Weak chemiluminescence at an early stage of the Maillard reaction

Because a fairly stable free radical was observed to form at an early stage of the Maillard reaction, the development of chemiluminescence (CL) in the Maillard reaction was examined by using a highly sensitive photon counting system. It was demonstrated that marked CL developed generally at an early stage in the Maillard reaction prior to browning. The CL intensity followed the order methylglyoxal is much greater than glyoxal is greater than xylose is greater than glucose in the carbonyl compounds and alkylamine is greater than lysine is greater than beta-alanine is greater than alpha-alanine in the amino compound. CL was observed even at pH 6.0 and increased markedly with increase in pH. These features are almost the same as those observed in the free-radical formation and browning. CL was negligibly weak in Ar or N2 and greatly increased in air; it was suppressed by addition of ascorbic acid and cysteine. The mechanism by which CL develops is not yet clear but is assumed to be attributable to the production of electronically excited species through the reaction of oxygen with the free-radical product(s) of low molecular weight dicarbonyl products or pyrazinium compounds and these may also directly contribute to browning