Free glutamine as a major precursor of brown products and fluorophores in Maillard reaction systems

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Abstract
Glutamine is one of the most abundant free amino acid found in raw food. In this study, the contribution of free glutamine to nonenzymatic browning and fluorescence was investigated using an aqueous model system with methylglyoxal. The results indicated that glutamine contributed to the Maillard reaction via two pathways. First, the hydrolysis of the amide bond of glutamine led to the release of ammonia which was implicated in the formation of brown color and fluorescence. Among other nitrogen donors tested (asparagine, glutamic acid and urea) our results demonstrated that free glutamine was a major source of ammonia during heating. When heated at 120 and 180 °C, 100% of ammonia was released from glutamine after 60 and 10 min, respectively. The second pathway involved a direct Maillard reaction with the α-amino group of glutamine. Both pathways led to a rapid and complete destruction of glutamine when heated in the model systems. With reference to the Maillard browning (absorbance at 420 nm) glutamine turned out to be the most reactive amine, followed by asparagine, glutamate, ammonia and urea. Maximum fluorescence (excitation and emission wavelengths at 330 and 450 nm, respectively) was also observed with glutamine followed by urea and ammonia. Overall this study suggested that free glutamine predominantly contributes to the color and fluorescence formations of foodstuffs.