Phenolic composition of grape stems

Crape stems contain significant amounts of polyphenolic compounds, especial ly phenolic acids, flavonols, and flavanonols such as astilbin. The tannin content was characterized after the depolymerization reaction thiolysis. Ta nnins consisted of polymeric proanthocyanidins (up to 27 units) mainly cons isting of (-)-epicatechin units along with smaller amounts of (+)-catechin, (-)epicatechin gallate, and (-)-epigallocatechin. Flavanonols (astilbin) h ave been identified for the first time in stem and characterized by LC/MS a nd NMR, All phenolic compounds in grape stems were quantified by HPLC: quer cetin 3-glucuronide was the most important, followed by catechin, caffeoylt artaric acid, and dihydroquercetin 3-rhamnoside (astilbin), Comparison was made of proanthocyanidin characteristics in different while and red grape v arieties and also among parts of the cluster (skin, seed, and stem). Stem-c ondensed tannins were qualitatively intermediate between seed and skin but could not be differentiated between red and white varieties.