Oxidation of Grape Juice 2‐S‐Glutathionyl Caffeoyl Tartaric Acid by Botrytis cinerea Laccase and Characterization of a New Substance: 2,5‐di‐S‐Glutathionyl Caffeoyl Tartaric Acid

Abstract

ABSTRACT The evolution of caftaric acid derivatives in grape juice samples exposed to air was monitored by high performance liquid chromatography. 2‐S‐glutathionyl caftaric acid was shown to be oxidized by laccase from Botrytis cinerea but it was not a substrate for grape polyphenoloxidase. In the presence of glutathione, 2,‐5‐di‐S‐glutationyl caftaric acid was formed; this was identified by proton NMR. Its production involved laccase oxidation of 2‐S‐glutathionyl caftaric acid into the corresponding orthoquinone and substitution of the latter by glutathione. When no glutathione was available, polymerization of the quinones led to browning of the juice.