Differential hydrolysis of alk(en)yl cysteine sulphoxides by alliinase in onion macerates: flavour implications

• Published in: Journal of the science of food and agriculture 1998-11, Vol.78 (3), p.367-372
• Main Authors: Lancaster, Jane E, Shaw, Martin L, Randle, William M
• Format: Article
• Language: English
• Subjects: alk(en)yl cysteine sulphoxides; alliinase; Allium cepa; Biological and medical sciences; flavour; Food industries; Fruit and vegetable industries; Fundamental and applied biological sciences. Psychology; hydrolysis; pyruvate; γ-glutamyl peptides
• ISSN: 0022-5142; 1097-0010
• DOI: 10.1002/(SICI)1097-0010(199811)78:3<367::AID-JSFA127>3.0.CO;2-N

Intact cells of Allium spp have no odour, but when cells are disrupted, the enzyme alliinase (EC 4.4.1.4) hydrolyses the S‐alk(en)yl sulphoxides (ACSOs) to produce pyruvate, ammonia and the many volatile sulphur compounds associated with flavour and odour. In onions, there are three main sulphoxides: methyl, propyl and 1‐propenyl, which gives rise to onion's tear‐producing effect. The relative proportions of these are a factor in determining the subsequent flavour. γ‐Glutamyl peptides, which are biosynthetic intermediates to ACSOs, are also present in significant amounts, but their contribution to flavour is not known. There is little work on the reaction of alliinase in vivo. The hydrolysis of ACSOs by alliinase was studied in rapidly macerated bulbs at intervals between 5 s and 2 h. ACSO and γ‐glutamyl peptide levels were measured by HPLC, and pyruvate levels were measured spectrophotometrically. Although the hydrolysis of propenyl cysteine sulphoxide (PrenCSO) was immediate and almost 100% between 5 and 20 s after bulb maceration, the hydrolysis of propyl cysteine sulphoxide (PCSO) and methyl cysteine sulphoxide (MCSO) was incomplete. After 5 s maceration, about 50% PCSO and MCSO remained and thereafter no further hydrolysis occurred. The levels of ACSOs and the extent to which they were hydrolysed were dependent on the cultivar and sulphur environment in which it was grown. The addition of pyridoxal phosphate cofactor enhanced the extent of MCSO and PCSO hydrolysis. Additional purified alliinase per se had no effect. Substantial hydrolysis of γ‐glutamyl peptides also occurred after maceration, and this was unexpected. Levels of pyruvate product were between 15 and 25% of the expected amount from ACSO hydrolysis assuming a stoichiometric relationship. The incomplete hydrolysis of MCSO and PCSO, enhancement of activity by additional pyridoxal phosphate and non‐stoichiometric production of pyruvate are evidenced that reaction inhibition of alliinase may be occurring in onion macerates. © 1998 Society of Chemical Industry.