Rice (Oryza sativa L.) is one of the leading food crops in the world and the staple food for more than half the world’s population. Starch, as the main component in rice, contributes to approximately 20 – 50 % of daily human food energy. In some regions of Asia, up to 71 % of the daily energy comes from rice. As the staple carbohydrate in most diets, digestibility studies of rice are of particular interest due to an increasing concern on health complications that are often associated with long-term consumption of high-starch food. Diabetes, cardiovascular diseases, obesity, colo-rectal cancers and other problems related to over-nutrition, rather than inadequate nutrition, have become epidemic in developing countries, as well as in fully developed ones. Therefore, studies on starch digestibility have been extended to include health promotion and disease prevention.
Differences in starch digestibility have been ascribed to a variety of factors, including starch structure. To date however, only limited studies on the relationship between starch structure and digestibility properties have been reported, especially on real food model systems. The relationship between starch molecular architecture and the degree of starch digestibility needs to be investigated to further elucidate the relationship between structure and digestibility properties. In order to accurately characterize starch structure, an optimized method for isolation and dissolution of starch was developed. Currently available methods either inadequately removed all non-starch components or caused structural damage during the isolation and dissolution process. A complete dissolution without degradation that also removes non-starch components is important for the accurate characterisation of starch molecular structure. Current milling and dissolution methods have limitations, including incomplete dissolution, molecular degradation, and starch loss. An improved multi-step extraction/dissolution method was devised and tested, involving cryo-grinding, protease pre-treatment, dissolution in dimethyl sulfoxide solution containing 0.5 % (w/w) LiBr (DMSO/LiBr) at 80 °C, centrifugation, ethanol precipitation, and finally, re-dissolution in the DMSO/LiBr solution at 80 °C. Cryo-grinding and dissolution in DMSO/LiBr solution were found to have negligible effects on the size distribution of starch molecules measured by size-exclusion chromatography (SEC). The peaks of non-starch components were removed or separated from the amylose and amylopectin peaks in the SEC distributions. The amylopectin component had a larger hydrodynamic radius than that obtained by conventional wet-milling and also than that obtained without protease pre-treatment, suggesting that molecular degradation and aggregation were reduced with the new method. This new extraction/dissolution method allows a more accurate structural analysis of starch molecules from grains than conventional treatments.
A causally meaningful relationship was established between starch molecular structures (obtained by size-exclusion chromatography, proton NMR and multiple-angle laser light scattering) and digestibility of cooked rice grains (measured by in vitro digestion). Significant correlations are observed between starch digestion rate and molecular structural characteristics, including fine structures of the distributions of branch (chain) lengths in both amylose and amylopectin. The in vitro digestion rate seems to increases with longer amylose branches and smaller ratios of long amylopectin and long amylose branches to short amylopectin branches, although the statistical analyses show that further data are needed to establish this unambiguously. These new relationships between fine starch structures and digestibility of cooked rice grains are mechanistically reasonable, but suggestive rather than statistically definitive.
The time evolution of starch molecular structure during in vitro digestion of cooked rice grains with varying amylose contents was analysed to gain a mechanistic understanding of starch digestion and the origin of dextrins with slow digestion properties. Both amylose and amylopectin molecules were hydrolysed by digestive enzymes, forming dextrins with two peaks, at hydrodynamic radius Rh ~ 2.5 and ~ 5 nm, which have much smaller molecular sizes than native amylose and amylopectin (peak Rh ~ 25 and ~ 200 nm, respectively). The dextrins with a peak at Rh ~ 5 nm seem to contain few short branches, similar to the branch-chain length of amylopectin molecules. The dextrins with a peak at Rh ~ 2.5 nm are less susceptible to enzymatic hydrolysis than those with a peak Rh ~ 5 nm and seem to have a predominantly linear structure (equivalent to DP ~50), originating from amylose molecules. This study provides better insight in the starch digestion process in real food system.
The findings of this thesis have produced new insight on starch structure at the molecular level, and have resulted in a greater understanding of structure-digestibility properties of cooked rice. An understanding of the relationship between structural characteristics and functional properties of rice starches is important for optimizing industrial applications as well as allowing consumers to select suitable rice varieties for health benefits; this improved understanding is also useful for food technologists, plant breeders and molecular biotechnologists to improve the management of health problems originating from a high intake of starchy foods, especially cooked rice grains.