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

•Two cysteine S-conjugates are formed in the Maillard reaction of xylose and cysteine.•The structures are S-furfuryl-l-cysteine and S-(2-methyl-3-furyl)-l-cysteine.•The identification is based on 1H NMR, 13C NMR and LC–MS2 data.•This is the first report on the formation of cysteine S-conjugates in Maillard reaction.•Both compounds are cleaved by microbial oral β-lyases releasing odorant thiols. 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 1H NMR, 13C NMR and LC–MS2. 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.