Occurrence of l-γ-glutamyl-S-(1-hydroxy-2-methyl-3-pentanyl)-l-cysteine and S-(1-ethyl-3-hydroxy-2-methylpropyl)-l-cysteine in fresh and processed Allium caepa L. cultivar

ABSTRACT The long‐lasting presence of onion flavour in the mouth is due to the action of mouth microflora on non‐volatile flavour precursors. The key odorant for the fresh green onion tonality is (±)‐2‐methyl‐3‐sulfanyl‐pentan‐1‐ol and it should originate from its corresponding cysteine‐S‐conjugate. This paper describes the analysis of onion powder and fresh onion, focusing on the discovery of S‐(1‐ethyl‐3‐hydroxy‐2‐methylpropyl)‐l‐cysteine. This compound is a precursor of a potent volatile sulfur compound, (±)‐2‐methyl‐3‐sulfanyl‐pentan‐1‐ol. Authentic samples of the cysteine‐S‐conjugate (S‐(1‐ethyl‐3‐hydroxy‐2‐methylpropyl)‐l‐cysteine) and the γ‐glutamyl‐cysteine‐S‐conjugate (l‐γ‐glutamyl‐S‐(1‐hydroxy‐2‐methyl‐3‐pentanyl)‐l‐cysteine) were prepared and fully characterized. In the onion powder extract, the presence of l‐γ‐glutamyl‐S‐(1‐hydroxy‐2‐methyl‐3‐pentanyl)‐l‐cysteine was confirmed; the average concentration calculated from four extractions was 1.1 ± 0.3 μg/kg. The occurrence of S‐(1‐ethyl‐3‐hydroxy‐2‐methylpropyl)‐l‐cysteine was also confirmed and the concentration was about 1.3 ± 0.4 μg/kg. In fresh onion, the occurrence of both compounds was also confirmed; the γ‐glutamyl‐cysteine‐S‐conjugate was present at a higher concentration (1.0 μg/kg) compared with cysteine‐S‐conjugate (< 0.1 μg/kg). The formation of (±)‐2‐methyl‐3‐sulfanyl‐pentan‐1‐ol from an onion powder fraction by means of a β‐lyase enzyme (apotryptophanase) confirmed the natural occurrence of these precursors. This analysis is another example showing the role of the glutathione cycle to ultimately release volatile organic sulfur compounds. Copyright © 2011 John Wiley & Sons, Ltd.