The grape reaction product (GRP, GRP1 or 2-S-glutathionyl caftaric acid[1]) is a phenolic compound explaining the disappearance of caftaric acid from grape must during processing.

[3] Its enzymatic production by polyphenol oxidase is important in limiting the browning of musts,[4] especially in white wine production.

[5][6] Determining its concentration in wine is possible by mass spectrometry.

[8] It is not a substrate for grape polyphenol oxidase, but laccase from Botrytis cinerea can use it to form GRP2.

[9] Other related molecules are trans-caffeoyltartrate derivatives like GRP o-quinone[10] and 2,5-di-S-glutathionyl cafteoyl tartrate (GRP2)[11] or adducts with anthocyanidins.