Effect of glutathione and Maillard reaction products prepared from glucose or fructose with glutathione on polyphenoloxidase from apple—I: Enzymatic browning and enzyme activity inhibition

The effect of unheated and heated glutathione (GSH) and Maillard reaction products (MRPs), derived from equimolar mixtures (0.25 M) glucose or fructose with GSH, on purified apple polyphenoloxidase (PPO) activity was investigated by polarography and spectrophotometry, using 4-methylcatechol as the main phenolic substrate. When assayed alone, glutathione interacted with enzyme-generated o-quinones, giving rise to colourless conjugates, as demonstrated by high pressure liquid chromatography (HPLC). By polarography, increasing concentrations (0–300 mM) of GSH, resulted in a high inhibitory effect on PPO activity, mostly due to a drop of pH of the reaction solutions to acidic values. Upon heating GSH at 90 °C, thermal degradation product formation was responsible for a partial PPO inhibition. GSH-derived MRPs highly inhibited PPO activity, inhibition efficiency increasing with heating time (2–39 h) and temperature (80–110 °C). Compared with MRPs prepared from hexose with cysteine, those from GSH exhibited a more potent inhibitory effect, due to the presence of an additional carboxylic function on the tripeptide. Therefore, these MRPs could be considered as potential natural inhibitors for use with minimally processed fruits.