Graded inclusions of sodium metabisulphite in sorghum-based diets: II. Modification of starch pasting properties in vitro and beneficial impacts on starch digestion dynamics in broiler chickens

Abstract

Seven sorghum-based broiler diets containing the reducing agent, sodium metabisulphite (SMBS) at levels of 0, 1.50, 2.25, 3.00, 3.75, 4.50 and 5.25g/kg were prepared. The in vitro starch pasting properties of the sorghum-based diets and starch extracted from the diets were assessed by rapid viscosity analysis (RVA). The seven dietary treatments were offered to 294 male Ross 308 chicks from 10 to 24 days post-hatch to determine SMBS effects on starch and nitrogen digestion dynamics. Digesta samples were collected, freeze-dried and weighed for determination of digestibility coefficients and mean retention times in the proximal jejunum, distal jejunum, proximal ileum and distal ileum. The digestion kinetics of starch and nitrogen (N) were determined using an exponential mathematical model to relate digestibility coefficients to mean retention times in each small intestinal segment. SMBS inclusions linearly reduced apparent starch digestibility coefficients in the proximal jejunum (r=−0.444, P<0.01), distal jejunum (r=−0.371, P=0.010) and predicted glycaemic responses (r=−0.474; P<0.001). SMBS linearly decreased (r=−0.413; P<0.01) starch disappearance from the proximal jejunum (abruptly digestible starch) and increased (r=0.483; P<0.001) starch disappearance from the three caudal small intestinal segments (gradually digestible starch). SMBS significantly (P<0.001) reduced final starch viscosity in diets from 2567 to 1935 cp and final extracted starch viscosity from 1035 to 441 cp. The SMBS-induced reductions in final viscosity of extracted starch were correlated with apparent metabolisable energy (AME) (r=−0.347; P<0.05), N-corrected AME (AMEn) (r=−0.376; P<0.01) and feed conversion ratio (FCR) (r=0.337; P<0.05). Also, gradually digestible starch was correlated (r=−0.291, P<0.05) with FCR improvements and abruptly digestible starch was negatively correlated (r=−0.282; P<0.05) with AMEn. It appears that the previously reported positive responses in AME, AMEn and FCR generated by SMBS stem from reductions in final starch viscosity that are probably attributable to SMBS-induced oxidative–reductive depolymerisation of starch polysaccharides. The small intestinal sites of starch digestion (and glucose absorption) were altered with higher levels of gradually digestible starch disappearing from the distal jejunum, proximal and distal ileum. Consideration is given to the possible mechanisms whereby SMBS generated more gradually digestible starch and how gradually digestible starch enhanced energy utilisation and efficiency of feed conversion.