Starch microstructure and functional properties in waxy rice (Oryza sativa L.)

Rice starch contains two types of glucose polymers, mainly linear amylose and hyper-branched amylopectin. Waxy rice has been characterised by the lack of amylose, the proportion of which being one of the most important parameters measured for rice quality. Germplasm collection work conducted in the Lao People’s Democratic Republic has previously demonstrated the diversity of this type of rice in terms of quality. The definition of a waxy rice variety is dependent on the meaning of amylose. The conventional method for determining amylose content has shown that waxy rice could have up to 5% amylose. However, including a 0% amylose standard in the standard curve causes the amylose content of these varieties to become 0-2%. In this work, the absence of amylose in waxy rice has been determined through three different approaches. Granule-bound starch synthase I (GBSS1), the enzyme which synthesises amylose, was not detected in waxy rice. Long linear chains associated with amylose were also not detected by size exclusion chromatography (SEC). The absence of these long chains affected functional properties of waxy rice, as indicated by the differences in viscosity curves between waxy and non-waxy rice. Moreover, these waxy varieties themselves exhibit differences in their viscosity curves, another indication of the diversity in coking properties in these varieties. A new approach, the ‘lnP(N) technique’, in analysing chain length distributions was applied to varieties with known mutations in two of the enzymes involved in the synthesis of amylopectin, and in determining gelatinisation temperature. It was determined that the presence of a novel feature, an interruption to linearity at DP 18-24, of the lnP(N) plot was found in rice samples with mutations in the alk gene, which codes for starch synthase (SS) IIa, and in samples with inactive branching enzyme (BE) IIb. Single nucleotide polymorphisms (SNPs) in the gene coding for SSIIa have been associated with lowered gelatinisation temperature. On the other hand, non-functionality of BEIIb changes the amylopectin structure such that gelatinisation temperature is increased. The novel feature of the lnP(N) plot is found when either or both SSIIa and BEIIb are non-functional. Waxy rice starch has hot-water-soluble (HWS) and insoluble (HWI) components. It has been confirmed that the soluble polysaccharides are structurally different from phytoglycogen, and are similar to amylopectin. Structural differences between the two fractions, which can account for their differences in solubility, were determined. At the level of the chain length distribution, the HWI fraction contained long chains not found in the HWS fraction. Considerable amounts of sucrose and glucose were found in the HWS fraction. At another level of structure, the degree of branching of the HWS components was higher than in the HWI fraction. On the other hand, the whole molecules of the HWS fraction were smaller than those of the HWI fraction. These structural differences between the two fractions potentially affected their physical behaviour, particularly solubility. The amount of leached material appears to be a property of the method, as varying cooking conditions changed the amount of HWS components. This amount reaches equilibrium at certain conditions, indicating the limited amount of the HWS material. On the other hand, the HWI component contains molecules that are insoluble in water, rather than molecules that solubilise slowly. Given the stability of the HWS fraction in solution, the ratio of the HWS fraction to the HWI fraction could potentially be used in measuring quality if the amount of the HWS fraction is variety-specific. However, the quantity of the HWS fraction appears to be a feature of the starch, rather than of the variety. Nevertheless, the fact that the soluble fraction is structurally and thermodynamically different from the insoluble fraction could presumably be grounds to classify the soluble component as a group of molecules distinct from amylopectin