Starch digestion kinetics in pigs : The impact of starch structure, food processing, and digesta passage behaviour

Abstract

The nutritional and energetic value of a pig's diet depend on the rate of starch digestion. At the moment, however, the reasons behind variation in in vivo digestion rates of different starches are not fully understood. The main aim of this thesis was to quantify the contribution of intrinsic starch structure, feed processing, and digesta passage behaviour on the kinetics and mechanisms of starch digestion in the upper gastrointestinal tract (GIT) of pigs. The relation between intrinsic starch properties and in vitro digestion kinetics was studied in a large set of starches from various botanic origins. Across botanic sources, increased concentrations of A-type crystalline structure and short amylopectin side-chains increased hydrolysis rate. Within botanic sources, additional variation in in vitro hydrolysis kinetics was explained by other properties, such as the amylose content and the number of pores. Based on this in vitro work, three starch sources were selected (barley, maize, high amylose maize) and included each in three forms (isolated starch, ground cereals, extruded cereals) in experimental pig diets. Starch hydrolysis and disappearance was measured in the stomach and several parts of the small intestine, in addition to the rheological and physical behaviour and mean retention time (MRT) of digesta. Combining those results, an in vivo starch hydrolysis rate was determined for each diet. Consistent with our in vitro findings, the hydrolysis rate of starch in pigs was increased by extrusion and a decreased amylose content of maize starches. Starch originating from ground barley was fully hydrolysed in pigs, whereas 16% of starch ingested as ground maize was resistant to digestion. Starch hydrolysis in the proximal small intestine was underestimated by our in vitro method (by 20% on average), whereas the amount of starch resistant to hydrolysis exceeded our in vitro predictions (by 9% on average). Consequently, glucose release from slowly digestible starch was less gradual than expected. Gastric bacteria were found to degrade granular starch in the stomach of pigs. Bacterial enzymes, extracted from stomach digesta, hydrolysed up to 29% of starch in a dynamic in vitro stomach model with a step-wise pH gradient from 6.5 to 2.0. Porcine salivary α-amylase, which has an optimum pH around 7.8, degraded 10% of gelatinized starch under these in vitro stomach conditions, but barely degraded any native starch. The rate at which glucose, originating from starch, appears in the portal circulation does not only depend on the starch hydrolysis rate, but also on the transit time through the upper GIT. The MRT of digesta solids in the stomach of continuously fed pigs was longer (129 to 225 min) than in the small intestine (86 to 124 min). In addition, liquids remained around 60 min shorter in the stomach than digesta solids. Consequently, retention in the stomach will largely affect the appearance rate of glucose in the blood. The MRT in the stomach depended, in turn, mostly on the amount of water in stomach digesta as fraction of the theoretical maximum held by the digesta matrix. In conclusion, the difference between the in vitro and in vivo situation is dominated by the initial rate of starch digestion, which was higher in vivo than in vitro. Gastric starch digestion and predigestion seem to contribute to the more rapid initial starch digestion in vivo and is a key factor in an accurate prediction of starch digestion rates.