14B-13 |
Reaction kinetics of an acid-base catalyzed reaction: Modeling the pH-lowering effect of sucrose in phosphate buffer solutions |
S. Chuy, Nutrition and Food Science, Auburn University, 328 Spidle Hall, Auburn, AL 36849 and L. N. BELL, Dept. of Nutrition & Food Science, Auburn Univ., 328 Spidle Hall, Auburn, AL 36849.
The addition of sucrose to phosphate buffer lowers the pH and apparent pKa of the buffer. The effects of these pH and pKa changes on food chemical stability have not been investigated. The objective of this project was to determine the significance of the polyol-induced pH lowering of phosphate buffer with respect to the rates of acid-base catalyzed reactions. The kinetics of aspartame degradation, an acid-base catalyzed reaction, were evaluated in phosphate buffer of various concentrations and pH values, with and without 2 molal sucrose or glycerol. HPLC was used to quantify aspartame. Aspartame loss was modeled via pseudo-first order kinetics. A detailed kinetic model was developed to predict the rate constants, kobs, in phosphate buffer alone, and with added sucrose or glycerol, incorporating the measured pH, apparent pKa, fraction of unprotonated aspartame (fNH2) and buffer concentration. Sucrose-containing phosphate buffer solutions had a lower pH than that of phosphate buffer alone (e.g., pH 4.57 compared to 4.85, respectively). The lower pH caused a reduction in aspartame reactivity. The kinetic model developed for predicting aspartame stability at 25ºC was kobs=(1E-07) + (1.46E-03)[H+] + (79.8)[OH-] + (0.272)(fNH2)[HPO4=] + (0.0222)(fNH2)[H2PO4-] + (4E-05)[H2PO4-]. Linear regression of the predicted rate constants versus experimental rate constants (n=20) gave a linear correlation of 0.996 and a slope of 0.96, indicating excellent predictability of the model. Aspartame degradation rate constants in the sucrose-containing solutions were also predicted successfully when corrections (i.e., lower pH, lower apparent pKa value, buffer dilution from the polyol) were applied to the kinetic model. The change of buffer properties (pH, pKa) from adding sucrose to phosphate buffer does impact food chemical stability. These effects can be successfully incorporated into predictive kinetic models. Therefore, pH and pKa changes from adding polyols to solutions containing buffer should be considered during food product development.
Session 14B, Food Chemistry: Physicochemical properties
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