Caftaric Acid Disappearance and Conversion to Products of Enzymic Oxidation in Grape Must and Wine

Enzymic oxidation during crushing and must preparation causes major losses of caftaric and coutaric acids in large part by conversion to a single specific reaction product quantifiable by HPLC. Similar reaction products are produced by oxidation in the presence of cysteine and other sulfhydryl compounds, and the identical one is produced by the interaction of the four components: glutathione, caftaric (or coutaric) acid, active polyphenoloxidase, and oxygen. This is believed to be the first proof of rather high levels (typically about 160 mg/L) of free glutathione in crushed grapes. While it is a product of oxidation, S-glutathionyl caftaric acid is itself oxidizable, but is not a substrate for grape polyphenoloxidase. It is not brown, but has an absorption maximum at about 325 nm sufficiently similar to that of caftaric acid that it appears satisfactorily quantitated by the same relative extinction. Its production is important in limiting the browning of musts. It survives into commercial wines and is important in explaining the nonflavonoid phenol content of wines and the oxidation of white wines.