The importance of amylase action in the porcine stomach to starch digestion kinetics

Starch digestion in the proximal small intestine of pigs exceeds in vitro predictions, suggesting a currently underestimated role for the stomach in starch digestion kinetics. This study aimed to investigate the role of amylase action in the porcine stomach on starch digestion kinetics, including hydrolysis by porcine saliva and degradation by bacterial enzymes present in the stomach. We studied the hydrolysis of starch in pigs fed barley based diets, in which starch was included as isolated powder, ground barley, or extruded barley. We identified soluble maltodextrins originating from starch breakdown in stomach digesta, especially in pigs fed extruded barley. Furthermore, we observed bacterial cells embedded in granular starch with electron microscopy, for pigs fed diets containing isolated barley starch. These observations lead us to measure starch hydrolysis in a dynamic stomach model over the course of a 225 min incubation, in which the pH was step-wise decreased from 6.5 to 2.0. Using this method, feed was either exposed to an enzyme extract obtained from stomach digesta or to porcine saliva. Up to 30 % of starch was hydrolysed into maltodextrins with a degree of polymerisation (DP) < 6 when starch was incubated with the enzyme extract obtained from the stomach. Under the same conditions, saliva amylase, with an optimum pH around 7.8, hydrolysed up to 10 % of processed starch into maltodextrins with DP < 6. We conclude that a substantial part of starch may be degraded into oligomers in the porcine stomach by both salivary and bacterial amylases. This implies a considerable role for the stomach on starch digestion kinetics, which is overlooked in current feed evaluation systems.