Relating starch structure in breakfast cereals and rice to their digestibility

Abstract

Grain based food products are rich in the complex carbohydrate starch and represent a major source of dietary energy for the majority of the world's population. These products play a significant role in digestive health, making them ideal targets for weight management and the prevention and management of obesity related illnesses such as type 2 diabetes and cardiovascular disease. Understanding the digestibility of grain products and improving their dietary quality is important for improving dietary management. However, the complex matrices of these products have resulted in poorly understood digestibility mechanisms. This project aims to characterise starch structure in grain foods to improve our understanding of their digestibility. In this work starches were employed as model samples with which to set up and optimise characterisation methods. Rice flour samples were then characterised. Rice has a relatively simple composition (90% starch) allowing for a less complex model with which to understand the relationship between starch structure and functional properties of starch such as digestibility. Breakfast cereals were also explored, allowing for an investigation into the characterisation of starch in a more complex sample matrix. Starch has multiple hierarchical structural levels on both the molecular and supramolecular level that need to be understood to comprehend the various models for starch digestibility. This has involved the mastering of a variety of techniques to characterise different structural levels of starch. Free solution capillary electrophoresis (CE) was employed in characterising the molecular of starch structure through the determination of amylose content and the investigation of heterogeneity of branching. The supramolecular structure of starch must also be characterised, with supramolecular arrangements introducing steric factors to digestibility. Both short- and long-range crystallinity were explored by a variety of techniques. Short-range order was investigated by Fourier-transform infrared (FTIR) spectroscopy, while long-range order was explored using X-ray diffraction (XRD) and small angle X-ray scattering (SAXS). CE is a powerful separation technique that has been shown to be useful for the separation of amylose and amylopectin, the two macromolecular components of starch, by taking advantage of iodine binding and visible light detection. The amylose content of starch has been linked with digestive properties; therefore, accurate characterisation is an important step in understanding digestive properties. Separation approaches overcome the interference caused by overlapping absorbance bands in traditional approaches, allowing for a more accurate quantification. Previous applications of the CE method in the literature had employed non-ideal dissolution conditions, introducing inaccuracy into analysis. In this thesis the incorporation of improved dissolution conditions in the methodology was explored. A reduced sample concentration was found to be ideal, reducing aggregation. The use of anhydrous DMSO with the addition of a hydrogen bond disruptor, as well as high dissolution temperatures were also determined to be essential in obtaining a complete dissolution thus allowing for accurate characterisation. Using a new methodology developed by our research team, the heterogeneity of branching in starch was also explored by CE, assessing the broadness of resulting electrophoretic mobility distributions through the value of their dispersity. Results using this methodology indicated an extremely high degree of heterogeneity of branching in starch. Further optimisation of the separation method will allow the heterogeneity of amylose and amylopectin to be investigated independently. The determination of short-range order by FTIR spectroscopy is based on the ratio of infrared bands; however, the assignments of these bands are poorly understood. In this work, the improvement of spectral resolution, and the correlation of crystalline index values with published crystallinity values were explored. Transmission mode FTIR spectra measured at cryogenic temperatures yielded the best resolution of the peaks of interest with deconvolution also yielding slight improvements to spectral resolution. The influence of improved spectral resolution on the determined crystalline index was highly variable. Crystalline index values had a loose correlation with published crystallinity values in some cases; however, as a measure of crystallinity, this method was deemed to be suitable in cases where only an estimate is required. The determination of long-range order by XRD was also explored, investigating the influence of data processing and software packages in peak fitting approaches for crystallinity determinations. The different algorithms that the software packages employed impacted on the peak fitting, baseline fitting and resulting crystallinity determinations. SAXS has proven to be a valuable tool in the analysis of lamellar structures in starch. The relation between lamellar structure of maize starches and amylose content was explored. A decrease in the relative amount and size of semi-crystalline structure was observed with increasing amylose content. This trend is similar to those found in the literature in other plant starches. The same trend between varying amylose content and semi-crystalline structural features was also observed in rice flour for the first time. Degree of branching (as measured by another member of our research team) was also investigated in relation to other structural features. Degree of branching showed a trend with varying amylose content as well as a relationship with semi-crystalline structure. These relationships are expected to rely heavily on the amylose content itself and also on the branching structure of the amylases present. In conclusion, the starch structure is complex and every structural level influences digestive properties. A number of characterisation methods were explored in this work, aiming to develop a set of tools that may be used in understanding how starch structure at different levels relates to digestibility. Identifying the links between starch structure and digestibility creates opportunities for applications within the food industry to alter the digestion rates of foods and food ingredients. These relationships may then be used in more efficiently producing healthier food products to combat the issue of rising obesity rates and associated illnesses.

Date of Award	2016
Original language	English