30D-20 |
A mobility-based theory for the role of moisture on maillard browning reaction rate |
C. SHERWIN, Dept. of Food Science, University of Wisconsin, 1605 Linden Dr., Madison, WI 53706 and T. P. Labuza, Department of Food Science and Nutrition, University of Minnesota, 1334 Eckles Ave., Saint Paul, MN 55108. Water activity (aw) can be correlated to reaction rate, but only the moisture monolayer value exists as a theory linking the two. No mechanism describes a molecular basis for increased rate with increased aw. Some researchers use the difference between storage temperature and the glass transition temperature (Tg), modeled by WLF kinetics, as a mobility-based mechanism. This approach has been inconclusive for chemical reactions. A theory of local mobility, not directly tied to Tg, may yet be the molecular mechanism for the role of water in food chemical reactions. Both water and non-water solvents in foods can affect rate of NEB via molecular mobility. Our objective was to determine in a model system the effect of humectants, aw, and moisture content on Tg, the Maillard browning rate, and the local mobility of a reactant. A model system was developed containing caseinate, glucose, and a humectant: either glycerol (a solvent), sorbitol (a non-solvent), or a non-humectant control. The solutions were freeze-dried and re-equilibrated to a range of aw. Tg was determined by DSC. NEB was measured by Hunter L, a, b values and A420 of a filtered aqueous extract. Rotational mobility of 13C-glucose was measured by cross-polarization/magic angle spinning NMR. Translational mobility found to be too slow to measure. Glycerol and sorbitol formulations were rubbery at 25°C over all aw, as were the control samples, except for aw=0.11. Control sample showed a maximum NEB rate at aw=0.65. Glycerol formulations showed higher NEB over all aw, with a maximum rate at 0.33. Sorbitol showed a decrease over all aw with a maximum at 0.65. Rotational mobility generally increased with aw, and could not be correlated to the min-max trend of NEB. Neither water activity, the monolayer value, Tg, nor mobility were predictors for NEB reaction rate.
Session 30D, Food Engineering: Physical and chemical properties
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