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

TLDR

Enzymic oxidation during grape crushing and must preparation leads to major losses of caftaric and coutaric acids, converting them into a specific HPLC‑quantifiable product that is also formed when glutathione, caftaric (or coutaric) acid, polyphenoloxidase, and oxygen react in the presence of sulfhydryl compounds. The study demonstrates that crushed grapes contain ~160 mg L⁻¹ free glutathione, which reacts with caftaric acid, polyphenoloxidase, and oxygen to form S‑glutathionyl caftaric acid—a non‑brown, 325‑nm absorbing product that is not a polyphenoloxidase substrate, limits must browning, and persists into commercial wines, contributing to their nonflavonoid phenol content and oxidation.

Abstract

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.