Inhibition of polyphenoloxidase from apple by Maillard reaction products prepared from glucose or fructose with l-cysteine under various conditions of pH and temperature

The effects of Maillard reaction products (MRPs), synthesized from equimolar glucose or fructose with l-cysteine (1 mol l⁻¹) aqueous model mixtures, by modulating pH and temperature of heating, according to a two-factor and five-level experimental design, were investigated on polyphenoloxidase (PPO) activity from apple. Final pH and absorbance measurements at 350 nm were also selected as indicators of the Maillard reaction development and checked. In general, inhibitory potency (IP) of the mixtures increased with the increase in temperature (80-120°C) and the decrease in pH (pH 2.0-12.0) of the reaction medium. A linear relationship between the IP and heating time (0-48 h) or Abs.350 nm (0-70 AU) was demonstrated for glucose/cysteine system heated from 80°C to 120°C. Polarographic and spectrophotometric data were used to calculate kinetic constants and activation energy (Ea) values of inhibitory MRPs formation versus PPO activity and of those compounds absorbing at 350 nm. Ea values for these reactions were close, being 191 and 124 kJ mol⁻¹, respectively. The experimental design allowed to conclude that linear effects of both factors as well as a quadratic effect of pH were significant, leading to optimum conditions for the production of glucose-derived MRPs inhibitors. In most cases, glucose produced MRPs with higher IP compared to counterpart fructose-cysteine MRPs.