Grape Marc Skin Valorization: from Waste to Valuable Polyphenol Source

In Tunisia, wine industry is unavoidably associated with a huge production of pomace (grape stalks and berry skin, pulp and seeds) that represents an environmental threat due to lacking sustainable management strategies. Even though, within pomace components, berry skins could represent a source of bioactive compounds such as phenolics, if optimally processed. In this context, grape marc skins (GMS) of 5 grapevine ( Vitis vinifera ) cultivars (Chardonnay, Muscat Alexandria, Verdejo, Syrah and Carignan) were collected and skin samples were separated and dried at 40 °C, 45 °C and 50 °C prior to phenolics (total polyphenols -TP, total flavonoids-TF and condensed tannins-CT) characterization using spectrophotometric, spectroscopic (FTIR) and chromatographic (UPLC) analyses. Significant differences in TP, TF and CT contents were observed between cultivars and temperatures. The highest contents of GMS phenolics were registered at 40 °C in all cultivars. On the other hand, UPLC results allowed the identification of syringic and caffeic acids, quercetin (flavonoid) and resveratrol (stilben) in studied GMS. Significantly higher contents of such phenolic compounds were observed in methanol extracts of skins processed at 40 °C than above temperatures. Besides, FTIR analysis of GMS samples dried at 40 °C, allowed to identify the functional groups specific to the investigated polyphenols. These results emphasize the importance of berry skin processing temperature (40 °C) for an optimal recovery of valuable phenolics such as syringic and caffeic acids, quercetin and resveratrol. Within a context of circular economy, grape marc skins could turn to a low-cost and a rich source of phenolics for food and pharmaceutical industries rather than be considered as an environmental threat.