Global warming inhibits grain filling in rice and leads to chalky grains, which are damaged in physical and cooking qualities. In the present paper, we evaluated 54 Japonica unpolished rice grains harvested in Japan in 2020, and these samples (original grains) were divided to two groups (whole grains and chalky grains). Using rice grains of 100% whole grains or those blended with 30% of chalky grains, we measured contents of sugars and amino acids, and textural properties of boiled rice grains. It was shown that the α-amylase activity and proteinase activity of raw chalky rice were significantly higher than those of whole rice grains, which led to the significant increase of low-molecular-weight sugars and free amino acids after boiling. Furthermore, hardness and toughness of the boiled rice grains were decreased markedly by blending chalky grains. The ratio of α-amylase activity of chalky grains to that of whole grains was shown to be a useful indicator for damage degree by high-temperature ripening. It became possible to estimate the degree of high-temperature damage of rice grains based on only the pasting properties of unpolished rice.

Background and objectives Although lipids are less abundant than carbohydrates and proteins in rice, they are important because they contribute to its nutritional, sensory, and functional qualities. In a previous study, we developed novel estimation formulae for starch properties on the basis of the iodine absorption curve or the pasting properties. The objectives of the present research are to characterize the fatty acid compositions of the various rice cultivars and to develop the estimation formulae for fatty acid contents based on the pasting properties. Findings We found that high degree of polymerization of amylopectin cultivars showed a high Pt, and a higher degree of unsaturated fatty acid (USF) (20:0, 20:1, 22:0 and 24:0) and low CD of amylopectin samples showed a high linoleic and linolenic acid content, while that of Pt and Cons showed a low value. Conclusions We developed novel estimation formulae for oleic and linoleic acid content based on pasting properties, and these equations showed determination coefficients (R2) of 0.75, and 0.68 for calibration and 0.65, and 0.76 for the validation test, respectively. Significance and novelty These formulae would be useful to select the promising rice cultivars in terms of biofunctionality, such as prevention of lifestyle diseases.

With rice being one of the most important crops worldwide, rapid and objective quality evaluation methods based on physicochemical measurements of rice are necessary. We compared the pasting properties of various rice samples using three different heating and cooling programs (maximum temperatures were 93, 120, and 140 °C, respectively) in a newly developed high-temperature-type Rapid Visco Analyzer (RVA, RVA 4800). Furthermore, we investigated the relationship between the different pasting properties measured by the three programs, with starch microstructure measured by iodine scanning analysis, the physical properties of the cooked rice measured by a Tensipresser after 2 h at 25 °C or after 24 h at 6 °C, and prolamin ratio measured by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The consistency value (final viscosity–minimum viscosity) yielded by a new program of maintenance for 2 min at 120 °C using RVA 4800 had a higher positive correlation with retrograded surface hardness H1(R) (r = 0.92), retrograded overall hardness H2(R) (r = 0.90), and the absorbance at λmax (Aλmax) of cooked rice (r = 0.88) and resistant starch (r = 0.80) than those by the conventional program at 93 °C. We developed estimation formulae for H1(R) for various kinds of rice, of which the determination coefficient was 0.86. It led to an easy and rapid assay method for the cooking properties of the various rice samples.

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