Tannin-containing faba beans ( Vicia faba L.): effects of methods of processing on ileal digestibility of protein and starch for growing pigs

Abstract

Studies were carried out with pigs (35 kg liveweight) fed diets containing 30% raw or processed whole faba beans. The processing methods studied were dehulling, dehulling followed by steam processing and reconstitution/extrusion of beans. The effects of treatments on the in vivo apparent ileal and faecal digestibility of dry matter (DM), nitrogen and starch were investigated. Dehulling improved the apparent ileal digestibility of DM and nitrogen of the beans to about 8% of an absolute level of approximately 0.75, while the digestibility of starch was not improved. These results indicate a depressive effect of faba bean cell wall related material and/or condensed tannins. Steam processing of bean cotyledons at 100 °C failed to improve the ileal digestibility of protein. Obviously, proteinaceous factors such as trypsin inhibitors and lectin proteins present in cotyledons only play a minor role in depressing the digestion unless residual levels still have antinutritional effects and are eliminated only at much higher temperatures. Reconstitution followed by extrusion decreased the ileal digestibility of nitrogen to a large extent. The faecal digestibility of DM was significantly higher for beans which were dehulled and steam-heated, but was lower for reconstituted/extruded beans. The faecal digestibilities of nitrogen starch were not affected by the different processing methods. The results of the present study indicate that, from a protein nutritional point of view, dehulling is an interesting treatment for improvement of the digestibility of faba beans. The economic feasibility associated with the dehulling process is discussed.