Free radical processes in non-enzymatic browning of glucose and lysine: influence of temperature and unsaturated lipids

Formation of dialkylpyrazinium radical cations in aerated 70 aqueous glycerol solutions of glucose and lysine during heating resulting in browning (90, 110, and 130C, investigated) was more dependent on temperature than formation of brown colour. Activation energy (Ea) for radical formation was 83 kJ mol-1, compared with 70 kJ mol-1 for browning, and was unaffected by methyl linolenate. Low-temperature browning was influenced by non-radical degradation of Amadori products, whereas radical processes were prominent at higher temperatures and were unaffected by unsaturated lipids. In contrast, methyl linolenate reacts with lysine in the absence of glucose to form fluorescent products at a slow rate (Ea 25 kJ mol-1). Glucose increased the rate of formation of fluorescent products (Ea 60 kJ mol-1), in agreement with Maillard reactions at low temperatures involving glucose as a rate-determining reagent. Lipid oxidation does not have a direct effect on lysine and glucose browning reactions at conditions relevant for food; effects of lipids on Maillard reactions are matrix-related.