Formation of cysteine-S-conjugates in the Maillard reaction of cysteine and xylose.

Cysteine-S-conjugates (CS-conjugates) occur in foods derived from plant sources like grape, passion fruit, onion, garlic, bell pepper and hops. During eating CS-conjugates are degraded into aroma-active thiols by β-lyases that originate from oral microflora. The present study provides evidence for the formation of the CS-conjugates S-furfuryl-l-cysteine (FFT-S-Cys) and S-(2-methyl-3-furyl)-l-cysteine (MFT-S-Cys) in the Maillard reaction of xylose with cysteine at 100°C for 2h. The CS-conjugates were isolated using cationic exchange and reversed-phase chromatography and identified by (1)H NMR, (13)C NMR and LC-MS(2). Spectra and LC retention times matched those of authentic standards. To the best of our knowledge, this is the first time that CS-conjugates are described as Maillard reaction products. Furfuryl alcohol (FFA) is proposed as an intermediate which undergoes a nucleophilic substitution with cysteine. Both FFT-S-Cys and MFT-S-Cys are odourless but produce strong aroma when tasted in aqueous solutions, supposedly induced by β -lyases from the oral microflora. The perceived aromas resemble those of the corresponding aroma-active thiols 2-furfurylthiol (FFT) and 2-methyl-3-furanthiol (MFT) which smell coffee-like and meaty, respectively.