Formation of aldehydes from S-alk(en)ylcysteines and their sulfoxides

Abstract.

Fourteen S-alk(en)ylcysteines, S-methyl, S-ethyl, S-propyl, S-isopropyl, S-allyl, S-butyl, S-isobutyl, S-(2-butenyl), S-(3-butenyl), S-pentyl, S-isopentyl, S-prenyl, S-hexyl and S-(methylthiomethyl) derivatives and the corresponding sulfoxides, including those naturally occurring in alliaceous and brassica vegetables, were oxidised in model solutions with either glyoxal or potassium peroxodisulfate. The Strecker aldehydes, 2-[alk(en)ylthio]ethanals, preferentially formed by oxidation of the individual S-alk(en)ylcysteines were identified and their sensory properties and odour threshold values evaluated and described. These Strecker aldehydes were accompanied by the corresponding 2,4-bis[alk(en)ylthio]but-2-enals formed by subsequent aldolisation/dehydration reactions of the 2-[alk(en)ylthio]ethanals. 2-(Methylthio)ethanal and 2-(allythio)ethanal have been identified as the minor constituents among cooked broccoli and cooked garlic volatiles for the first time. Despite being formed in relatively small amounts, they might contribute to the overall aroma of processed vegetables.