This work compared the molecular structures and physicochemical properties of starches obtained from chalky and translucent kernels of six rice cultivars. Starch samples were prepared according to a modified alkali steeping method. Crystallinity, pasting characteristics, and thermal properties were studied by X-ray diffraction, rapid viscosity analysis, and differential scanning calorimetry, respectively. Starch molecular size fractions (amylopectin, amylose, and intermediate material) were estimated by high-performance size exclusion chromatography, and the chain length profiles of isoamylase-debranched amylopectin were evaluated by high-performance anion-exchange chromatography with pulsed amperometric detection. Starches from chalky kernels contained less amylose (more amylopectin) and more short branch-chain amylopectin (less long branch-chain amylopectin) compared with the translucent kernel starches. Differences in starch structural features significantly correlated with observed variation in grain translucency, starch X-ray diffraction patterns, thermal properties, and pasting characteristics. Starch synthesis in chalky kernels may slightly favor glucan chain branching over chain elongation.

Rough rice (RR) is the conventional feedstock for parboiling. The use of brown rice (BR) instead of RR is gaining interest because it results in shorter processing time and lower energy requirement. This study compared the functional properties of milled parboiled rice under different parboiling conditions from RR and BR. Presoaked RR and BR from cultivars Bolivar, Cheniere, Dixiebelle, and Wells were parboiled under mild (20 min, 100 degrees C, 0 kPa) and severe (20 min, 120 degrees C, 98 kPa) laboratory-scale conditions. Head rice yield improved on the RR and BR samples subjected to severe parboiling and was comparable to that of a commercially parboiled sample. Mild parboiling of BR resulted in lower head rice yields. Parboiling generally resulted in decreased head rice whiteness, decreased apparent amylose, increased total lipid, and sparingly changed protein content. Under the same parboiling conditions, the extent of starch gelatinization was higher for BR compared to RR as manifested by some distinct differences in pasting and thermal properties. The cooking characteristics (water uptake ratio, leached materials, and volumetric expansion) and cooked rice texture (hardness and stickiness) of RR and BR subjected to severe parboiling were fairly comparable. Differences in parboiled rice functional properties due to cultivar effect were evident.

Among cereal grains, rice is the world's leading source of caloric energy in the human diet. About 90% of a milled rice kernel is starch (dry weight basis), thus justifying the need to understand the impacts of factors affecting starch composition and behavior when processed. This review presents past and current perspectives regarding the effects of growing environment on the macro, micro, and nano level structures of rice starch. The environmental elements considered in this work are: air temperature, atmospheric carbon dioxide, light, water, and soil nutrients. All these elements are essential for plant development and reproduction. The effect of air temperature is the most widely explored aspect of the environment in relation to rice production and grain quality. This could be attributed in part to the recently purported and highly publicized global warming phenomenon. Amylose content has remained a major predictor of rice starch quality but emerging instrumental techniques have facilitated more thorough research addressing the fine structure of amylopectin, which constitutes ∼70% of the starch present in a milled rice kernel. Kernel chalkiness appears to be the most common visual manifestation of environment‐related stress. Typically, there are genotype by environment interactions; this may result in inconsistent temporal and spatial findings. However, such interactions may prove useful in identifying particular genotypes that are adaptable and stable across growing locations.

The molecular-level features of starch in relation to the changes in rice functionality during storage are not yet fully elucidated. This work investigated the effects of rough rice storage conditions on starch fine structures and physicochemical properties. Dried rough rice samples (medium-grain Bengal and long-grain Cypress) were stored at 4, 21, and 387C in temperature-controlled chambers and then periodically removed and evaluated after 1, 3, 5, 7, and 9 months. Flour (powdered head rice) and starch (extracted from head rice by alkali steeping) samples were evaluated for pasting and thermal properties. High-performance size-exclusion chromatography and high-performance anion exchange chromatography were used to characterize starch molecular size and amylopectin chain-length distribution, respectively. Significant changes in starch fine structure were observed primarily on the 387C lots, and to some extent on the 217C lots. The decreased amylose: amylopectin ratio, shortened amylopectin average chain length, and the shift in chain-length distribution to shorter branch chains were implicative of molecular-level starch degradation. The flour and starch samples showed inconsistent trends in pasting and thermal properties, thus suggesting the role of not only starch but also its interaction with non-starch components in rice aging.

Cluster, correlation, and multivariate regression analyses were used to rationalize the effects of grain composition, starch fine structure, and leaching characteristics on cooked rice texture (hardness and stickiness). The head rice grain composition of 23 U.S. long-grain cultivars was evaluated in terms of apparent amylose content, crude protein, and surface lipids. Starch samples were prepared by extraction with dilute alkali and amylopectin fine structure was characterized by high-performance anion-exchange chromatography with pulsed amperometric detection. Hardness and stickiness of head rice samples cooked in optimum water were measured with a texture analyzer. The amylose-amylopectin ratio (AAR) of the material that leached out of the grains on cooking was evaluated by high-performance size-exclusion chromatography (HPSEC). Simple correlation and multivariate linear regression analyses pointed to AAR as the main indicator of cooked rice hardness and stickiness. Cluster analysis showed that the leached starch from soft-cooking, high-amylose cultivars (e.g., Jodon and L-202) generally had a higher proportion of amylopectin than amylose (AAR<1). In contrast, dry-cooking, high-amylose cultivars (e.g., Newrex and L-205) leached out starch with a higher proportion of amylose than amylopectin (AAR>1) during cooking. The amount of leached materials itself was also higher for the soft-cooking cultivars than the dry-cooking counterparts. Cultivar differences in leaching characteristics were attributed to variations in apparent amylose content, crude protein, and amylopectin chain-length distribution.