The inhibitory effects of hull polysaccharides and tannins of field beans (Vicia faba L.) on the digestion of amino acids, starch and lipid and on digestive enzyme activities in young chicks

Abstract

The effects of polysaccharides and tannins present in the hulls of field beans (Vicia faba L.) on the digestion of amino acids, starch and lipid were studied in poultry. A control diet without hulls and the same diet substituted with 400 g hulls/kg diet from three different varieties of beans were fed to 3-week-old chicks for 4 d. Digestibility coefficients for amino acids, starch and lipid were calculated from measurements made of these nutrients in the diets and the freeze-dried excreta with the aid of titanium dioxide as a marker. Activities of trypsin (EC 3.4.21.4), α-amylase (EC 3.2.1.1), and lipase (EC 3.1.1.3) in digesta removed from the upper jejunum, sucrase (EC 3.2.1.48) in the gut mucosa from the upper jejunum, and α-amylase and lipase in the pancreas were measured. The hulls were analysed for their polysaccharide and tannin contents. Results showed that the hulls were mostly carbohydrate in composition, with cellulose the predominant polysaccharide. Tannins present in the hulls of two coloured-flowering varieties (Brunette and Minica) were of the condensed type. The diet with tannin-free hulls (white-flowering variety Medes) lowered slightly the digestion of amino acids, starch and lipid compared with the control diet. This effect was believed to be due to inhibition of digestive enzymes, possibly through their adsorption onto the hulls. Diets with tannin-rich hulls (varieties Brunette and Minica) caused a large reduction in the digestion of amino acids, starch and lipid compared with the control diet mainly due to inactivation of digestive enzymes by the formation of tannin–enzyme complexes in the digestive tract. Enzyme activities could be partially restored by the addition of polyvinylpyrrolidone to the digesta. Tannins inactivated trypsin the most, α-amylase to a lesser extent and lipase the least and as a consequence lowered the digestion of amino acids the most, starch to a lesser extent and lipid the least. Tannins did not induce an increased pancreatic production of digestive enzymes, nor did they affect activity of jejunum mucosal sucrase. Condensed tannins from Brunette and Minica hulls were partially extractable in methanol alone, but required acidic methanol for fuller extraction. The vanillin: anthocyanidin ratio suggested that tannins were polymerized to the same degree in the Brunette and Minica varieties, both in the methanol and acidic methanol extracts. Hulls from the variety Minica contained a greater amount of methanol-extractable tannins, the quantity of remaining tannins that required acidic methanol for extraction being the same for both varieties.