ. of New Jersey, 65 Dudley Rd., New Brunswick, NJ 08901-8520 The rates of chemical and physical change in amorphous solids determine the stability of most dried or frozen solid foods. Although it is widely recognized that these rates are modulated by molecular mobility, the details of this modulation are largely unknown, in part because the modes and rates of molecular mobility in amorphous materials and their relation to chemical structure are largely unknown. Our objective is to develop novel luminescent probes of molecular mobility and use them to characterize the mobility of amorphous solid sugars in the glassy and viscous liquid states. We have examined how the fluorescence and phosphorescence of the amino acid tryptophan can report on the local mobility of amorphous thin films of sucrose, maltose, and trehalose. Tryptophan fluorescence emission red-shifts over the temperature range from 0-100C, providing evidence for thermally acivated local mobility on the ~10 ns time scale in both glassy and liquid sugars. The emission red-shifts upon excitation at the red edge of the absorption band, indicating that the sugars are dynamically heterogeneous on the ns time scale. The effect of temperature on fluorescence intensity is greater above than below Tg. Phosphorescence intensity and lifetime are effectively quenched by local motions within the amorphous sugars at temperatures below Tg. Tryptophan phosphorescence lifetimes vary as a function of wavelength across the emission band, indicating that the glassy sugars are dynamically heterogeneous on the millisecond time scale at 20C. These results demonstrate that tryptophan luminescence is sensitive to dynamic heterogeneity within amorphous sugars in both glassy and liquid states and that this dynamic heterogeneity is evident on time scales ranging over seven orders of magnitude. Models of how molecular mobility modulates food stability must thus account for both mobility and dynamic heterogeneity on a wide range of time scales.
Session 67D, Food Chemistry: Lipid and carbohydrate chemistry
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