Molecular characterisation of breakfast cereals and rice to understand their digestibility

Abstract

Grains and grain products remain the main source of energy for the majority of the world's population. These carbohydrate-rich foods also have a significant influence on digestive health which renders them ideal target foods for weight management and the prevention and management of obesity-related illnesses, especially type II diabetes and colorectal cancer. However, due to the complexity of their matrices, the mechanism behind the digestibility of grains and grain products is poorly understood. In this project, commonly consumed grain foods were analysed for sugar composition and starch structure to improve our understanding of their digestibility. A new free-solution capillary electrophoresis (FS-CE) method with direct ultraviolet (UV) detection had been applied by our research team to plant fibre and ethanol fermentation samples. In this thesis, the FS-CE method was applied to a complex matrix food sample: breakfast cereal (BC). Sucrose was detected in all BCs, while lactose, maltose, glucose and fructose were detected in some. The quantification for 11 BCs was grouped into three categories for total sugar content: low (for example, 'Weet-Bix'), medium (for example, 'Corn Flakes') and high (for example, 'Nutri-Grain'). For total sugar content, FS-CE was significantly more repeatable and reproducible than traditional colorimetric (Fehling and 3,5-dinitrosalicyclic acid) and enzymatic (glucose oxidase-peroxidase) methods, likely due to automation and lack of sample degradation before analysis. Currently, the gold standard method for individual sugar analysis in food products is high performance liquid chromatography (HPLC). However, FS-CE has a much lower running cost compared to HPLC and was shown to detect and quantify more sugars for most BCs (excluding those containing fruit). This is likely because the robust FS-CE method has minimal sample preparation, which only involves sample suspension in water, and therefore has a reduced risk of sample loss. At present, glycaemic index (GI) is the only accepted measure of the digestibility of foods by the food industry (allowed on food packaging) and is the only way consumers can predict whether a product will induce a rapid or slow release of energy upon consumption. However, the measurement of GI is very costly (both money- and time-wise) and is therefore highly ineffective as a selection tool for the screening of breeding lines or during product development. Moreover, the values are not statistically significant for the population as they are based on an average blood glucose response, with a minimum requirement of only 10 people. The FS-CE method was successfully applied to digesta of three breakfast cereals. Glucose measurements by FS-CE were shown to be significantly more repeatable than enzymatic methods (glucometry and glucose oxidase-peroxidase) used to monitor in vitro digestion. Moreover, FS-CE is able to produce digestograms of sugars other than glucose, which can contribute to increased understanding of the processes involved in by enzymatic degradation of grain foods. There is great potential for future research to use the FS-CE method for online in vitro digestibility studies. Starch digestibility of grain foods, whether measured by in vivo or in vitro methods, is related to the source and nature of starches. The structure of starch in rice flour samples (3 varieties) was analysed and associated with digestibility using an international GI database. The amylose content and gelatinisation temperature (GT) was measured by the Department of Primary Industries (Yanco, NSW, Australia). Using quantitative 1H nuclear magnetic resonance (NMR) spectroscopy the degree of branching (DB) of the same rice flour samples was determined with 2-3 % precision. Many studies have found a link between high-amylose rice starches and slower digestibility. Whether grown at high or low temperatures, the samples of one variety had similar amylose content, DB and GT. The consistent structural properties suggest that this variety would be able to maintain a low GI value across different climates, which meets the criteria for which it was designed. For other 2 varieties (varieties B and C) grown at high temperatures, decreased DB was linked with increased amylose content and increased GT. As these varieties were also associated with a higher GI value, DB was determined to be a useful tool in the prediction of future digestibility profiles of particular rice varieties grown in a specific environment. To improve our understanding of starch digestibility the analysis of molecular and supramolecular structure (chain length distribution, single and double helices, crystalline order and lamellar spacing) should be included in future research. Useful methods for analysis of supramolecular structure include solid-state NMR spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), and small-angle-X-ray scattering (SAXS). In conclusion, the quantification of sugars by FS-CE was successfully applied to BCs and proof-of-concept was established for monitoring in vitro digestion. The DB of 3 varieties of rice flours was measured by 1H NMR and had associations with amylose content and GT. Thus, alongside in vitro digestion, the key to understanding digestibility of grain foods and their effects on health will require a multi-method approach to characterising starch structure.

Date of Award	2015
Original language	English