Quantitative analysis of the S‐alk(en)yl‐L‐cysteine sulphoxides in onion (Allium cepa L.)

Quantitative analysis of each of the S‐alk(en)yl‐L‐cysteine sulphoxides was achieved by the following procedure. A methanol: chloroform: water extract of onion tissue was purified by electrophoresis. Alk(en)yl‐L‐cysteine sulphoxides were separated by thin‐layer chromatography (t.l.c.) on commercial silica‐gel plates and were reacted with ninhydrin, the spot intensity being measured by an integrating densitometer. The synthetic analogue (±)‐S‐1‐butyl‐L‐cysteine sulphoxide was used as an internal standard in each extract. In white bulb onion (cv. Southport White Globe), (±)‐S‐1‐propyl‐L‐cysteine sulphoxide is the predominant flavour precursor at a concentration of 2.9 ± 0.4 mg g −1 fresh weight. ±‐S‐1‐methyl‐L‐cysteine sulphoxide and (±) trans ‐S‐1‐propenyl‐L‐cysteine sulphoxide are present at the lower concentrations of 0.9 ± 0.25 mg g −1 fresh weight and 0.6 ± 0.15 mg g −1 fresh weight respectively. Stable n‐butyl, trifluoroacetyl derivatives of standards and of the alk(en)yl‐L‐cysteine sulphoxides in extracts, were formed. Subsequent g.c.‐m.s. enabled parent ions and fragmentation patterns to be recorded and allowed the conclusive identification of the endogenous alk(en)yl‐L‐cysteine sulphoxides.