日本調理科学会誌（J.Water Absorption Cookery Rates Sci. of Jpn.） Vol. at Different 52，No. Soaking 3，159～168（2019） Temperatures〔Note〕

The Reverse Phenomenon in Water Absorption Rate in 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 of Japonica rice, Kinuhikari, , and Hatsushimo, based on the modified water absorption rate calculation method for rice. The grains of each 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 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

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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 experi- ments were selected from the descendants of three main cultivars: Kinuhikari rice（from the 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 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 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. The tests were conducted area of a rice grain was calculated by the following for- for 8, 16, 20, and 24 h to reach the equilibrium water mula: absorption rates. The soaking times of the rice flour sam- ples were 20, 90, and 240 min. The surface area of a rice grain（cm 2）＝ 4 π（（ a pbp Soaking methods and dehydration methods ＋apcp＋bpcp） / 3）1/p The soaking and dehydration of the rice grains was per- （2a, 2b, 2c are the lengths of each rice grain, p＝ formed as previously reported（Sakamoto et al., 2015）. 1.6075） After certain periods of the soaking time, the water and Rice moisture control soaked rice grains were separated by centrifugal dehydra- As reported（Brewer’s Rice Research Group, 1996）, if tor（H-112, KOKUSAN Co., Ltd., Saitama, Japan）（3,000 the water content of the polished rice is different by 1%, rpm（500×g） for 5 min）. Subsequently, the soaked rice the water absorption rate fluctuates by 3% over 120 min. grains were weighed, and the soaking water was col- Therefore, rice moisture was controlled prior to conduct- lected. The soaking water was dried in an automatic oven ing the experiments, and the water content was measured at 105℃ until the residue reached a constant mass. From using a moisture analyzer. this data, the dry weight of the suspended rice content at

The moisture content of the rice was measured using a any given absorption time（D t） was calculated. moisture analyser（MOC63u, SHIMADZU Corp., Kyoto, After the water absorption test, the soaked rice flour Japan）, which analysed approximately 2 g of sample at was separated from the water using a centrifuge（TX- 135℃ for 3 h. The moisture content of the rice was main- 201, TOMY SEIKO Co., Ltd., Tokyo, Japan）（3,000 rpm tained at approximately 15.5%, which is similar to that of （500×g） for 5 min）, and the excess supernatant fluid was polished rice, as described in the Standard Tables of Food subsequently drained by placing the tubes at 45°angle for Composition in Japan （2015）, by keeping a beaker of 5 min, followed by the precipitate being weighed warm water（50-60℃） and a non-woven fabric bag con- （Matsumoto and Abe, 1962; Naganuma, 2003）. All the taining the rice grains inside a well-closed container at above the tests were repeated three times during these room temperature（Brewer’s Rice Research Group, 1996）. studies. As a result, when the rice samples underwent water Calculation of the water absorption rate absorption tests, the moisture content of each rice variety In this study, we regarded “the adjusted water absorp- was between 15.1% and 15.8%（Table 1）. tion rate”（AA t） calculated based on the weight contain- Water absorption test ing that of the suspended rice content at any given

In this study, rice samples were not washed with water absorption time（S t） not only that of the rice grains at the so that the water absorption rate and extracted solid con- same absorption time（W t）, as the water absorption rate tent in the rice washing would not be affected. of the rice, according to our previous report（Sakamoto et The rice grain samples（10 g） were soaked in 30 mL of al., 2015）. tap water in a 50-mL beaker, and the water temperature AAt was obtained by the following formulas: was maintained at 5℃ using a refrigerator, 20℃ using a At＝（ W t－W0） / W0×100 cold water bath or 40℃ using a hot water bath. Three cultivars of polished rice grains were each pulverized by where At （%） is the standard absorption rate at any using a pot mill, on the rotating base, into which a set given absorption time（t）, Wt（g） is the weight of the rice amount of alumina balls was placed with each sample, fol- grains at any given absorption time, and W0（g） is the lowed by filtration with screen filters of 150-µm pore size. initial weight of the rice grains. The rice flour samples（0.5 g） were soaked in 20 mL of St＝Dt×（1＋At / 100） tap water in a 30-mL centrifuge tube, and the water tem-

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where St（g） is the weight of the suspended rice content Scheffe’s method or t-test. We analyzed the relationships at any given absorption time, and Dt（g） is the dry weight between the rice grain surface area and water absorption of the suspended rice content at any given absorption rate using Spearman’s rank correlation. time. S was calculated by D plus the moisture content t t RESULTS and DISCUSSION obtained by multiplying Dt by At/100. The characteristics of the rice samples: brown rice, 70% AAt＝（ W t＋St－W0） / W0×100 polished rice and polished rice grains of three types of where AAt（%） is the adjusted water absorption rate. It is Japonica rice the water absorption rate of the sum of Wt plus St. Table 2 shows the comparison of the sizes of the milled The water absorption rate of rice flour was obtained by rice grains of the three cultivars of rice, Kinuhikari, the following formulas: Sasanishiki, and Hatsushimo. Although the width and thickness of the grains of the three cultivars of the two AFt＝（WFt－WF0） / WF0×100 types of milled rice were approximately equal or similar where AFt（%） is the standard absorption rate at any to each other, the lengths of each of the polished and 70% given absorption time（t）, WFt（g） is the weight of the polished rice grains of the Hatsushimo cultivar were sig- rice flours at any given absorption time, and WF0（g） is nificantly longer than those of the other cultivars, and the initial weight of the rice flour. their surface areas were also significantly greater than X-ray diffraction （XRD） analysis of the rice samples those of the other cultivars（Table 2）. The 1000-kernel The rice grain samples soaked in water were air-dried weights of the Hatsushimo grains were significantly and ground into powder with a mortar and pestle, fol- heavier than those of the other cultivars at two milling lowed by filtration through a 150-µm stainless steel yields: 70% polished rice and polished rice （Table 1）. screen. The rice flour that was applied to the soaking test From these results, Hatsushimo grains are regarded as was also air-dried and filtrated with the screen. The XRD the largest and the heaviest among the three cultivars. analyses of the rice samples were carried out using an Water absorption curves of the three cultivars of rice X-ray diffractometer （MiniFlexII, Rigaku Co., Ltd., Tokyo, soaked at different temperatures Japan）. The samples were exposed to X-ray beams at 15 The water absorption curves of the three cultivars of mA and 30 kV. The data were recorded over a diffraction polished rice and 70% polished rice soaked in water at dif- angle（2 θ） range of 5° to 40°. ferent temperatures（5℃, 20℃, and 40℃） as a function of Statistical methods time are shown in Figures 1 and 2, respectively. In our The data are reported as the mean ± standard devia- previous report（Sakamoto et al., 2015）, we found that at tion. For the statistical analyses, we used Excel Statistics the beginning of the soaking process, the absorption rate 2012（Social Survey Research Information Co., Ltd., Tokyo, of commercial Koshihikari polished rice soaked in warm Japan）, and the level of significance was assumed to be water was higher than that of the same rice soaked in 0.05. The measurements of the rice grains, the water cold water. However, at the final stage of soaking, rice absorption rates of rice samples after soaking for 30 min soaked in cold water showed a higher absorption rate and the maximum water absorption rates of those soaked than rice soaked in warm water, causing their time- for a long time were verified statistically using one-way dependent absorption curves to intersect. We call this analysis of variance followed by multiple comparisons by effect the “reverse phenomenon”.

Table 2. Comparison of the sizes of the milled rice grains of the three types of Japonica rice Length of Width of Thickness of Surface area Milling rate Cultivar grain（mm） grain（mm） grain（mm） （mm2） Kinuhikari 5.0±0.1a 2.9±0.1 2.0±0.1 33.5±2.4a Polished rice Sasanishiki 4.9±0.1a 2.9±0.1 2.0±0.1 32.4±1.5a Hatsushimo 5.5±0.1b 3.0±0.2 2.0±0.1 36.0±1.6b Kinuhikari 5.0±0.2a 3.0±0.1 2.0±0.1 33.5±1.5a 70% Polished rice Sasanishiki 4.9±0.2a 2.8±0.2 2.0±0.1 32.0±2.1a Hatsushimo 5.4±0.1b 3.0±0.0 2.1±0.1 36.6±0.9b a-b: Mean ± SD values in a column with different letters are significantly different,p ＜0.05, n＝5

34 （162） Water Absorption Rates of Rice at Different Soaking Temperatures

30 30 （a）Kinuhikari （b）Sasanishiki 25 25 ） ） （ %

20 （ % 20

15 15

10

ate r absorp ti on 10 ate r absorp ti on W W 5 5

0 0 0 30 60 90 120 150 180 210 240 0 30 60 90 120 150 180 210 240 Soaking time（min） Soaking time（min）

5℃ 20℃ 40℃ 5℃ 20℃ 40℃

30 30 （c）Hatsushimo 25 25 ） %

（ 20 20

15 15

10

ate r absorp ti on 10 W 5 5

0 0 0 30 60 90 120 150 180 210 240 4 8 12 16 Soaking time（min） Soaking time（h） 5℃ 20℃ 40℃

Fig. 1. Water absorption of polished rice grains（including wet rice solid contents in the soaking water）:（a） Kinuhikari,（b） Sasanishiki, and（c） Hatsushimo Mean ± SD, n＝3

30 30 30 （a）Kinuhikari （b）Sasanishiki 25 25 25 ） ） 20 （ %

（ % 20 20

15 15 15

10 10 10 ate r absorp ti on ate r absorp ti on W W 5 5 5

0 0 0 4 8 0 30 60 90 120 150 180 210 240 0 30 60 90 120 150 180 210 240 （ ） Soaking time（h） Soaking time min Soaking time（min）

5℃ 20℃ 40℃ 5℃ 20℃ 40℃

30 30 （c）Hatsushimo 25 25 ） % 20 （ 20

15 15

10 10 ate r absorp ti on W 5 5

0 0 0 30 60 90 120 150 180 210 240 4 8 12 16 20 24 Soaking time（min） Soaking time（h）

5℃ 20℃ 40℃

Fig. 2. Water absorption of 70% polished rice grains（including wet rice solid content in the soaking water）:（a） Kinuhikari,（b） Sasanishiki, and（c） Hatsushimo Mean ± SD, n＝3

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In this study, when polished and 70% polished rice tion rates. samples from the three cultivars were soaked in water at First, focusing on the effect of the milling rates of the 5℃, 20℃, and 40℃, the water absorption rates at the rice samples, the water absorption speed varied among beginning of the soaking process were higher for higher the different those of the same cultivar of rice（Table 3）. water temperatures than for lower temperatures（Fig. 1 After 30 min of soaking, the water absorption rates of pol- and 2）. As the soaking test progressed, the water absorp- ished rice soaked at 5℃, 20℃, and 40℃ were 9.9–16.6%, tion curves of the rice soaked at 20℃ intersected those of 13.9–20.1%, and 14.9–21.0%, respectively. The water the rice soaked at 40℃. Then, the absorption curves of absorption rates of 70% polished rice soaked at 5℃, 20℃, the rice soaked at 5℃ intersected those of the rice soaked and 40℃ were 8.2–11.8%, 13.9–16.6%, and 15.2– at 40℃. Finally, the absorption curves of the rice soaked 17.8%, respectively. For the first 30 min of soaking, the at 5℃ approached or intersected those of the rice soaked rates of water absorption of the rice samples with a lower at 20℃. When the polished and 70% polished rice samples milling yields may therefore be significantly lower than of all cultivars were soaked until the absorption rates those of rice samples with higher milling yields（polished reached equilibrium, the absorption curves of the rice rice vs. 70% polished rice; p＝0.039）. This result is consis- soaked at 40℃ intersected the curves of the rice samples tent with previously published studies. For example, in soaked at 5℃ and 20℃. The curves intersected despite Yamada , the absorption rate is positively cor- the fact that the absorption rates of the rice soaked at related with the milling yield （Yoshida and Horigane, 40℃ were the highest initially. The “reverse phenomenon” 2008）. When samples of three rice cultivars, Yamada can also be observed in the soaking tests of three culti- Nishiki, Koshihikari, and Takaneminori, with different mill- vars of polished and 70% polished Japonica rice. There ing yields were soaked in water at 20℃ for 30 min, the was a very low probability that it occurred due to the milling yield was also positively correlated with the water influence of the differences in the rice milling machines absorption rate（Onishi et al., 1997）, particularly for the which were used for the preparation of the rice samples, cultivars Koshihikari and Takaneminori. In addition, because it was observed not only in the rice samples pre- Kuwada et al.（2011） reported that regardless of the soak- pared using the machine in this study but also in the com- ing water temperature, polished rice absorbed water mercially prepared Japonica polished rice used in our slightly faster than 70% polished rice. They hypothesized previous study（Sakamoto et al., 2015）. that for higher milling yields, water may more easily soak Water absorption rates of rice soaked at different tem- into the center of the grain due to reductions in the con- peratures in the early stages of soaking tents of lipids and proteins with nonpolar groups and When cooking rice, soaking generally occurs over a changes in the structure of the grain surface induced by period of 30 min to 2 h. Water absorption rates after 30 milling. min of soaking were compared for rice samples of three Second, the water absorption speed varied among rice cultivars soaked under six experimental conditions（Table cultivars（Table 3）. Previous reports support this finding. 3）. Our results suggest that rice cultivars, milling rates, Ehara et al.（1996） reported that when soaked at 20℃, and soaking water temperatures affect the water absorp- the water absorption rates of Koshihikari and Hitomebore

Table 3. Water absorption rates of milled rice grain samples of the three rice cultivars soaked for 30 min at different temperatures

Soaking Water absorption rate（%） temperature（℃） Kinuhikari Sasanishiki Hatsushimo 5℃ 16.6±0.8a 11.5±0.2a 9.9±0.5a Polished riced 20℃ 20.1±0.8b 17.4±0.4b 13.9±0.2b 40℃ 21.0±0.5b 17.5±0.4b 14.9±0.3c 5℃ 11.8±0.5a 10.3±0.5a 8.2±0.3a 70% Polished ricee 20℃ 16.6±0.3b 15.4±1.7b 13.9±0.2b 40℃ 17.8±0.2c 16.3±0.1b 15.2±0.2c a-c: Mean ± SD values in a column at each milling rate of rice with different letters are significantly different, p＜0.05, n＝3 d-e: Mean ± SD values in all the cells at each milling rate of rice with different letters are significantly different, p＜0.05, n＝27

36 （164） Water Absorption Rates of Rice at Different Soaking Temperatures rice 30 min after soaking were lower than the absorption Maximum water absorption rates of rice soaked at dif- rate of Sasanishiki rice. Miwa et al.（2002） reported that ferent temperatures when rice was soaked at 15℃, the water absorption rates The maximum water absorption rates at equilibrium of of some cultivars changed little when soaked for more the three cultivars of rice with two degrees of milling than 10 min because by that time, the rice had already yields soaked at different temperatures are shown in reached 80-90% of its equilibrium absorption. Okuno and Table 4. We defined that the equilibrium water absorption Adachi（1992） reported that there was a 2% difference in rate was achieved when the increase in the water absorp- water absorption rates between different cultivars soaked tion rate per hour was less than 0.1%. These results show at 20℃ for 30 min. Hitomi et al.（2015） reported that in that for all cultivars, the equilibrium water absorption Himegonomi rice （a rice low in amylose）, the water rates of the polished rice soaked at 5℃ and 20℃ were absorption speed was faster, and the water absorption significantly higher than the equilibrium absorption rate of rate was higher than that of Koshihikari rice. In some rice soaked at 40℃, contrary to the above-mentioned reports, the authors considered the relationship between results of the soaking tests for 30 min in the previous sec- the surface area of the grain of rice and the water absorp- tion and common belief. Kinuhikari polished rice soaked at tion rate（Ueda, 1994; Kato et al., 2010）. However, Yoshida 5℃ and 20℃ showed the highest maximum water absorp- and Horigane（2008） proposed that at first, almost all of tion rate（23.7%） of the polished rice samples of the three the water permeates into the tissue from the part of the cultivars. For the Kinuhikari and Sasanishiki cultivars, the grain to which the embryo was adhered. Water then pen- maximum water absorption rates of the polished rice etrates through the cracks and soaks into the whole grain. samples soaked at 5℃ and 20℃ were significantly higher Finally, we can confirm that the water absorption rates than those of the polished rice samples soaked at 40℃ for of rice soaked at 40℃ were significantly higher than those 240 min（ Fig. 1 and Table 4）. Hatsushimo polished rice at 5℃ of all of the three cultivars of rice samples with soaked at 5℃ reach its maximum water absorption rate two degrees of milling rates（Table 3）（ p＜0.05）. （Fig. 1 and Table 4） in 16 h. The maximum absorption We have determined the correlation coefficients rate of rice soaked at 5℃ was significantly higher than between the total surface areas（calculated from the data those of rice soaked at 20℃ and 40℃. Our present in Table 1） of 10 g each of both the polished and 70% pol- findings confirm the results of our previous study on ished rice grains of the three types of Japonica rice and commercial Japonica rice（Sakamoto et al., 2015）. Similar the water absorption rates of the six rice samples, after to the results observed in polished rice, the water soaking for 30 min and at equilibrium, respectively. Sig- absorption rates of the 70% polished rice samples of all nificant correlation （r＝0.67, n＝18） was observed cultivars soaked at 5℃ were eventually significantly between the surface areas of the both of polished and 70% higher than those of the rice soaked at 40℃ by the end of polished rice grains and the water absorption rates of six the soaking period（Table 4）. rice samples after 30 min of soaking. Significant correla- Overall, we found that at equilibrium, the water absorp- tion（r＝0.81, n＝18） was also observed between the sur- tion rates of Japonica rice soaked at lower temperatures face areas and the maximum water absorption rates. were higher than those of rice soaked at higher tempera- Therefore, the surface area of the rice grains may influ- tures. This “reverse phenomenon” occurred in these three ence the water absorption rate. cultivars descended from the different rice lines of pol-

Table 4. Maximum water absorption rates of milled rice grain samples of the three rice cultivars soaked at different temperatures

Soaking Maximum water absorption rate（%） temperature（℃） Kinuhikari Sasanishiki Hatsushimo 5℃ 23.7±0.3a 20.5±0.1a 21.0±0.2a Polished rice 20℃ 23.7±0.1a 20.5±0.2a 19.5±0.3b 40℃ 22.6±0.2b 18.9±0.4b 18.7±0.3c 5℃ 22.0±0.1a 20.5±0.1a 21.1±0.2a 70% Polished rice 20℃ 21.7±0.3a 20.1±0.1a 19.2±0.2b 40℃ 20.5±0.3b 19.4±0.3b 18.5±0.3c a-c: Mean ± SD values in a column at each milling rate of rice with different letters are significantly different, p＜0.05, n＝3

（165） 37 日本調理科学会誌（J. Cookery Sci. Jpn.） Vol. 52 No. 3（2019） ished and 70% polished Japonica rice. It is suggested that analyses of the rice samples were performed to observe this occurs for many cultivars of Japonica rice. the changes in the crystalline structure of rice starch To elucidate whether or not the granular shape of the caused by water soaking. The XRD patterns of the three rice functionally or mechanically affects this trend, we cultivars of rice samples soaked in water for 24 h are compared the results of the soaking tests between grains shown in Fig. 4. The rice grains soaked at 5℃ and 40℃ of the three cultivars of rice and rice flour prepared from and the rice flour soaked at 5℃ displayed typical A-type each of the same cultivars of rice. The water absorption XRD patterns, indicating the crystalline structure of ordi- curves of the rice grains soaked at 5℃ and 20℃ inter- nary rice starch （Buléon et al., 1998）, whereas slight sected those of the rice grains soaked at 40℃（Fig. 1）, decreases in the peak intensities of the rice flour soaked whereas none of those of rice flour intersected（Fig. 3）. at 40℃ were observed. From these results, we can make The water absorption rates of the rice flour were substan- the following considerations. Soaking at 5℃ maintained tially higher than those of the rice grains, possibly because the crystalline structures of both the rice grains and the of the larger specific surface area of the rice flour com- rice flour. After soaking at 40℃, the rice flour became pared to that of the rice grains. The water absorption more amorphous while the rice grains maintained their rates of the rice flour soaked at 40℃ were higher than crystalline structures. The reason the XRD spectra of the those of the rice flour soaked at 5℃（Fig. 3）. The XRD rice grains and flour are different is that whereas changes in the crystalline structures of starch soaked in water 600 were exclusively on the surface of the rice grains, as ）

% 500 occurred in almost all parts of the rice flour, the crystal- （

400 line structures in the other parts of the rice grains were maintained. It is suggested that the intersections of the 300 water absorption curves of the rice grains soaked at 5℃ ate r absorp ti on 200 W and 40℃ occurred due to the disturbances of the water 100 absorption by the deterioration in the starch crystalline 0 structures followed by the swelling of the starch granules 0 60 120 180 240 Soaking Time (min) on the surface of the rice grains. Fig. 3. Water absorption of polished rice flour. The soaking tem- Furthermore, when soaked in warm water such as at peratures were as follows: 40℃ （（○） Kinuhikari, （□） 40℃ in this study, the action of endogenous enzymes, for Sasanishiki, and （△） Hatsushimo） and 5℃ （（●） Kinuhikari,（■） Sasanishiki, and（▲） Hatsushimo）. example, amylase in rice grains on the starch granules, is Mean ± SD, n＝3 expected. We could not determine how the enzymatic

Kinuhikari Sasanishiki Hatsushimo

(d)

(c) Intensity (cps) (cps) Intensity

(b)

(a)

2θ(°) 2θ(°) 2θ(°)

Fig. 4. XRD spectra of soaked rice grains at 5℃（a）, soaked rice grains at 40℃（b）, soaked rice flour at 5℃ （c）, and soaked rice flour at 40℃（d） for 24 h. The rice cultivars are indicated at the upper right edge of each spectrum.

38 （166） Water Absorption Rates of Rice at Different Soaking Temperatures activities influence the water absorption rate of the rice REFERENCES grains examined in this study. However, they should effect Brewer’s Rice Research Group（1996/10/15）, National Stan- some change in rice starch, and there is the potential for dard Analysis for Brewing （in Japanese）, http:// www.sakamai.jp/pdf/bunseki.pdf,（2018/11/08） its influence on the water absorption rate of rice grains. Buléon, A., Colonna, P., Planchot, V. and Ball, S.（1998）, Starch Moreover, kinds of enzyme and enzymatic activities vary granules: structure and biosynthesis, Int. J. Biol. Macromol., according to the cultivars of rice （Kishio and Aoyagi, 23, 85–112 2014）. The difference of the water absorption rate of rice Ehara, T., Ichikawa, T., Mitsumura, Y. and Shimomura, M. （1996）, Relationship between cooking conditions and tastes grains among the three rice cultivars might be due to the for three kinds of （in Japanese）, Nihon Chourikagaku varieties of enzyme, especially glycoside hydrolase. Kaishi（ J. Cookery Sci. Jpn.）, 29, 298-305 Many studies have characterized the relationship Hitomi, T., Ashida, N. and Niigaki, H.（2015）, Evaluation of the between the absorption rate of rice before cooking and mixed rice of COCORO, large-germ-rice, and Himegonomi, rice with low-amylose content（in Japanese）, Mimasaka the characteristics of the cooked rice（Matsumoto and Daigaku, Mimasaka Daigaku Tankidaigakubu Kiyou Suzuki, 1971; Seki and Kainuma, 1982; Seki and Kainuma, （Bulletin of Mimasaka University Mimasaka Junior Col- 1986; Kainuma and Ema, 1987; Tubone et al., 2006; lege）, 58, 111-115 Kainuma, 2009）. From these studies, we can deduce that Kainuma, Y. （2009）, Study on the cookery of rice （in Japanese）, Nihon Kaseigaku Kaishi（ J. Home Econ. Jpn.）, the differences in water absorption rates influence the 60, 775-783 quality of the cooked rice. Kainuma, Y. and Ema, S.（1987）, The effect of ratio of water to Further studies are needed to elucidate the mechanisms rice on cooking（in Japanese）, Nihon Kaseigaku Kaishi（ J. of the observed effects of soaking water temperature on Home Econ. Jpn.）, 38, 567-575 Kainuma, Y. （2012）, “Okome to Gohan no Kagaku” （in the water absorption rates. Such studies should focus on Japanese）, Kenpakusha, Tokyo, pp. 45-46 exploring the effects of the characteristics of the starch Kato, Y., Nakayama, S., Shirane, Y. and Ito, S.（2010）, Study on granules, the states of the starch in the surface and inside a cooking of purple- （in Japanese）, Hirosaki of the rice grains, and the effects of the endogenous Daigaku Kyouikugakubu Kiyou（ Bulletin of the Faculty of Education, Hirosaki University）, 104, 95-103 enzymes in rice grains on the water absorption rate. Kishio, S. and Aoyagi, Y.（2014）, Cultivar- and region-specific CONCLUSION differences in the starch-degrading enzymes produced dur- ing rice soaking（in Japanese）, Nippon Shokuhin Kagaku 61 We studied the influence of soaking temperatures （5℃, Kougaku Kaishi（ J. Jpn. Soc. Food Sci. Technol.）, , 232- 243 20℃, and 40℃） on the water absorption in three cultivars Kuwada, H., Teramoto, A., Jibu, Y., Tabuchi, M. and Fuchigami, of polished rice and 70% polished rice. Our experiments M.（2011）, Texture and fine structure of cooked brown rice showed that at equilibrium, the water absorption rates of （in Japanese）, Nihon Chourikagaku Kaishi（ J. Cookery 44 Japonica rice soaked at lower temperatures were higher Sci. Jpn.）, , 137-144 Maruyama, E. and Sakamoto, K.（1992）, A basic study on cook- than those of rice soaked at higher temperatures. This ing of rice（part 1）: the influence of rice soaking by hot phenomenon is found in these three cultivars descended water（in Japanese）, Nihon Kaseigaku Kaishi（ J. Home from the different rice lines used in this study. It is sug- Econ. Jpn.）, 43, 97-103 gested that this occurs for many cultivars of Japonica rice. Matsumoto, F. and Abe, S.（1962）, Studies on the cooking qual- ity of polished rice flour（in Japanese）, Kaseigaku Zasshi We confirmed that it was specific to the granular shape of （J. Home Econ. Jpn.）, 13, 311-316. the rice grain. We deduce that the quality of the cooked Matsumoto, F. and Suzuki, Y.（1971）, Method of cooking rice rice is influenced by the differences in the water absorp- （part 2）: effect of pre-soaking raw rice and cooking time （in Japanese）, Kaseigaku Zasshi（ J. Home Econ. Jpn.）, 22, tion rates. 29-34 ACKNOWLEDGEMENT Matsumoto, F.（1972）, “Chourigaku”（in Japanese）, Koseikan, Tokyo, p. 100 The authors acknowledge Tomiyoshi Hanioka in Ministry of Education, Culture, Sports, Science and Technology, The Standard Tables of Food Composition in Japan（in Ichikawa city, Hyogo, Miyagi Prefectural Furukawa Japanese）（2010/11）, http://www.mext.go.jp/component/ Agricultural Experiment Station, and Gifu Prefectural b_menu/shingi/toushin/__icsFiles/afieldfile/2011/01/25/ Agricultural Technology Center for providing the three 1299012_1.pdf,（2018/11/08） cultivars of Japonica rice used in this study. Miwa, S., Kuroda, A., Oda, H., Takaya, T. and Nishinari, K. （2002）, The relationship between the sensory properties of cooked rice and the water uptake of raw rice（in Japanese）,

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Nippon Shokuhin Kagaku Kougaku Kaishi（ J. Jpn. Soc. Japanese）, Kaseigaku Zasshi（ J. Home Econ. Jpn.）, 37, Food Sci. Technol.）, 49, 32-39 93-99 Naganuma, S.（2003）, Effects of granular size of rice powder on Toyoshima, H., Okadome, H. and Ohtsubo, K.（1994）, Food physicochemical and cooking properties （in Japanese）, Research Institute, NARO Report 1994 （in Japanese）, Akita Daigaku Kyouikubunkagakubu Kenkyuukiyou http://www.naro.affrc.go.jp/project/results/laboratory/ （Memoirs of Faculty of Education and Human Studies, nfri/1994/nfri94-07.html,（2018/11/08） Akita University（ Natural science））, 58, 29-35 Tubone, M., Wada, T. and Ogata, T.（2006）, Palatability and Onishi, M., Ogawa, N., Yamanaka, N. and Shoji, I. （1997）, physicochemical properties of cooked rice without soaking Physico-chemical properties and histo-chemical studies of （in Japanese）, Nihon Sakumotugakukai Kyushushibu rice milled under various conditions（in Japanese）, Nihon （Report of the Kyushu Branch of the Crop Science Society Kaseigaku Kaishi（ J. Home Econ. Jpn.）, 48, 303-313 of Japan）, 72, 56-57 Okuno, M. and Adachi, K.（1992）, Relationship between physi- Ueda, S. （1994）, A study on the taste of cooked rice （in cal properties and water absorption characteristics of milled Japanese）, Obihiro Otani Tankidaigaku Kiyou（ Bulletin of rice（in Japanese）, Nihon Sakumotugakukai Kiji（ Jpn. J. Obihiro Otani Junior College）, 31, 63-69 Crop Sci.）, 61, 244-250 Yoshida, M. and Horigane, A.（2008）, Observation of water Sakamoto, K., Morii, S. and Ueda, M.（2015）, Differences in penetration process into rice grains by MRI（in Japanese）, water absorption of rice when soaked in warm and cold Nihon Zhouzou Kyokaishi （ J. Soc. Brew. Japan）, 103, water （in Japanese）, Nihon Chourikagaku Kaishi （ J. 10-16 Cookery Sci. Jpn.）, 48, 193-199 Yoshizawa, K., Ishikawa, Y. and Hujie, I.（1975）, Study on sake Seki, C. and Kainuma, Y.（1982）, A study of rice cooking（part rice（part 10）: the influence of sake rice soaked by hot 2） soaking times as a factor controlling rice cooking（in water（in Japanese）, Nihon Zhouzou Kyokai Zasshi（ J. Japanese）, Kaseigaku Zasshi（ J. Home Econ. Jpn.）, 33, Soc. Brew. Japan）, 70, 910-912 228-234 Seki, C. and Kainuma, Y.（1986）, A study of rice cooking（part （Received Nov, 16, 2018 Accepted Jan, 16, 2019） 4） boiling time as a factor controlling rice cooking （in

ジャポニカ米の異なる温度での長時間浸漬における吸水率の逆転現象

森井沙衣子*，**，*** 坂 本 薫*，** 白杉（片岡）直子***§

和文抄録 3 品種のジャポニカ米（キヌヒカリ，ササニシキ，ハツシモ）を用いて，前報で改良した吸水率算出法により，米の吸 水率に与える浸漬温度の影響について検討した。搗精度は，それぞれ 93％搗精米（7 分つき米）および 91％搗精米（精白 米）とした。米の浸漬温度は 5℃，20℃，40℃とし，浸漬時間は 10 分から吸水率が平衡になるまでの最長 24 時間までと した。吸水曲線を比較したところ，3 品種のすべての精白米と 7 分つき米において，平衡状態に達するまでに，5℃，20℃ 浸漬の吸水曲線が 40℃浸漬の吸水曲線を超える現象が観察された。この「逆転現象」は米が粒状であることに起因するこ とがわかった。本研究で用いた系の異なる 3 品種に起こった吸水率の逆転は従来の通説に反する現象と言える。ジャポニ カ米の多くの品種で，この現象が起こる可能性が示唆された。

キーワード：ジャポニカ米，精白米，浸漬温度，吸水

* 兵庫県立大学環境人間学部環境人間学科 ** 兵庫県立大学先端食科学研究センター *** 神戸大学大学院人間発達環境学研究科人間環境学専攻 § 責任著者連絡先 神戸大学大学院人間発達環境学研究科人間環境学専攻 〒 657-8501 兵庫県神戸市灘区鶴甲 3-11 E-mail：[email protected]

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