The grape reaction product (GRP, GRP1 or 2-S-glutathionyl caftaric acid) is a phenolic compound explaining the disappearance of caftaric acid from grape must during processing.Caftaric acid in grapes and conversion to a reaction product during processing. V.L. Singleton, J. Zaya, E. Trousdale and M. Salgues, Vitis, 1984, pages 113-120 (article) It is also found in aged red wines. Its enzymatic production by polyphenol oxidase is important in limiting the browning of musts, especially in white wine production. The product can be recreated in model solutions.
Determining its concentration in wine is possible by mass spectrometry.Straightforward Method To Quantify GSH, GSSG, GRP, and Hydroxycinnamic Acids in Wines by UPLC-MRM-MS. Anna Vallverdú-Queralt, Arnaud Verbaere, Emmanuelle Meudec, Veronique Cheynier and Nicolas Sommerer, J. Agric. Food Chem. 2015, 63, 142−149,
S-Glutathionyl caftaric acid is itself oxidizable.Caftaric Acid Disappearance and Conversion to Products of Enzymic Oxidation in Grape Must and Wine. V. L. Singleton, M. Salgues, J. Zaya and E. Trousdale, Am. J. Enol. Vitic, 1985, volume 36, number 1, pages 50-56 'abstract) It is not a substrate for grape polyphenol oxidase, but laccase from Botrytis cinerea can use it to form GRP2.
Related molecules Other related molecules are trans-caffeoyltartrate derivatives like GRP o-quinone and 2,5-di-S-glutathionyl cafteoyl tartrate (GRP2) or adducts with anthocyanidins.
See also Phenolic compounds in wine
References