The use of synthetic (±)‐ s ‐1‐propyl‐l‐cysteine sulphoxide and of alliinase preparations in studies of flavour changes resulting from processing of onion ( allium cepa l.)

Large losses of onion flavour components result from certain post‐harvest processes such as cooking by boiling, dehydration, freezing etc. The mechanisms by which these losses occur have been investigated on the basis of the alliinase‐precursor system and shown to include: (a) complete or partial enzyme destruction; (b) partial non‐enzymic destruction of precursors; and (c) partial enzymic hydrolysis of precursors with loss of volatile reaction products. A stable, freeze‐dried alliinase preparation and synthetic (±)‐ S ‐1‐propyl‐L‐cysteine sulphoxide ((±)‐PCSO) were prepared by known methods. Addition of an excess of enzyme to the processed product under optimal conditions was used in the determination of residual native substrate in the processed products and of an excess of the synthetic substrate for assay of enzymic activity. As compared with fresh onion as 100, enzymic activities of the processed products were as follows: freeze‐dried 45, laboratory‐frozen 18, hot‐air dried 8.8–10, commercially‐frozen 6.6, boiled 5.2 and pickled 0.01. Non‐enzymic destruction of substrate (precursor) was greatest in the hot‐air dried product ( ca 80%) and least in freeze‐dried onion (2.0%) Model experiments based on the alliinase preparation, (±)‐PCSO and cis‐S ‐1‐propenyl‐L‐cysteine sulphoxide (the major component of the native onion precursors is the trans ‐isomer) exaggerated destruction of enzyme at 60 and 100 °C as compared with the relevant processed products, yet underestimated the substrate losses at these temperatures. It was concluded that factors other than those embodied in the model conditions were also involved.