The Reverse Phenomenon in Water Absorption Rate in Japonica Rice Grains Caused by Long-Term Soaking in Water at Different Temperatures

日本調理科学会誌（J.Water Absorption Cookery Rates Sci. of Jpn.） Vol.Rice at Different 52，No. Soaking 3，159～168（2019） Temperatures〔Note〕 The Reverse Phenomenon in Water Absorption Rate in Japonica Rice Grains Caused by Long-term Soaking in Water at Different Temperatures Saeko Morii*,**,*** Kaoru Sakamoto*,** Naoko Shirasugi（Kataoka）*** § We investigated the influence of soaking temperature on the water absorption in three cultivars of Japonica rice, Kinuhikari, Sasanishiki, and Hatsushimo, based on the modified water absorption rate calculation method for rice. The grains of each cultivar were processed under different conditions, i.e., 70% polished and polished rice. We com- pared the water absorption curves of grains soaked in water at 5℃, 20℃, and 40℃ for 10 min to 24 h to ensure that the absorption had reached equilibrium. The absorption curves of grains soaked at 5℃ and 20℃ intersected those of grains soaked at 40℃ in all cultivars of both 70% polished rice and polished rice. This “reverse phenomenon” was specific to the granular shape of the rice grain. The findings for these three cultivars descended from the different rice lines contradict the established theories in the field of cookery science. It is suggested that this occurs for many cultivars of Japonica rice. Keyword：Japonica rice, polished rice, soaking temperature, water absorption and Adachi, 1992）. The water absorption has also been INTRODUCTION reported to increase when the rice is soaked in water for Japonica rice, a staple of Japanese food, is shorter in a long time, even if the soaking temperature is low length and more viscous than Indica rice. The Japanese （Toyoshima et al., 1994）. In our previous report mainly use nonwaxy rice, which has a lower viscosity than （Sakamoto et al., 2015）, we found that the water absorp- waxy rice, in everyday life. Rice is usually soaked, as a tion rate of rice soaked in cold water exceeded that of rice process of water absorption, prior to boiling. An ordinary soaked in warm water, and the water absorption curves rice cooking method in Japan is “the takiboshi method”, of rice soaked in cold water intersected those of rice and the taste of the rice is impacted by small changes in soaked in warm water. The modified dehydration method various steps in the cooking process: washing the rice, of measuring soaked rice was used to more accurately soaking the rice, boiling the rice, and “murashi”, i.e., allow- estimate the equilibrium water absorption rate of the rice. ing the boiled rice to settle. Rice cooked by the above This finding was not consistent with the established theo- method contains a suitable amount of water because of ries described in cookery science textbooks. We call the the adequate soaking time and the use of the appropriate above results the “reverse phenomenon”. amount of water for soaking the rice prior to boiling. However, the water absorption properties of rice are Some studies（Maruyama and Sakamoto, 1992; Miwa et variety-dependent. A study by Miwa et al.（2002） showed al., 2002） have shown that the eating quality of cooked that there are two types of cultivars of rice for which the rice is affected by the water absorption rate of the rice water absorption rate moderately increases or greatly soaked in water prior to cooking. The water absorption increases after soaking the polished rice in water at 15℃ rate of rice has been reported to increase with increasing for 10 min. In addition, there are two types of cultivars the soaking water temperature （Matsumoto, 1972; with higher water absorption rates at the beginning of Yoshizawa et al., 1975; Kainuma, 2012）; however, other soaking than at 120 min in to the soaking process for pol- studies have reported that the maximum water absorption ished rice of these cultivars soaking in water at 20℃ of rice decreases as water temperature increases（Okuno （Ehara et al., 1996）. The report（Kuwada et al., 2011） showed that both the rice milling yield and the cultivar * University of Hyogo ** Research Institute for Food and Nutritional Sciences, University of affect the water absorption rate of the rice. Hyogo There have been no reports that calculated a detailed *** Graduate School of Human Development and Environment, Kobe University water absorption rate that included the soaking of the § Inquiry Graduate School of Human Development and Environment, minute solid content of various rice cultivars at various Kobe University milling yields. In this study, using the previously reported 3-11 Tsurukabuto, Nada-ku, Kobe 657-8501, Japan E-mail：[email protected] modified water absorption rate calculation method for （159） 31 日本調理科学会誌（J. Cookery Sci. Jpn.） Vol. 52 No. 3（2019） commercial Japonica rice （Sakamoto et al., 2015）, the The term “1000-kernel weight” refers to the weight of time-dependent changes in the water absorption rate of 1,000 extracted rice grains measured in grams, and this three rice cultivars descended from the different Japonica value was calculated by the following formula: rice lines at the two different milling yields soaked in 1000-kernel weight （g）＝the weight of perfect warm or cold water were studied. grains （g） / the number of perfect grains To elucidate whether or not the granular shape of rice （grains）×1000 is functionally related to the “reverse phenomenon,” we also conducted soaking tests for rice flour that had been The real milling yield was calculated by the following prepared from the three cultivars of rice. formula: MATERIALS and METHODS Real milling yield（%）＝the 1000-kernel weight of milled rice（g） / the 1000-kernel weight of brown Materials rice（g）×100 The Japonica rice（paddy rice） used in these experiments were selected from the descendants of three main cultivars: Kinuhikari rice（from the Koshihikari cultivar）, According to the Standard Tables of Food Composition which was harvested from Ichikawa City, Hyogo in 2013; in Japan（2015）, the milling yield of polished rice is gener- Sasanishiki rice（from the Sasanishiki cultivar）, which was ally 90–92%, and the milling yield of 70% polished rice is provided by Miyagi Prefectural Furukawa Agricultural 92–94%. In this study, the polished rice prepared at a Experiment Station, Miyagi in 2014; and Hatsushimo rice milling yield of 91% in “polished rice” mode with the rice （from the Asahi cultivar）, which was provided by Gifu milling machine is referred to as “the polished rice”, and Prefectural Agricultural Technology Center, Gifu in 2014. the 70% polished rice prepared at a milling yield of 93% in These brown rice samples were stored at 12℃, and the “70% polished rice” mode is referred to as “the 70% pol- experiments were conducted at room temperature. ished rice.” Calculation of rice milling and real milling yields Table 1 shows the 1000-kernel weights, real milling Rice samples were milled by two different milling yields, and rice moisture contents of the nine types of rice modes with a rice milling machine（the rice polishing samples used in this study: three different milled grains, apparatus）（BT-AF05, ZOJIRUSHI Corp., Osaka City, i.e., unpolished（brown）, 70% polished and polished rice of Japan） to produce the polished rice and 70% polished rice. the three cultivars. The three cultivars of brown rice “Perfect grains” refers to rice grains that remained after were polished at milling yields of 93% and 91% to prepare the other kinds of grains, i.e., cracked rice grains, broken 70% polished rice and polished rice, respectively. The real rice grains, dead rice grains, and immature rice grains, milling yields of the rice samples were estimated and cal- were removed by filtering through a wire mesh net culated to confirm that the error ranges were maintained （openings of the sieve mesh: 2×2 mm）. The perfect within 1% based on the set values of the machine（Table grains were obtained by sifting the broken rice grains 1 ）. from the milled rice with a sieve（mesh size of 2 mm）. Table 1. Comparison of 1000-kernel weights and rice moisture contents of brown rice and milled rice grains 1000-kernel Real milling Rice moisture Milling rate Cultivar weight（g） yield（%） content（%） Kinuhikari 20.0±0.2a 91.5±0.7 15.1±0.4 Polished rice Sasanishiki 19.9±0.1a 91.3±0.3 15.5±0.3 Hatsushimo 23.6±0.2b 91.4±0.7 15.6±0.2 Kinuhikari 20.4±0.2a 93.2±0.9 15.8±0.6 70% Polished rice Sasanishiki 20.5±0.1a 93.3±0.5 15.4±0.5 Hatsushimo 24.1±0.1b 93.3±0.5 15.6±0.2 Kinuhikari 21.9±0.1a — 15.5±0.2 Brown rice Sasanishiki 21.8±0.1a — 15.1±0.3 Hatsushimo 25.8±0.2b — 15.3±0.0 a-b: Mean ± SD values in a column at each milling rate with different letters are significantly different,p ＜0.05; 1000-kernel weight（g） and real milling yield（%）: n＝5, rice moisture content（%）: n＝3 32 （160） Water Absorption Rates of Rice at Different Soaking Temperatures Measurement of the grain size and surface area of the perature was maintained at 5℃ or 40℃ by following the milled rice grain samples above described methods. The variations in the water The sizes of the grains of each type of the milled rice temperature for the soaking tests of the rice grains and grain samples were determined by measuring the length, rice flour were maintained within ±1℃ and ±2℃, width, and thickness of five grains of the polished and 70% respectively. polished rice with calipers. In this study, the rice grains The soaking times of the rice grain samples were 10, were assumed to be spheroid in shape, and the surface 20, 30, 60, 90, 120, and 240 min.

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