Stability of polyphenolic extracts from red grape skins after thermal treatments

Considering the potential of applying grape skin extract (GSE) as functional ingredients in products that must be thermally processed, the aim of this study was to evaluate the stability of individual polyphenols in grape skin extracts submitted to heating at different temperatures. Polyphenolic compounds were extracted by applying 20% aqueous ethanol containing 1% acetic acid for 1 h at 50 °C on a magnetic stirrer. The obtained extract was divided into aliquots and submitted to heating at different temperatures for a various time. The content of individual phenolic compounds in all incubated extract was determined by the HPLC method. All studied compounds were very stable during heating at 40 °C. Among analyzed anthocyanins, diglucosides were more stable than corresponding monoglucosides. Gallocatechin and procyanidins B1 and B2 contents were decreased, while catechin and epicatechin contents were increased during thermal treatments. At both incubation temperature contents of caftaric and coutaric acid were decreased, while in the same time contents of caffeic and coumaric acids were raised which could be due to hydrolysis. A significant increase of gallic, protocatechuic, vanillic, and syringic acid contents was due to thermal degradation of delphinidin-3-O- glucoside, cyanidin-3-O-glucoside, peonidin-3- O-glucoside, and malvidin-3-O-glucoside, respectively. trans-Piceid showed high stability toward thermal treatments.