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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., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

5-Hydroxy-3-mercapto-2-pentanone is claimed in the scientific literature as a key intermediate in the degradation of thiamine and the related generation of aroma compounds; however, there are no analytical NMR and MS data available. We have identified the compound in a thermally treated mixture of thiamine, cysteine, and xylose and characterized it by MS and NMR.

The influence of different pH values, ranging from 4.0 to 7.0, on the formation of sulfur volatiles in the Maillard reaction was studied using a model system with [13C5]xylose, cysteine, and thiamin. The use of 13C-labeled xylose allowed, by analysis of the mass spectra, volatiles that incorporated xylose carbons in the molecule from other carbon sources to be discerned. For 2-furaldehyde and 2-furfurylthiol, which were favored at low pH, the labeling experiments clearly indicated that xylose was the exclusive carbon source. On the other hand, xylose was virtually not involved in the formation of 3-mercapto-2-butanone, 4,5-dihydro-2-methyl-3-furanthiol, and 5-(2-hydroxyethyl)-4-methylthiazole, which apparently stemmed from thiamin degradation. Both xylose and thiamin seemed to significantly contribute to the formation of 2-methyl-3-furanthiol, 3-mercapto-2-pentanone, and 2-mercapto-3-pentanone, and therefore different formation pathways must exist for each of these molecules. In general, the pH determined strongly which volatiles were formed, and to what extent. However, the relative contribution of xylose to the C-skeleton of a particular compound changed only slightly within the investigated pH range, when both xylose and thiamin were involved in the formation.

The volatiles formed from [1-(13)C]-ribose and cysteine during 4 h at 95 degrees C in aqueous phosphate buffer (pH 5) were analyzed by headspace SPME in combination with GC-MS. The extent and position of the labeling were determined using MS data. The identified volatiles comprised sulfur compounds such as 2-[(13)C]methyl-3-furanthiol, 2-[(13)CH(2)]furfurylthiol, [1-(13)C]-3-mercaptopentan-2-one, [1-(13)C]-3-mercaptobutan-2-one, [4-(13)C]-3-mercaptobutan-2-one, and 3-mercaptobutan-2-one. The results confirm furan-2-carbaldehyde as an intermediate of 2-furfurylthiol, as well as 1,4-dideoxypento-2,3-diulose as an intermediate of 2-methyl-3-furanthiol and 3-mercaptopentan-2-one. Loss of the C-1 and C-5 carbon moieties during the formation of 3-mercaptobutan-2-one suggests two different mechanisms leading to the key intermediate butane-2,3-dione.

The headspace volatiles produced from a phosphate-buffered solution (pH 5) of cysteine and a 1 + 1 mixture of ribose and [(13)C(5)]ribose, heated at 95 degrees C for 4 h, were examined by headspace SPME in combination with GC-MS. MS data indicated that fragmentation of ribose did not play a significant role in the formation of the sulfur aroma compounds 2-methyl-3-furanthiol, 2-furfurylthiol, and 3-mercapto-2-pentanone in which the carbon skeleton of ribose remained intact. The methylfuran moiety of 2-methyl-3-(methylthio)furan originated from ribose, whereas the methylthio carbon atoms came partly from ribose and partly from cysteine. In 3-mercapto-2-butanone one carbon unit was split from the ribose chain. On the other hand, all carbon atoms in 3-thiophenethiol stemmed from cysteine. In another trial cysteine, 4-hydroxy-5-methyl-3(2H)-furanone and [(13)C(5)]ribose were reacted under the same conditions. The resulting 2-methyl-3-furanthiol was mainly (13)C(5)-labeled, suggesting that it stems from ribose and that 4-hydroxy-5-methyl-3(2H)-furanone is unimportant as an intermediate. Whereas 2-mercapto-3-pentanone was found unlabeled and hence originated from 4-hydroxy-5-methyl-3(2H)-furanone, its isomer 3-mercapto-2-pentanone was formed from both 4-hydroxy-5-methyl-3(2H)-furanone and ribose. A new reaction pathway from ribose via its 1,4-dideoxyosone is proposed, which explains both the formation of 2-methyl-3-furanthiol without 4-hydroxy-5-methyl-3(2H)-furanone as an intermediate and a new way to form 3-mercapto-2-pentanone.