Tannins antioxidant properties come from their hydroxyl groups
and more specifically from their catechol moieties (i.e. –OH groups
onortho positions on aromatic rings). Grape seed tannins are galloylated
and contain more catechol units than the apple ones, which
may explain that they are also more antioxidant. When oxidation
takes place, it does not lead to the destruction of catechol moieties
(Guyot, Vercauteren, &Cheynier, 1996) (at least not systematically),
explaining why oxidized apple tannins A15ox still have antioxidant
activity. This has practical implications, since tannin fractions
obtained from distilleries are rarely unoxidized. It is important that
they still have antioxidant properties.