Complexation of starch and phenolic compounds induced by extrusion and its impacts on in vitro digestion of extruded noodles : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Diabetes ranks as the first of the world's top ten chronic diseases. Excessive consumption of rapidly digestible starch can lead to postprandial hyperglycaemia. Lowering the digestion properties of starchy foods is a non-negligible aspect of reducing the risk of Type 2 diabetes mellitus. Pure phenolic compounds or phenolic extracts, as functional food ingredients, can reduce the glycaemic response through forming starch-phenolic complexes or reducing digestion enzyme activity, which is interfered with by natural aspects of phenolic compounds ((i.e. available concentration, structure), food processing conditions, and food models. In extruded starchy noodles enriched in phenolics, there is limited evidence of the contributions of phenolics to the digestion of extruded starch. The intensity and manner of starch-phenolic complexation through extrusion are to study yet. Dietary phenolic compounds must be released from the food matrix in the gastrointestinal tract to play the enzyme indicators, the release of which is affected by food structure. However, the relationship between the release of phenolics from extruded noodles and the complexation behaviours remains lacking. In this thesis, the phenolic extract from the buckwheat hull was selected as the functional ingredient incorporated into extruded noodles. Extruded noodles supplemented with starch and various concentrations of phenolic extracts (0.5-2.0 %) were prepared to investigate the inhibitory effects of phenolics on the digestion of noodles. Cold and hot extrusion were utilized to achieve the starch-phenolic complexation with different intensities and manners, which were characterised by the complexation index, thermal properties, crystalline structure, status of functional groups, and chain length distributions. The relationship of starch-phenolic complexes through an extrusion and in vitro starch digestion of extruded noodles was established. The in vitro inhibitory activities of phenolics on digestive enzymes were conducted to evaluate the roles of unbound in the digestion properties of noodles. To be specific, rutin is the dominant profile in the buckwheat hull extract and extruded starchy noodles enriched with rutin were prepared to evaluate the inhibitory effects of unbound rutin and starch-rutin complexes on starch digestion. Cold extruded noodles enriched with 2.0% phenolic extract prepared using a pasta extruder showed a significantly inhibited release of reducing sugar (p < 0.05), attributed to the phenolics being released during gastric digestion decreasing the activity of enzymes such as α-amylase (0.102 mg/mL of IC50). The existing forms of phenolics interfered with their release from extruded noodles during simulated in vitro gastrointestinal digestion. Cold and hot extruded noodles enriched with phenolic extract (2.0%) were prepared using a twin screw extruder to give different forms of phenolics (unbound and bound). Following the simulated in vitro digestion, unbound phenolics were released mainly in the gastric phase and the first 20 min in the small intestinal phase, and bound phenolics were released more slowly and mainly in the simulated small intestinal phase. More importantly, the V-type inclusion complexes may be retained in the large intestinal tract. Cold and hot extruded phenolic-enhanced noodles consisted of the physically wrapped mixtures, the non-inclusion complexes, and the V-type inclusion complexes between starch and phenolics. Compared with cold extrusion, hot extrusion induced phenolics to form complexes with gelatinised or degraded starch to a larger extent, particularly the V-type inclusion complexes. This is because hot extrusion caused the gelatinisation and degradation of amylopectin crystallites, along with the disrupted crystallites and orderliness, to a higher level than cold extrusion. Such molecular changes may aid phenolic molecules to penetrate the starch matrix more easily and some phenolics complex with starch, especially amylose (500 < X < 5000), thereby improving the formation of V-type crystalline structure. Starch-phenolic complexes, especially the V-type inclusion complexes, and structural alteration, such as the increased V-type crystallites, contributed to the slower hydrolysis of starch. Rutin was the most abundant phenolic profile in the digesta of extruded noodles. A higher intensity of complexation between starch and rutin, and more V-type inclusion complexes through hot extrusion were responsible for the inhibited starch digestion. Released rutin during simulated digestion hardly inhibited starch digestion, possibly owing to the relatively low concentration in digestion liquids (<0.014 mg/mL). Considering the inhibition of released phenolics against starch digestion, other phenolic profiles instead of rutin may be the main inhibitors of digestive enzymes. This thesis provides strategies for developing hot extruded buckwheat noodles with a reduced glycaemic index from the view of starch-phenolic complexes and controlled release of phenolics.