Thermal degradation of S-methylcysteine and its sulfoxide -: Important flavor precursors of Brassica and Allium vegetables

Two naturally occurring S-containing amino acids, S-methyl-L-cysteine sulfoxide and its biochemical precursor S-methyl-L-cysteine, were heated in closed model systems at different temperatures (from 80 to 200 degrees C) in the presence of variable amounts of water (0-98%) for 1-60 min. The arising volatile compounds were extracted with diethyl ether, analyzed, and identified by means of GC and GC/MS. It was found that thermally generated breakdown products of either S-methyl-L-cysteine or particularly S-methyl-L-cysteine sulfoxide can significantly contribute to the typical aroma of culinary processed Brassica and Allium vegetables. Nevertheless, degradation pathways of these closely related amino acids seem to be quite different. Dimethyl disulfide was identified as the predominant volatile compound generated by thermal degradation of both S-methylcysteine and its sulfoxide. Dimethyl trisulfide, dimethyl thiosulfinate, dimethyl thiosulfonate, and alkyl- and alkylthio-substituted pyridines were identified as minor volatile breakdown products arising from S-methylcysteine sulfoxide. Formation of dimethyl trisulfide, the major off-flavor principle of overcooked Brassica vegetables, was investigated in detail.