Evidence for protein-tannin interaction in legumes: Implications in the antioxidant properties of faba bean tannins

Abstract

The nutritional consequence of the absence of tannins in faba bean and lentil seeds was evaluated by comparing tannin-containing and tannin-free varieties for in vitro protein digestibility, cystine chemical reactivity and extent of proteolysis. In vitro protein digestibility and cystine reactivity (susceptibility to reduction) were found to be significantly increased in the tannin-free compared to the tannin-containing variety. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of faba bean and lentil proteins, before and after proteolytic digestion, showed that proteins in the tannin-free variety underwent better digestion than those in the tannin-containing variety of the same species. This suggested that interaction between tannin and protein impaired the susceptibility of seed proteins to proteolysis. Preliminary evaluation of the antioxidant properties of faba bean tannins indicated that the antioxidant activity found is accounted for by the direct interaction of tannin with hydroxyl radical rather than to a metal chelating activity. On the basis of these data it is possible to hypothesise that, in spite of the known adverse action on protein digestibility, legume seed tannins might exert a beneficial nutritional antioxidant activity. © 1996 Academic Press Limited.