Exploring extraction/dissolution procedures for analysis of starch chain-length distributions

Instrumental measurement of cooked rice texture by dynamic rheological testing and its relation to the fine structure of rice starch

Increasing demands for better instrumental methods to evaluate cooked rice texture is driving innovations in rice texture research. This study characterized cooked rice texture by descriptive sensory analysis and two instrumental methods (texture profile analysis (TPA) and dynamic rheological testing) using a set of 18 varieties of rice with a wide range in amylose content (0-30%). The panellists' results indicated that hardness and stickiness were the two most discriminating attributes among 13 tested textural attributes. The consistency coefficient (K(*)) and loss tangent (tan δ) from a dynamic frequency sweep were used to compare with hardness and stickiness tested by TPA and sensory panellists, showing that using K(*) to express hardness, and tan δ to express stickiness, are both statistically and mechanistically meaningful. The instrumental method is rationalized in terms of starch structural differences between rices: a higher proportion of both amylose and long amylopectin branches with DP 70-100 causes a more elastic and less viscous texture, which is readily understood in terms of polymer dynamics in solution.

Improved understanding of rice amylose biosynthesis from advanced starch structural characterization

BACKGROUND

It has been shown from the chain length distributions (CLDs) that amylose chains can be divided into at least two groups: long and short amylose chains. These molecular structures influence some functional properties of starch, such as digestibility and mouth-feel. GBSSI is the key enzyme for the elongation of amylose chains; however, the effect of other starch biosynthesis enzymes in amylose synthesis is still not fully understood. Two advanced starch characterization techniques, size exclusion chromatography (SEC) and fluorophore-assissted carbohydrate electrophoresis (FACE), together with a newly developed starch biosynthesis model, are used to improve understanding of amylose biosynthesis.

RESULTS

SEC and FACE were used to determine the CLD of amylose and amylopectin in various native and mutant rice starches. The types of starch branching enzymes (SBEs) involved in the synthesis of the distinct features seen for shorter degrees of polymerization, DP, < 2000, and longer (DP > 2000) amylose chains are identified by combining these data with a mathematical model of amylopectin biosynthesis. The model enables each feature in the amylopectin CLD to be parameterized in terms of relative SBE activities, which are used to explain differences in the genotypes.

CONCLUSIONS

The results suggest that while GBSSI is the predominant enzyme controlling the synthesis of longer amylose chains, some branching enzymes (such as BEI and BEIIb) also play important roles in the synthesis of shorter amylose chains.

The importance of amylose and amylopectin fine structure for textural properties of cooked rice grains

Statistically and causally meaningful relationships are established between starch molecular structure (the molecular distribution of branched starch and the chain length distribution of debranched starch) and texture (hardness and stickiness) of cooked rice grains. The amounts of amylose chains with degree of polymerization (DP) 100-20,000, and of long amylopectin chains, positively correlated with hardness, while amylopectin chains with DP<70 and amylose molecular size both showed negative correlations with hardness (p<0.05). There was also a significant negative correlation between stickiness and the amounts of long amylopectin chains (p<0.01). For rices with similar amylose content, the amount of amylose chains with DP 1000-2000 positively correlated with hardness while size negatively correlated with hardness (p<0.05). This indicates for the first time that, regardless of amylose content, rice varieties with smaller amylose molecular sizes and with higher proportions of long amylose chains have a harder texture after cooking.

Diurnal changes in Sorghum leaf starch molecular structure

Control of the fine structure of transitory starch synthesized during the day in leaves is required for its normal degradation during the subsequent night. In this study, the molecular structure of transitory starch from Sorghum leaves over the diurnal cycle was characterized using size-exclusion chromatography. This is the first study of diurnal changes in the chain-length distribution (CLD) of amylopectin and amylose over the entire range of chain lengths, and in the size distribution of whole starch molecules. It was found that the outer layers of leaf starch granules, which were synthesized during the daytime and degraded during the night, contained more large molecules, including amylopectin with more short chains and more branching, than those in the inner layers. The outer layers also had lower amylose content. Starch molecular sizes in leaves are much smaller than in grain starch. The starch structures observed are likely to give optimal energy control during plant growth. Lack of this control may contribute to poor plant growth.

The biosynthesis, structure and gelatinization properties of starches from wild and cultivated African rice species (Oryza barthii and Oryza glaberrima)

The molecular structure and gelatinization properties of starches from domesticated African rice (Oryza glaberrima) and its wild progenitor (Oryza barthii) are determined and comparison made with Asian domesticated rice (Oryza sativa), the commonest commercial rice. This suggests possible enzymatic processes contributing to the unique traits of the African varieties. These have similar starch structures, including smaller amylose molecules, but larger amounts of amylose chains across the whole amylose chain-length distribution, and higher amylose contents, than O. sativa. They also show a higher proportion of two- and three-lamellae spanning amylopectin branch chains (degree of polymerization 34-100) than O. sativa, which contributes to their higher gelatinization temperatures. Fitting amylopectin chain-length distribution with a biosynthesis-based mathematical model suggests that the reason for this difference might be because O. glaberrima and O. barthii have more active SSIIIa and/or less active SBEIIb enzymes.

The characterization of modified starch branching enzymes: toward the control of starch chain-length distributions

Starch is a complex branched glucose polymer whose branch molecular weight distribution (the chain-length distribution, CLD) influences nutritionally important properties such as digestion rate. Chain-stopping in starch biosynthesis is by starch branching enzyme (SBE). Site-directed mutagenesis was used to modify SBEIIa from Zea mays (mSBEIIa) to produce mutants, each differing in a single conserved amino-acid residue. Products at different times from in vitro branching were debranched and the time evolution of the CLD measured by size-exclusion chromatography. The results confirm that Tyr352, Glu513, and Ser349 are important for mSBEIIa activity while Arg456 is important for determining the position at which the linear glucan is cut. The mutant mSBEIIa enzymes have different activities and suggest the length of the transferred chain can be varied by mutation. The work shows analysis of the molecular weight distribution can yield information regarding the enzyme branching sites useful for development of plants yielding starch with improved functionality.

Roles of GBSSI and SSIIa in determining amylose fine structure

This study examines the relationships between genetics (single nucleotide polymorphisms (SNPs) in GBSSI and SSIIa genes), starch structure (amylose and amylopectin fine structures), and starch properties (relating to gelatinization). GBSSI and SSIIa SNPs did not alter the starch content of rice grains. GBSSI SNPs can affect the amylose content, but they are incapable of altering the chain-lengths of amylopectin and amylose. The amounts of both long and short amylose branches changed with the same trend as amylose content, and they appeared to affect starch gelatinization properties. SSIIa synthesizes intermediate single-lamella amylopectin chains (DP 16-21), and consequently impacts the gelatinization temperature. Mathematical modelling suggests that the reduction in SSIIa activity significantly increases the activity of SBEII, resulting in a decreased activity ratio of SS to SBE in the enzyme set governing an appropriate chain-length distribution range. This application of the genetics-structure-property paradigm provides selection strategies to produce rice varieties with improved qualities.