A rapid spectrophotometric method of determination of thiopropanal s-oxide (lachrymator) in onion (allium cepa l.) and its significance in flavour studies

The method depends upon rapid extraction of the lachrymator (thiopropanal S-oxide) into hexane at 0 °C and observation of the absorbance maximum at 254 nm as compared with that of the synthetic compound as a standard. Thiopropanal S-oxide was synthesised by dehydrochlorination of 1-propanesulphinyl chloride by a published method. Evidence that the observed absorbance was due to the presence of thiopropanal S-oxide was based mainly on agreement of the properties of the synthetic compound with those of hexane extracts of fresh onion juice with respect to λ_max values, reaction with L-cysteine reagent, and thin-layer chromatography. Further confirmation was afforded by the properties of alliinase fission products of synthetic (±)-S-l-propenyl-L-cysteine sulphoxide. A series of applications of the method in determination of onion flavour intensity, including non-destructive sampling, is described.