2019 Volume 25 Issue 2 Pages 227-235
In this study, we investigated the starch, pasting, thermo-physical, and iodine absorption spectrum properties of Chou 2418, a new amylopectin long-chain (ALC) rice cultivar developed by crossing a high amylose rice cultivar Koshinomenjiman and amylose extender mutant rice cultivar EM10. Chou 2418 exhibited high apparent amylose content, high pasting temperature, and low breakdown viscosity. The iodine absorption spectrum indicates that Chou 2418 contains long-chain glucans of amylopectin and also has high amylose content. Boiled rice grains of Chou 2418 contained more resistant starch (RS) than those of high amylose rice cultivars. The RS content of Chou 2418 rice grains increased following the retrogradation of starch granules, and the grains became hard and non-sticky. The RS content of Chou 2418 was 25 times higher than that of Koshihikari, the most popular rice cultivar. Taken together, these data suggest that Chou 2418 is a suitable raw material for bio-functional foods that help prevent diabetes.
Rice is a major staple food across the world, and more than 90% of rice is processed and consumed in Asia. Although usually consumed as boiled rice, it is also used as a raw material for preparing other foods, such as rice bread (Nishita, 1977; Nakamura et al., 2009), rice sponge cake (Ishida and Morohashi, 2014) and batter for fried food (Nakamura et al., 2010a). Rice is the only crop for which Japan is self-sufficient, although rice consumption has steadily decreased since the 1970s. Therefore, to increase consumption of rice, breeding of new rice cultivars has been conducted not only for improved taste but also higher yield (Sakai, 2010) and greater health functionality (Miura et al., 2007).
More than 400 million people worldwide are reported to have diabetes, and prevention and treatment of this disease are extremely important (The World Health Organization (WHO), 2016). The WHO and Food and Agriculture Organization of the United Nations recommend foods with low glycemic index (GI) to prevent diabetes (Nantel, 2003). Many factors influence the GI of foods, including the type of sugars, structure of starch, processing technique, and cooking method (Arvidsson-Lenner et al., 2004). Indigestible dextrin is made from hydrolyzed cornstarch residues by amylase, and it is used in various foods such as “low-GI drinks” (Okuma et al., 2006). Resistant starch (RS) is a type of starch that is tolerant to α-amylase and is recognized as a functional ingredient to control postprandial blood glucose level and to reduce the GI of foods (Englyst et al., 1987).
Rice starch is generally highly digestible and abruptly increases the postprandial blood glucose level. Therefore, one of the objectives of rice breeders is to develop rice cultivars with lower carbohydrate bioavailability. The digestibility of rice starch depends on amylose content and the molecular structure of amylopectin and cell walls in the embryo of rice grains. It has been reported that high amylose rice cultivars reduce postprandial glucose response due to the high RS content (Ohtsubo et al., 2010; Zenel et al., 2015; Ohtsubo et al., 2016). Boiled grains of the sugary rice cultivar Ayunohikari exhibit low digestibility (Arai et al., 2010) and attenuated postprandial blood glucose levels in humans (Arai et al., 2011) because of the strong endosperm cell wall.
Previously, Sato (1994) developed amylose extender (ae) mutant rice cultivars via chemical mutagenesis using N-methyl N-nitrosourea (MNU) at Kyushu University in Japan. The ae mutants lack starch branching enzyme IIb which increases the number of short-chain glucans and decreases the gelatinization temperature (Nishi et al., 2001). The representative ae mutant rice cultivar EM10 is promising as a raw material for low-GI foods, such as bread and noodles (Nakamura et al., 2010b). Ohtsubo and Nakamura (2015) reported that amylopectin long-chain (ALC) rice cultivars, namely ae mutants, contain a substantial amount of RS.
In recent years, new ALC rice cultivars have been developed via cross-fertilization between ae mutant rice and rice cultivars with excellent processing properties (Wada et al., 2018). At the Crop Research Center of the Niigata Agricultural Research Institute in Japan, a new ALC rice cultivar Chou 2418 has been developed through cross-fertilization between EM10 and a high amylose rice cultivar suitable for rice noodles, Koshinomenjiman (Ishizaki et al., 2011).
In this study, we evaluated and compared the components and starch characteristics of Chou 2418 and conventional rice cultivars. In addition, we prepared Chou 2418 as boiled rice and examined the change in the RS and physical properties during storage. We found Chou 2418 to be a promising raw material for low-GI foods, and it is necessary to research processing techniques based on starch properties.
Materials Seven rice cultivars were used in this study. Medium and low amylose content rice cultivars Koshihikari and Milkyqueen, respectively, and the glutinous rice cultivar Koganemochi were purchased from a local rice store in Niigata, Japan. A high yielding rice cultivar Akidawara, high amylose rice cultivars Koshinomenjiman and Koshinokaori, and ALC rice cultivar Chou 2418 were cultivated in 2017 at the Crop Research Center, Niigata Agricultural Research Institute, Japan.
Preparation of polished rice and white rice flour Grains of all brown rice, except Chou 2418, were polished with a friction type rice polisher (VP-32, Yamamoto Seisakusyo Co., Ltd., Yamagata, Japan) to a milling yield of 90%. Grains of brown Chou 2418 rice were polished with an abrasive type rice polisher (Grain Testing Mill™, SATAKE Corp., Hiroshima, Japan) to prevent breakage on milling, and to a milling yield of 88% for the complete removal of bran. White rice flour was prepared for analysis using a cyclone mill (Tecator Cyclotec 1093; FOSS JAPAN, Yokohama, Japan) fitted with a screen with 0.5-mm diameter pores.
Analyses of nutritional components The nutritional components of white rice were analyzed as follows. Protein content was measured by the macro-Kjeldahl method using a Kjeltec™ 8400 Analyzer Unit (FOSS JAPAN, Yokohama, Japan); a conversion factor of 5.95 was used to convert nitrogen into protein. Lipid content was measured by the Soxhlet extraction method using diethyl ether as the solvent. Ash content was calculated from the difference in mass of the sample before and after incineration in a furnace at 550 °C. Carbohydrate content was calculated by subtracting the weight of protein, lipid, and ash contents from the total weight. All nutritional components of white rice were calculated on a dry matter basis.
Measurement of apparent amylose contents (AAC) The AAC of white rice was measured by the iodine colorimetric method (Juliano, 1971). Potato amylose type III (Sigma Chemical Co., St. Louis, MO, USA) and waxy rice starch (prepared from Koganemochi) were used as standards for measuring amylose and amylopectin content, respectively. Iodine absorption was measured at 620 nm using a spectrophotometer (UV-2700; Shimadzu Corporation, Kyoto, Japan).
α-Amylase activity The α-amylase activity of white rice was measured using the α-amylase activity assay kit (Megazyme Ltd., Wicklow, Ireland). White rice flour (3.0 g) was extracted with 20 mL of extraction buffer (pH 5.4) at 40 °C for 20 min. The supernatant (0.1 mL) and substrate (blocked p-nitrophenyl maltoheptaoside, 0.1 mL) were pre-incubated at 40 °C for 5 min. The supernatant and substrate were mixed, and the solution was incubated at 40 °C for 20 min. The reaction was stopped by addition of the stopping reagent (1.5 mL), and absorbance was measured at 400 nm.
Pasting properties The pasting properties of white rice flour were measured using a Rapid Visco-Analyzer (RVA, model 4; Perten Instruments, Stockholm, Sweden). The viscosity of 11.8% aqueous suspensions of rice flour were analyzed following this processing: hold at 50 °C for 1 min, heating from 50 to 93 °C at a rate of 10.8 °C/min, holding at 93 °C for 7 min, cooling from 93 to 50 °C at a rate of 10.8 °C/ min, and finally holding at 50 °C for 3 min. The pasting temperature, maximum viscosity, minimum viscosity, final viscosity, break down (maximum viscosity – minimum viscosity), setback (final viscosity – maximum viscosity), and consistency (final viscosity – minimum viscosity) were determined using the software supplied with the RVA.
Differential scanning calorimetry (DSC) The thermo-physical properties of each sample were analyzed using a differential scanning calorimeter (DSC8000; Perkin Elmer Japan, Yokohama, Japan). White rice flour (15 mg) and distilled water (45 µL) were encapsulated in a stainless pan (Perkin Elmer Japan, Yokohama, Japan), which was then placed in the DSC instrument and scanned from 4 °C to 130 °C at a heating rate of 10 °C/min. The onset temperature of gelatinization (Tgel), peak temperature (Tpeak), end temperature of gelatinization (Tend), gelatinization temperature range (Tend – Tgel, Huang et al., 2015) and endothermic enthalpy (ΔH) were determined with the software supplied with the DSC8000. ΔH was calculated on a dry weight basis (J/g).
Analysis of iodine absorption spectrum and degree of polymerization (DP) The iodine absorption spectrum of each sample (100 mg) was measured by a colorimetric method with measurement from 250 to 750 nm using a spectrophotometer. Distilled water was used as a control. Absorbance was measured at λmax, i.e., the peak wavelength in the visible light region of the iodine-starch reaction that was highly correlated with the DP of glucose and amylopectin and molecular size of amylose. The new λmax (used for estimating amylopectin fraction) and DP (Fa, DP ≤ 12; Fb1+2, 13 ≤ DP ≤ 36; Fb3, DP ≥ 37) of amylopectin were calculated as follows (Nakamura et al., 2015):
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Preparation of boiled rice Rice grains (100 g) with a moisture content of 15.5% (w/w) were mixed with 1.4 volumes of water; the moisture content in rice grains was subtracted from the amount of water needed. The mixture was soaked at room temperature for 1 h. The soaked rice grains were boiled using an electric rice cooker (NS-NE05; Zojirushi Corp., Osaka, Japan), and the boiled rice was examined either immediately or after storage at 20 °C for 6 h or at 5 °C for 24 h. Subsequently, the boiled rice was instantaneously frozen in liquid nitrogen, freeze-dried, and pulverized.
RS in boiled rice The RS in boiled rice was measured according to the AOAC method using an RS assay kit (Megazyme Ltd., Wicklow, Ireland). Flour (100 mg) prepared from boiled rice was digested with pancreatin and amyloglucosidase at 37 °C for 6 h, and glucose was measured using a spectrophotometer at 510 nm.
Analysis of physical properties of boiled rice After storing boiled rice under different conditions described above, the physical properties of a single rice grain were measured according to the low-compression/high-compression method using a Tensipresser (TTP-50BXII2006, TAKETOMO ELECTRIC, Inc., Tokyo, Japan). The measurement conditions were as follows: plunger, 4.9 cm2; bite speed, 2 mm/s; distance, 20 mm; 1st deformation, 25%; 2nd deformation, 90%; and load cell, 10 kg. Values of whole grain hardness (H2) and stickiness (S2) were measured 25 times using the software supplied with the Tensipresser (Okadome et al., 1999).
Statistical analysis Data were analyzed using analysis of variance and Tukey's test with Excel Statics 2010 (Social Survey Research Information Co., Ltd., Tokyo, Japan). Statistical significance of differences was defined at p < 0.05.
Nutritional composition of white rice samples The nutritional composition of white rice samples is summarized in Table 1. The protein content of Chou 2418 was comparable with that of Koganemochi and Milkyqueen. The lipid and ash contents of Chou 2418 were significantly higher than those of other cultivars (p < 0.05). By contrast, the carbohydrate content in Chou 2418 was the lowest among all seven rice cultivars (p < 0.05). A similar observation has been previously reported in a Korean ae mutant rice cultivar, Goami2 (Noro et al., 2018).
| Protein | Lipid | Ash | Carbohydrate | AAC | α-amylase activity | |
|---|---|---|---|---|---|---|
| (%) | (%) | (%) | (%) | (%) | (CU) | |
| Koganemochi | 5.19 ± 0.03 ab | 0.78 ± 0.01a | 0.34 ± 0.021a | 93.7 ± 0.06 ab | - | 0.03 ± 0.002 ab |
| Milkyqueen | 5.04 ± 0.02 ac | 0.59 ± 0.04 b | 0.37 ± 0.015 a | 94.0 ± 0.03 ac | 6.7 ± 0.094 a | 0.03 ± 0.001 ac |
| Koshihikari | 4.46 ± 0.05 d | 0.89 ± 0.03 c | 0.48 ± 0.005 b | 94.2 ± 0.08 c | 15.0 ± 0.745 b | 0.03 ± 0.001 c |
| Akidawara | 4.82 ± 0.02 c | 0.71 ± 0.04 d | 0.34 ± 0.018 a | 94.1 ± 0.04 c | 17.6 ± 0.555 b | 0.04 ± 0.001 b |
| Koshinomenjiman | 5.33 ± 0.01 be | 0.72 ± 0.04 d | 0.37 ± 0.008 a | 93.6 ± 0.03 b | 30.9 ± 0.696 c | 0.02 ± 0.000 d |
| Koshinokaori | 5.57 ± 0.25 e | 0.81 ± 0.03 ac | 0.42 ± 0.018 c | 93.2 ± 0.29 d | 33.0 ± 1.033 c | 0.01 ± 0.001 d |
| Chou2418 | 5.07 ±0.03 bc | 2.17 ± 0.01 e | 0.58 ± 0.007 d | 92.2 ± 0.03 e | 56.8 ± 2.154 d | 0.22 ± 0.004 e |
The nutritional composition is calculated in terms of dry weight basis. Each value represents the average±standard deviation (n = 3). Different letters indicate signifcant differences among rice samples, as shown by Tukey's test (p < 0.05).
AAC was the highest in Chou 2418, followed by Koshinokaori and Koshinomenjiman. AAC is a good indicator of processing applications and cooking performance of rice grains (Nakamura et al., 2016) and is related to the actual amylose content and structure of amylopectin. Because the long-chain glucans of amylopectin also reacted with iodine, the actual amylose content was lower than the AAC. From the genetic background of Chou 2418, we presume that the AAC is involved with many long-chain glucans in amylopectin.
The α-amylase activity of Chou 2418 was 0.22 Ceralpha Unit per gram (CU/g), which was 5- to 22-times higher than that of other rice cultivars. Previously, Nakamura et al. (2011) reported a very high α-amylase activity of EM10 (0.19 CU/g) among ae mutant rice cultivars. These data suggest that the α-amylase activity of Chou 2418 is derived from its parent rice cultivar, EM10.
Pasting properties of rice samples The pasting properties of white rice flour are summarized in Table 2. The pasting properties measured by RVA are good indicators of the suitability of rice cultivars for processing (Toyoshima et al., 1997). Among the seven rice cultivars, Chou 2418 showed the highest pasting temperature. In addition, Chou 2418 showed very low maximum viscosity and breakdown values compared with non-glutinous rice cultivars, except Koganemochi, suggesting that the starch granule is resistant to gelatinization and breaking down due to stirring at maximum temperature.
| Maxi | Mini | Break.d | Final | Setback | Past.t | Consis | |
|---|---|---|---|---|---|---|---|
| (RVU) | (RVU) | (RVU) | (RVU) | (RVU) | °C | (RVU) | |
| Koganemochi | 130.8 ± 0.88 a | 71.1 ± 0.42 a | 59.6 ± 0.48 a | 106.0 ± 0.05 a | −24.7 ± 0.05 a | 71.7 ± 0.34 a | 34.9 ± 0.44 a |
| Milkyqueen | 401.6 ± 0.63 b | 150.2 ± 0.88 b | 251.4 ± 0.46 b | 222.6 ± 2.74 b | −179.0 ± 2.49 b | 69.2 ±0.15 b | 72.4 ± 2.02 b |
| Koshihikari | 421.4 ± 2.20 c | 221.6 ± 2.48 c | 199.8 ± 3.56 c | 341.5 ± 2.04 c | −79.9 ± 2.46 c | 69.1 ± 0.62 b | 119.9 ±1.19 c |
| Akidawara | 366.0 ± 1.22 d | 185.3 ± 3.47 d | 180.7 ± 2.73 d | 300.3 ± 4.55 d | −65.6 ± 3.53 d | 66.6 ±0.10 c | 115.0 ±1.54 d |
| Koshinomenjiman | 233.6 ± 1.15 e | 135.1 ± 1.64 e | 98.4 ± 1.65 e | 283.2 ± 1.67 e | 49.6 ± 0.80 e | 66.1 ± 0.72 c | 148.0 ± 2.45 e |
| Koshinokaori | 309.1 ± 1.09 f | 177.2 ± 0.50 f | 131.9 ± 0.59 f | 343.1 ± 1.37 f | 34.1 ± 0.94 f | 77.8 ± 0.28 d | 165.9 ± 1.01 f |
| Chou2418 | 176.7 ± 1.43 g | 152.6 ± 1.37 b | 24.2 ± 0.09 g | 235.7 ± 1.42 g | 59.0 ± 1.42 g | 80.7 ±0.12 e | 83.2 ± 1.51 g |
Each value represents the average± standard deviation (n = 3). Different letters indicate signifcant differences among rice four samples, as shown by Tukey's test (p < 0.05).
The α-amylase activity of cereals is known to affect the gelatinization viscosity (Matsukura et al., 2004). The measurement of viscosity using copper sulfate aqueous solution instead of distilled water showed that the increase in maximum viscosity of Chou 2418 was lower than that of Koganemochi (Chou 2418, 22.9 Rapid Visco Units (RVU); Koganemochi, 129.7 RVU). This suggests that the α-amylase activity of Chou 2418 does not remarkably affect its gelatinization viscosity.
The final viscosity, a parameter of the degree of starch retrogradation, of Chou 2418 was identical to that of the low amylose rice cultivar Milkyqueen but significantly lower than that of the high amylose rice cultivars Koshinomenjiman and Koshinokaori. However, as described below, boiled rice grains of Chou 2418 were certainly easy to retrograde. It is possible that the final viscosity of Chou 2418 was low because the starch granules in rice grains did not completely gelatinize under the experiment conditions (maximum temperature = 93 °C). Inouchi et al. (2005) showed that the setback value (final viscosity – maximum viscosity) also affects the degree of starch retrogradation. In this study, Chou 2418 showed the highest setback value among all rice cultivars, indicating that starch of Chou 2418 is easily retrograded. Partial gelatinization suppresses the swelling of starch granules and reduces the final viscosity. However, the setback value calculated from viscosity under the same gelatinization condition, which reduced the influence of gelatinization on viscosity, may reflect the original properties of the starch granules. Nakaura et al. (2012) demonstrated a positive correlation between the pasting temperature and AAC in non-glutinous rice cultivars. In this study, significant correlation was not observed between these two parameters (r = 0.76; p > 0.05); instead, the pasting temperature of Koshinomenjiman was very low despite the high AAC.
Thermo-physical properties of rice samples All rice samples showed a typical endothermic enthalpy peak, indicating the gelatinization of crystalline regions of starch granules. The DSC endothermic profiles of rice samples are summarized in Table 3. The Tgel of Chou 2418 was the second highest among all rice cultivars. In general, the Tgel of rice starch increases with increasing AAC. The higher Tgel value of Chou 2418 may be due to a strong interaction between long-chain glucans of amylopectin, which is also estimated as AAC. The lowest Tgel among all rice cultivars was observed for Koshinomenjiman, which is consistent with its pasting temperature measured by RVA. Analysis of ΔH showed that hydrogen bonds in starch granules were disrupted by gelatinization. Among all cultivars, Chou 2418 showed the highest ΔH and the most numbers of hydrogen bonds. Nakaura et al. (2011) reported that the Tpeak value shows a positive correlation with the ΔH value in the endosperm starch of rice grains. In this study, significant correlation was observed between the two values (r = 0.98; p < 0.05). Among all cultivars, the Tend − Tgel value was the highest for Chou 2418 and lowest for Koshinokaori. The high Tend − Tgel value of Chou 2418 indicates the diversity of the crystal structure in its starch granules. In addition, the high Tend value of Chou 2418 showed that the high temperature condition was necessary for complete gelatinization.
| Tgel | Tpeak | Tend | Tend-Tgel | ΔH | |
|---|---|---|---|---|---|
| (°C) | (°C) | (°C) | (°C) | (J/g) | |
| Koganemochi | 70.3 ± 0.3 a | 77.6 ± 0.28 a | 86.5 ± 0.5 a | 16.3 ± 0.28 ab | 13.9 ± 0.54 a |
| Milkyqueen | 67.5 ± 0.04 b | 74.9 ± 0.29 b | 82.6 ± 0.76 b | 15.1 ± 0.78 ac | 12.4 ± 0.65 ab |
| Koshihkari | 67.4 ± 0.25 b | 75.2 ± 0.50 b | 83.3 ± 0.73 b | 16.0 ± 0.61 a | 13.5 ± 0.38 a |
| Akidawara | 64.7 ± 0.62 c | 72.6 ± 0.49 c | 80.1 ± 0.59 c | 15.4 ±0.18 ad | 12.2 ± 0.77 ab |
| Koshinomenjiman | 62.1 ± 0.66 d | 69.8 ± 0.53 d | 79.0 ± 0.61 c | 16.9 ± 0.98 ab | 10.9 ± 0.56 bc |
| Koshinokaori | 76.5 ± 0.52 e | 82.9 ± 0.51 e | 90.5 ± 0.92 d | 14.0 ± 0.44 cd | 15.8 ± 1.08 d |
| Chou2418 | 73.7 ± 0.50 f | 82.9 ± 0.47 e | 91.8 ± 0.91 d | 18.1 ± 1.06 b | 16.1 ± 0.14 d |
Each value represents the average ± standard deviation (n = 3). Different letters indicate significant differences among rice samples, as shown by Tukey's test (p < 0.05).
Iodine absorption characteristics and estimation of DP The iodine absorption characteristics and DP of rice samples are listed in Table 4. λmax of Chou 2418 was higher than that of other rice cultivars. In a previous study, the λmax value of ae mutant rice cultivars was not as high as that of high amylose rice cultivars. Fukahori et al. (1996) reported that high molecular weight amyloses have a higher λmax value, indicating the actual amylose content of starch granules is high. This suggests that the actual amylose content of Chou 2418 is equivalent to that of Koshinomenjiman and Koshinokaori. Igarashi et al. (2009) reported a positive correlation between λmax and AAC in rice starch. In this study, a positive correlation was observed between λmax and AAC (r = 0.81; p < 0.05). On the other hand, the iodine absorption of Koganemochi was very low and did not show peak absorbance at 400–750 nm. The absorbance at λmax of Chou 2418 was higher than that of Koshinomenjiman or Koshinokaori. Additionally, Chou 2418 showed depletion of short-chain glucans (DP ≤ 12) and enrichment of long-chain glucans (DP ≥ 37) in comparison with other rice cultivars. These data suggest that Chou 2418 is a new type ALC rice cultivar containing not only long-chain amylopectin but also more amylose.
| λmax | Aλmax | New λmax | Fa DP ≤ 12 | Fb1+2 13 ≤ DP ≤ 36 | Fb3 DP ≥ 37 | |
|---|---|---|---|---|---|---|
| (nm) | (%) | (%) | (%) | |||
| Milkyqueen | 533.0 ± 3.6 a | 0.240 ± 0.004 a | 0.725 ± 0.240 a | 26.02 | 42.34 | 31.64 |
| Koshihikari | 553.0 ± 1.0 b | 0.301 ± 0.003 b | 0.499 ± 0.011 a | 28.37 | 50.10 | 21.53 |
| Akidawara | 561.7 ± 2.1 c | 0.306 ± 0.003 b | 0.454 ± 0.017 a | 28.83 | 51.64 | 19.53 |
| Koshinomenjiman | 575.3 ± 1.2 d | 0.393 ± 0.003 c | 0.592 ± 0.015 a | 27.20 | 47.14 | 25.66 |
| Koshinokaori | 575.3 ± 0.6 d | 0.414 ± 0.007 d | 0.631 ± 0.013 a | 26.76 | 45.84 | 27.40 |
| Chou2418 | 575.7 ± 0.0 d | 0.610 ± 0.006 e | 1.078 ± 0.033 b | 21.66 | 30.93 | 47.41 |
λmax, absorbance at λmax (A λmax) and New λmax represent the average ± standard deviation (n = 3). Different letters indicate signifcant differences among rice samples, as shown by Tukey's test (p < 0.05). Degree of polymerization (DP) was calculated from New λmax value according to the method of Nakamura et al. (2015).
RS content of boiled rice grains The RS content of boiled rice prepared from various rice cultivars is shown in Fig. 1. In the case of boiled rice just after cooking, the RS content of Chou 2418 (5.30%) was significantly higher than those of other rice cultivars (p < 0.05), specifically 25 times higher than that of Koshihikari (0.21%) and 3.8 times higher than that of Koshinomenjiman (1.41%). The RS content of Koganemochi and Milkyqueen was very low, i.e., 0.06% and 0.12%, respectively. In general, the RS in rice grains increases as the AAC increases (Zhu et al., 2011) or as the ungelatinized starch increases. Since higher pasting temperature in RVA measurement is likely to cause the partial gelatinization of starch granules during cooking, we think that these factors led to an increase of RS in Chou 2418.
RS of boiled rice prepared from various rice cultivars.
RS is calculated in terms of dry weight. Different letters indicate significant differences among boiled rice (just cooking), as shown by Tukey's test (p < 0.05). Symbol indicate significant differences among storage conditions, as shown by Tukey's test (p < 0.05).
Storage of boiled rice grains at 5 °C for 24 h increased RS content by up to 0.44% in Koshihikari, 0.82% in Akidawara, 2.81% in Koshinomenjiman, 3.94% in Koshinokaori, and 13.9% in Chou 2418; no significant change was observed in the RS content of Koganemochi and Milkyqueen. These data suggest that boiled rice grains retrograde, resulting in an increase in the amount of RS. Although these results indicate that stored boiled rice of Chou 2418 is suitable as a source of RS, it is important to consider the changes induced in the taste and physical properties of boiled rice by storage.
Physical properties of boiled rice grains The hardness (H2) and stickiness (S2) of whole grains of boiled rice under high compression (90%) are shown in Fig. 2. The hardness of boiled rice grains not subjected to storage was comparable between Chou 2418 and Koshinokaori; the values of H2 of both these cultivars were significantly higher than that of Koshihikari (p < 0.05). Storage of boiled rice grains at 5 °C for 24 h increased the hardness of grains of all rice cultivars because of starch retrogradation. Hardness showed a trend to increase with the increase of RS content in almost all cultivars, except for Koganemochi and Milkyqueen. The degree of increase in the RS content varied among rice cultivars, for example Koshinokaori vs. Chou 2418. These results indicate that changes in the digestibility and physical properties of rice grains could not be compared directly owing to differences in the structure of retrograded starch.
Change in hardness (A) and stickiness (B) of whole grains of boiled rice.
Different letters indicate significant differences among boiled rice (just after cooking), as shown by Tukey's test (p < 0.05). Symbol indicate significant differences among storage conditions, as shown by Tukey's test (p < 0.05).
The stickiness of boiled rice grains of Chou 2418 was the lowest among all rice cultivars and sharply declined during storage. Boiled rice grains of Chou 2418 showed the same change in hardness and stickiness as those of Koshinomenjiman and Koshinokaori during storage, and they may not be suitable for consumption as boiled rice. Adjusting the amount of water added during boiling may change the physical properties of boiled rice grains. Our data suggest that a large amount of water is necessary to obtain soft and sticky textured rice grains in the case of Chou 2418. In this case, although the RS content of Chou 2418 may decrease a little, the high moisture content of boiled rice will lead to lower calorie consumption.
Nakamura et al. (2013) reported that soaking ae mutant rice in a miso suspension improves the physical properties of boiled rice. In addition, Maeda et al. (2015) showed that high-pressure treatment and soaking of rice grains of ALC cultivars in unsalted miso improves the taste and physical properties of boiled rice. To make full use of the bio-functionality of Chou 2418, further research is necessary to study utilization methods and processing technology.
In this study, we investigated nutrient composition, pasting and thermal properties, and iodine absorption characteristics of a new ALC rice cultivar, Chou 2418, and compared these characteristics with those of ordinary rice cultivars. The higher AAC and pasting temperature of Chou 2418 led to an increase in RS. Iodine absorption data indicate that Chou 2418 is rich in amylose and long-chain amylopectin. The RS content of boiled Chou 2418 rice not subjected to storage was 5.3%, which was 25 times higher than that of Koshihikari. Thus, we propose Chou 2418 as a promising food material for diabetes prevention. It is necessary to examine the applications of rice cultivars with reference to pasting properties and starch structure of rice grains.
Acknowledgements The authors are grateful to Dr. Kazuyuki Kobayashi, Mr. Hironobu Kasaneyama (Crop Research Center, Niigata) and Dr. Noriyuki Honma (Food Research Center, Niigata) for helpful discussions. We also wish to thank Ms. Misato Saitoh for her technical assistance.
