One new pathway for Allium discoloration

(+)-S-(1-propenyl)-l-cysteine S-oxide (1-PeCSO) is a key compound associated with Allium discoloration. The pathway by which 1-PeCSO participates in Allium discoloration in the presence of alliinase has been established, but whether 1-PeCSO results in the discoloration in the absence of this enzyme remains unknown. In the present study, one new model reaction system containing 1-PeCSO alone in a phosphate buffer was established to simulate discoloration of white onion tissue and garlic puree. Results showed that the model system generated yellowish-green colour at pH 3.0 and red colour at pH 5.0. Two red species were isolated and identified by high-resolution multistage MS analysis, molecular formulae of which are C24H30N2O5S (red species 1) and C27H36N2O6S2 (red species 2). Both of them are sulphur-containing pyrrole derivatives and have UV/visible maximum absorbances at 520 (species 1) and 523 nm (species 2). The discoloration reactions occurring in this new system might represent a new pathway correlated with both onion reddening and garlic greening, emphasising that protein amino acids and alliinase are not necessary during the discoloration.