Degradation of tryptophan in heated beta-lactoglobulin-lactose mixtures is associated with intense Maillard reaction

The Maillard reaction that occurs during food processing is believed to induce the formation of radical species at high temperatures. This study was carried out to determine whether radical-sensitive Trp is affected during the glycation of P-lactoglobulin (PLG) by lactose in the presence of iron and vitamin C at 115 degrees C for 6 min. The early glycation, measured as furosine, increased nonlinearly with time, but the formation of fluorescent advanced glycated products (AGEs) was almost exponential, explaining most of the blockage of lysine residues (up to 70%). At the same time, 46-55% of Trp residues were lost, and the Trp concentration was negatively correlated with the fluorescent AGEs. The Trp fluorescence quantum yield decreased considerably due to changes in the protein conformation upon heating with lactose. Trp degradation and AGEs formation were significantly greater in proteic fraction soluble at pH 4.6 than in insoluble fraction. The radicals released during the advanced Maillard step could be responsible for Trp oxidation.