Effect of urea on volatile generation from Maillard reaction of cysteine and ribose

Urea occurs naturally in many food products, and its presence affects food quality. However, little is known about its impact on flavor generation in food production. In this study, the urea contents in beef, pork, and chicke n were determined. The effects of urea and pH on thermal flavor generation were investigated using the model system of cysteine with ribose, which was heated to the roasting temperature of 180 degrees C for 2 h at pH 5 and pH 8.5. The results revealed relatively large amounts of urea in these meats and demonstrated that pH affects aroma generation. Volatiles identified fro m the reaction system of ribose and cysteine showed that sulfur-containing compounds such as thiophenes, thiazoles, and thiophenethiols were the most abundant compounds. The addition of urea into the reaction mixture caused t he disappearance or reduction in content; of some sulfur-containing compoun ds but resulted in the generation of several important nitrogen-containing volatiles, like pyrazine, methylpyrazine, 2,5- (and 2,6-)dimethylpyrazine a nd other alkylpyrazines, which are known to elicit roasty, nutty flavor not es. A plausible explanation for this phenomenon is that ammonia can be rele ased from urea upon heating and the formed ammonia competes with hydrogen s ulfide to react with Maillard reaction precursors to produce nitrogen-conta ining compounds such as alkylpyrazines.