Characterization of Dietary Fiber and the In Vitro Indigestible Fraction of Grape Pomace

Abstract

Grape pomace red grape skins (RGS), and white grape skins (WGS), and seeds (WGSe) were analyzed and the chemical composition of their in vitro indigestible fraction characterized. All samples were rich in protein (12%-14% dry matter), fat (7%-12% dm), and minerals (6%-9% dm), with smaller amounts of soluble sugars (about 3% dm) and polyphenols (4-5% dm). Analysis of the indigestible fraction showed that about 17% to 21% of the dry matter were non-starch polysaccharides (NSP), mainly cellulose and pectins. Condensed tannins (CT) content was very high, ranging from 16.0% in WGSe to 26.9% in RGS. A very high percentage of the protein (up to 80%) was also indigestible in vitro, appearing as resistant protein (RP) in the fiber residue. In conclusion, over 60% of the grape pomace dry matter was indigestible in vitro. This indigestible fraction was composed of dietary fiber (NSP plus lignin) as well as CT and RP. The presence of high amounts of CT and RP, unusual in most of the dietary fiber-rich vegetable products, would provide grape pomace with peculiar physiological and nutritional properties.