Both the amount and source of dietary carbohydrates have important physiological roles in human health and disease. However, characterisations based solely on chemical attributes do not adequately describe the physiologic fate of carbohydrates. The glycemic index (GI) has been proposed as a measure of carbohydrate quality, but does not necessarily distinguish between different types of low GI foods that have different nutritional properties. There is a need to establish a better description of the physiological attributes of carbohydrates, which is the main focus of this thesis. This thesis describes a physicochemical characterisation of carbohydrates based on their chemical composition and likely gastrointestinal fate. Modifications were made to existing methodology for the measurement of rapidly (RAG) and slowly (SAG) available glucose, and resistant starch. Using this method, with improved precision and quality control, a database of physicochemical carbohydrate fraction profiles for 189 foods was established. The content of the SAG fraction, a marker of slow release carbohydrates, ranged from <5% in potatoes, bakery products and breakfast cereals to >35% in some legumes, pasta and whole kernel cereals. The superiority of the RAG and SAG measures over conventional carbohydrate classification schemes in explaining glycemic responses was demonstrated in two studies. In one study (n = 23 cereal products) the RAGS:SAG model explained 69% of the variation in GI values for a selection of cereal products, which was considerably greater than other models (e.g. 34% for the starch and sugar model). The other study, incorporating two carbohydrate portion sizes, found that the RAG fraction explained 70% of the within subject variation in the glycemic responses to test meals, significantly greater than the 42% explained by the total carbohydrate intake. This study also demonstrated that the magnitude of the glycemic response was largely determined by RAG intake, rather than SAG intake which had a minimal effect. It appears that SAG is a marker of slow release carbohydrates that do not elevate the glycemic potential of foods. An epidemiological study (n=684) was used to establish the intake of the physicochemical carbohydrate fractions: RAG, 62%; SAG, 4%; RS, 2%; fructose, 21%; other (mainly lactose), 11%. Although the average intake of SAG was small, it had a 13 fold intake range (1.5-19.5g/day) indicating that it could be utilised in identifying dietary patterns. Indeed, it was shown that the physiochemical carbohydrate fractions were significant determinants of dietary GI and glycemic load values. The initial investigation of relationships with metabolic markers for disease risk found that correlations were generally poor, both for the physicochemical carbohydrate fractions and for other nutrients with previously established associations. An extended study adjusting for confounding factors is warranted. In conclusion, the RAG and SAG measures provide a new perspective on the physiological role of carbohydrates, which can be applied to metabolic and epidemiological studies investigating the complex role of carbohydrate quality in nutrition.

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The aim of this study was to use a low commercial value by-product of the oil palm industry as an oligosaccharide source, identifying in the process those molecular weights that would have the highest prebiotic efficacy. Oil palm empty fruit bunches (OPEFB) fibre were subjected to autohydrolysis to obtain oligosaccharide-rich liquor. The processed liquor was characterised to determine the operational conditions required to produce the highest xylooligosaccharide (XOS) concentration. Two different purification techniques were employed to remove the monosaccharides and non-saccharide compounds present in the liquor: gel filtration chromatography (GFC) and sequential charcoal adsorption and ultrafiltration. With GFC purification, six mixed XOS fractions with different average degree of polymer is at ion (avDP 4, 7,14,28,44,64) were obtained. The effect of avDP on the human faecal microbiota was determined in vitro, in pH controlled batch cultures using XOS as the main carbohydrate source. The most favourable fermentation profiles were observed with the lower avDP fractions (4, 7, and 14) which showed the highest selectivity for bifidobacteria. A broader DP distribution of XOS fraction with an avDP 9, obtained through ultrafiltration, was further tested in a three stage continuous culture system. The findings suggest that XOS is a potential prebiotic that combines the benefits of increasing health-associated bacteria in the vessel 3 of the model system that simulates the distal colon and reducing proteolysis therein with butyrate production, a metabolite that plays a central role in the regulation of colonic cell proliferation, differentiation and modulation of gene expression.

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β-lactoglobulin (βLg) is an important source of angiotensin I-converting enzyme (ACE) inhibitory peptides. These peptides have the potential to become ingredients in future functional foods, however appropriate technologies are needed for their cost-effective industrial production. Thus, this work was carried out to investigate the feasibility of an ion exchange process for the production of potent βLg-derived ACE inhibitors using Protease N Amano, a commercial food-grade proteolytic preparation. βLg was hydrolysed in solution and in situ using Protease N Amano and optimal hydrolysis conditions.

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