Development of a novel formula for estimating the amylose content of starch using japonica milled rice flours based on the iodine absorption curve

Effects of Hard Water Boiling on Chalky Rice in Terms of Texture Improvement and Ca Fortification

In the present paper, we investigated the characteristics of chalky rice grains generated by ripening under high temperature and compared them with whole grains. We evaluated 14 unpolished Japonica rice grains harvested in Japan in 2021, and these samples (original grains) were divided into two groups (a whole grain group and a chalky grain one). We found that not only activities of endogenous amylase and proteinase, but also cell wall-degrading enzymes, such as xylanase and cellulase, changed markedly between chalky grains and whole grains. Using rice grains blended with 30% of chalky grains as the material, we compared the sugar and mineral contents and textural properties of the rice grains soaked and boiled in either ordinary water or hard water, such as Evian or Contrex. It was shown that xylanase, in addition to amylase and proteinase, may play an important role in changing the texture of the boiled chalky rice grains. For the sake of preventing the above-mentioned deterioration in the texture of boiled grains of chalky rice, we tried to use hard water, such as Evian or Contrex, to soak and cook the chalky rice grains. It was shown that the hard water was useful for the prevention of texture deterioration of the boiled rice grains due to inhibition of the activities of endogenous hydrolytic enzymes, such as α-amylase, β-amylase, proteinase, and xylanase. Furthermore, we found that the hard water was useful in increasing the calcium absorption through the meal by 2.6 to 16.5 times.

Palatability and Bio-Functionality of Chalky Grains Generated by High-Temperature Ripening and Development of Formulae for Estimating the Degree of Damage Using a Rapid Visco Analyzer of Japonica Unpolished Rice

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.

Characteristics of Physicochemical Properties of Chalky Grains of Japonica Rice Generated by High Temperature during Ripening

Global warming has caused devastating damage to starch biosynthesis, which has led to the increase in chalky grains of rice. This study was conducted to characterize the qualities of chalky rice grains and to develop the estimation formulae for their quality damage degree. We evaluated the chalkiness of 40 Japonica rice samples harvested in 2019, in Japan. Seven samples with a high ratio of chalky rice grains were selected and divided into two groups (whole grain and chalky grain). As a results of the various physicochemical measurements, it was shown that the surface layer hardness (H1) of cooked rice grains from chalky grains was significantly lower, and their overall hardness was significantly lower than those from the whole grains. In addition, α- and β-amylase activities, and sugar contents of the chalky rice grains were significantly higher than those of the whole rice grains. The developed estimation formula for the degree of retrogradation of H1 based on the α-amylase activities and pasting properties, showed correlation coefficients of 0.84 and 0.81 in the calibration and validation tests, respectively. This result presents the formula that could be used to estimate and to characterize the cooking properties of the rice samples ripened under high temperature.

Evaluation of Hardness and Retrogradation of Cooked Rice Based on Its Pasting Properties Using a Novel RVA Testing

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.