The levels of S‐alk(en)yl‐ L ‐cysteine sulphoxides during the growth of the onion (Allium cepa l.)

Flavour precursors, the S‐alk(en)yl‐L‐cysteine sulphoxides, change considerably in both concentration and total amount during the growth of onions. For the brown onion cv. Pukekohe Long Keeper (PLK) and for the white onion cv. Dehyso, (±)‐S‐1‐propyl‐L‐cysteine sulphoxide (±PCSO) is the predominant flavour precursor, with (±)‐trans‐S‐1‐propenyl‐L‐cysteine sulphoxide (±PECSO) and (±)‐S‐1‐methyl‐L‐cysteine sulphoxide (±MCSO) in a minor role. For PLK the greatest concentration of all three precursors is just prior to bulbing (3.6±1.1 mg g−1 fresh weight); for Dehyso the greatest concentration is at bulbing (4.6±0.6 mg g−1 fresh weight); Both varieties show a mid‐season reduction in flavour precursor concentration (of about 45%), followed by a small but significant increase in ±PCSO concentration during the end of season leaf die‐down. The total amount of the three flavour precursors per bulb in both varieties shows a maximum in late January (360±107 mg for PLK, 602±144 mg for Dehyso). In the subsequent months there is a dramatic reduction in the total amount of ±PECSO and ±MCSO, and a slight reduction in ±PCSO. Both varieties show an increase in the amount of ±PCSO at leaf die‐down. The relative proportions of these flavour precursors change with bulb age and cultivar. These results suggest that white onions used for drying and processing could be harvested from bulbing onwards. Furthermore, when onion cultivars and onion plant selections are assessed for flavour, care needs to be given to the stage of maturity of the onion.