A thesis submitted to the Faculty or Science, University of the Witwatersrand Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. / Two carbohydrates predominate in modern diets namely sucrose and starch. They are metabolized by oral microorganisms to products implicated in dental caries. The influence of sucrose and its metabolites is well documented but starches have received less attention. This study investigated the secretion of starch hydrolysing enzymes by oral streptococci and the production of sugars, acids and polysaccharides from the two ccmponents of starch, amylose and amylopectin. ( Abbreviation abstract ) / AC2017

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The primary objectives of this study were to develop an effective reactive extrusion process to produce granular phosphorylated pea starches with enhanced enzyme resistance, and examine the effects of bulk phosphorylation conditions on the morphology, physicochemical and functional properties of extruded pea starch phosphates. Two types of commercially available pea starches (NutriPea and Meelunie) were chosen as the research subjects in this study with differing native resistance. A number of methods including optical microscopy, SEM, ICP-OES, Englyst method, DSC and rapid visco analysis (RVA) were used to characterize the morphology and properties of extruded pea starches. The effects of feed formulations and extrusion conditions on phosphorus incorporation and Englyst digestion profiles were examined systemically. The results showed that phosphorus content and digestion profiles were highly dependent on the feed moisture. Enzyme resistance did not positively correlate with phosphorus content for extruded pea starch phosphates in contrast to their counterparts prepared by conventional aqueous slurry. This was because extrusion processing can markedly increase the susceptibility of pea starch granules to enzymatic digestion. Lower feed moisture content (40%) gave lower phosphorus content, significantly lower RDS content, and higher SDS and/or RS content. Bulk phosphorylation in the extruder resulted in decreased RS2 content but increased RS4 content. Screw geometry with excessive mixing index was not desirable in terms of producing resistant starch. High screw speeds (150rpm and 200rpm) and low feed rate (1.02kg/h) brought about higher yields of SDS and RS in spite of lower phosphorus incorporation. Reactive extrusion of pea starches under optimized conditions achieved a significant but moderate increase either in RS content (from 18.67% to 22.57% for NutriPea) or in SDS content (from 37.18% to 42.23% for Meelunie) compared to their native counterparts. However, RS and SDS content could not be improved simultaneously at least based on these pea starches and the reactive extrusion processes in this study. The optical and SEM micrographs confirmed that the granule integrity was largely retained after optimized reactive extrusion process. DSC thermograms found no significant correlation between gelatinization characteristics and Englyst digestion profiles. Evidenced by RVA pasting profiles, NutriPea pea starch phosphates exhibited enhanced thermal and shear stability in comparison to their native counterpart. A novel foaming injection technology of cross-linking reagents solution was pioneeringly introduced to uniformly coat all starch particles at the lowest moisture level possible during the continuous production of granular NutriPea pea starch phosphates. Yet, the resulting phosphorus incorporation was much lower than expected and would require further studies. / Thesis / Master of Applied Science (MASc) / The Canadian food industry is increasingly interested in the potential to probe new avenues to produce enzyme-resistant food starches from pulses starches. Although extrusion cooking is commonly used for manufacturing cereals, snacks and other food products, no research has been reported on using an extruder to rapidly produce resistant pulse starches for functional food ingredients. This study aimed to develop an effective reactive extrusion process to produce phosphate cross-linked pea starches with enhanced enzyme resistance (i.e., increased slowly digestible starch (SDS) and resistant starch (RS) content ) based on an examination of the effects of reaction conditions on the properties of extrusion products. Two types of commercially available pea starches, NutriPea and Meelunie, were chosen as subjects of the research. The cross-linked pea starches under optimized conditions achieved a significant but moderate increase either in RS content (for NutriPea) or in SDS content (for Meelunie) compared to their native counterparts. However, RS and SDS content could not be improved simultaneously at least based on these pea starches and the reactive extrusion processes in this study.

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