Free radical scavenging, inhibition of polyphenoloxidase activity and copper chelating properties of model Maillard systems

Antioxidant properties of Maillard reaction products (MRP) obtained from aqueous mixtures of glucose (0.8 mol/l) with proline, glycine, arginine, lysine, cysteine or glutathione (0.5 mol/l) heated at 103 °C for 1–92 h were investigated using different in vitro tests. Free radical scavenging activity was determined by measuring their reactivity towards DPPH° and lipidic radicals produced by AAPH. Contrary to glucose–proline and glucose–glycine MRP, glucose-lysine and glucose–arginine MRP displayed high scavenging activities. The activity of glucose–lysine MRP peaked after 14 h of heating while the activity of glucose–arginine constantly increased during the whole heat treatment. The high scavenging capacity observed towards DPPH° for glucose–cysteine mixtures could be attributed to the sulfhydryl group of cysteine.The inhibitory effect of MRP on activity of two copper-oxidoreductases, apple polyphenoloxidase and mushroom tyrosinase, assessed by polarography, showed that thiol-derived MRP were the most potent inhibitors, even at very low levels in the reaction medium. Conversely, the other MRP were only slightly efficient at high levels. Unheated mixtures containing thiol compounds exhibited a potent copper chelating ability, as efficient as EDTA, when determined by the tetramethylmurexide (TMM) test. After heating, these mixtures lost part of their chelating efficiency, but it remained higher than that of the other MRP, suggesting that the sulfhydryl group also played a role in the copper chelating properties.