Thermal Degradation of the Lachrymatory Precursor of Onion

Summary The lachrymatory precursor of onion, S-(1-propenyl)-L-cysteine sulfoxide, was heated in a closed model system at different temperatures (from 80 to 200°C) in the presence of variable amounts of water (0-98%) for 1 to 60 minutes. The volatile compounds produced were isolated by extraction, analyzed, and identified by means of GC and GC-MS. The major volatile sulfur-containing compounds generated by thermal degradation of this amino acid were 2,4- and 3,4-dimethylthiophenes and alkyl substituted thiazoles. The most important non-sulfur volatile degradation products (formed only at water contents higher than 20%) were 2-methyl-2-butenal and 2-methyl-2-pentenal.