Influence of High Hydrostatic Pressure and pH on the Rate of Maillard Browning in a Glucose−Lysine System

Glucose−lysine solutions, initial pH 5.1, 6.5, 8.0, and 10.1, were incubated at temperatures in the range 40−60 °C under atmospheric pressure and 600 MPa. The rate of Maillard browning at 50 °C was shown to be retarded by pressure in solutions of initial pH 5.1 and 6.5 but significantly enhanced in solutions of initial pH 8.0 and 10.1, while the effect of pressure was negligible at pH 7.0−7.5. At pH 10.1 the activation energies for the two systems (high pressure and atmospheric pressure) were not significantly different. The rates of reaction of these systems carried out in phosphate and bicarbonate buffer, at 600 MPa and alkaline pH, were slower than expected. It is proposed that pressure-induced ionization of the carboxylate group on the lysine at low pH, or of the phosphate and bicarbonate groups at high pH, causes a decrease in pH and subsequent reduction in the rate of reaction. In systems in which the pH is unaffected by pressure, i.e., those buffered by the amino groups of lysine, pressure accelerates the rate of reaction. Preliminary HPLC and UV data suggest there is no difference between the chemical pathways followed with and without the application of elevated pressure.