EVIDENCE FOR PROTEIN-TANNIN INTERACTION IN LEGUMES - IMPLICATIONS IN THE ANTIOXIDANT PROPERTIES OF FABA BEAN TANNINS

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 re activity and extent of proteolysis. In vitro protein digestibility and cystine reactivity (susceptibility to reduction) were found to be sig nificantly increased in the tannin-free compared to the tannin-contain ing variety Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of faba bean and lentil proteins before and after proteolytic digestio n, showed that proteins in the tannin-free variety underwent better di gestion than those in the tannin-containing variety of the same specie s. This suggested that interaction between tannin and protein impaired the susceptibility of seed proteins to proteolysis. Preliminary evalu ation of the antioxidant properties of faba bean tannins indicated tha t the antioxidant activity found is accounted for by the direct intera ction of tannin with hydroxyl radical rather than to a metal chelating activity. On the basis of these data it is possible to hypothesise th at, in spite of the known adverse action on protein digestibility, leg ume seed tannins might exert a beneficial nutritional antioxidant acti vity. (C) 1996 Academic Press Limited