Investigations on the conversion of onion aroma precursors S-alk(en)yl-L-cysteine sulphoxides in onion juice production

Onion flavour develops when the cells are disrupted, allowing the enzyme alliinase (EC 4.4.1.4) to act upon the aroma precursors S-alk (en)yl-L-cysteine sulphoxides (ACSOs). For maximum flavour release, optimal cell disintegration and complete enzymatic conversion of ACSOs are desired. To assess an industrial scale onion juice extraction in regard to the extent of cell disintegration and the rate of enzymatic ACSO conversion, samples from all mass flows (ie onion, macerate, juice and pomace) were subjected to ACSO analysis. Applying a modified HPLC method, which allows baseline separation of the aroma precursors, three ACSOs, namely methyl-, propyl- and 1-propenyl-L-cysteine sulphoxides (MCSO, PCSO and 1-PeCSO), were determined. Initial ACSO ratios changed in the course of increasing degree of comminution from 17:83 to 100:0 (MCSO/1-PeCSO). Only negligible amounts of PCSO were detected (<9.2 nmol g -1 fresh weight (fw)), contributing less than 0.2% to the total ACSO content. For evaluation of the disintegration efficiency a chart was established consisting of an abscissa representing a percentage scale of ruptured cells and ordinates for the development of the marker parameters ACSOs and reaction product pyruvate relative to their initial contents. From this correlation a cell disintegration percentage after grinding of about 70% was deduced according to the determined ACSO contents. On a molar basis, macerate, juice and pomace showed 15, 0.6-2.4 and 0.7-1.0% of initial ACSO contents respectively. Only trace amounts of 1-PeCSO were found in the juice ((less than)16 nmol g^-1 fw). MCSO was converted to almost 90%, indicating that enzymatic transformation, in a practical sense, goes to completion.