Polyphenolic content and in vitro antioxidant characteristics of wine industry and other agri-food solid waste extracts

Solid by-products from white and red wine industry were subjected to evaluation as potential sources of antioxidant phytochemicals on the basis of their content in phenolics and in vitro antioxidant activity. Furthermore, several other common plant solid wastes, including apple, potato and onion peels, as well as carob pods and olive tree leaves were also considered, in order to carry out a comparative assessment. The results showed that extracts from grape seeds (either white or red) contain exceptionally high amounts of total polyphenols (10.3–11.1% on a dry weight basis), a great part of which is composed of flavanols. Red grape pomace and stems contained appreciable amounts of polyphenols, whereas potato and white grape peels were the tissues with the lowest polyphenol content. The in vitro antiradical activity and reducing power were shown to be highly dependent on the total flavonoid and total flavanol content (P<0.001)(P<0.001), but the hydroxyl free radical scavenging activity did not exhibit the same trend, suggesting dependence on particular structural features. The results indicate that wine industry by-products, including grape seeds but also red grape pomace and stems, are very rich sources of antioxidant polyphenols compared with other agri-food solid wastes, and therefore their exploitation as a source of added-value products may be more cost-effective and merits a profounder investigation.