Comparison of eating quality and physicochemical properties between Japanese and Chinese rice cultivars

Abstract

In this study, we evaluated 16 Japanese and Chinese rice cultivars in terms of their main chemical components, iodine absorption curve, apparent amylose content (AAC), pasting property, resistant starch content, physical properties, sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, and enzyme activity. Based on these quality evaluations, we concluded that Chinese rice varieties are characterized by a high protein and the grain texture after cooking has high hardness and low stickiness. In a previous study, we developed a novel formula for estimating AAC based on the iodine absorption curve. The validation test showed a determination coefficient of 0.996 for estimating AAC of Chinese rice cultivars as unknown samples. In the present study, we developed a novel formulae for estimating the balance degree of the surface layer of cooked rice (A3/A1: a ratio of workload of stickiness and hardness) based on the iodine absorption curve obtained using milled rice.

Key words:

• apparent amylose content
• iodine absorption curve
• cooked rice

Rice (Oryza sativa L.) is one of the main food crops throughout the world and the staple food for over half of the global population. Therefore, it is necessary to improve the quality of rice. Rice production is heavily concentrated in Asia, where only four countries (China, India, Indonesia, and Bangladesh) account for nearly 70% of the global rice production.¹⁾ China is the largest rice-producing country, accounting for 32% of the global production from 20% of the global rice-growing area. China produces the variety indica mostly in the south and japonica mostly in the north (Heilongjiang, Liaoning, eastern and southern Jilin, Jiangsu, Zhejiang, and Yunnan), whereas the other three countries primarily grow indica rice. China expanded the production of japonica rice by over 31.5% during 2012s, when both the area and yield increased, primarily in response to government programs designed to expand the plantation area planted of japonica rice.²⁾ Recently, changes in diet, lifestyle, and a growing awareness of agricultural food safety and health due to economic growth have enhanced interest in the production of high-quality grain and rice with good palatability. In China, consumers are now choosing japonica rice based on its shape and color as well as its texture and taste. Zhang et al. performed a sensory test of Chinese japonica cultivars.³⁾ In which a Chinese panel mainly determined the overall eating quality based on the stickiness and hardness of cooked rice grains. According to Ohtsubo,⁴⁾ grain quality evaluations aim to select high-quality rice simply and accurately. Thus, Nakamura et al.^5,6) developed formulae for estimating the amylose content, amylopectin chain length distribution, and resistant starch (RS) content based on the iodine absorption curve and pasting properties measured using a Rapid Visco Analyzer (RVA). Quality evaluations for rice are performed using a sensory test and based on physicochemical measurements. The former is a basic method but requires a large amount of samples and many panelists. Therefore, methods and instruments need to be developed to assess palatability. Thus, Mikami⁷⁾ developed equipment for evaluating the quality of rice based on the absorbance values in the visible and near-infrared spectrum Chinese indica rice cultivars. In the study, we conducted physicochemical evaluations of some Chinese and Japanese japonica rice cultivars using traditional and novel indicators based on the iodine absorption curve and RVA.

Materials and methods

Materials

High-quality premium japonica rice (Koshihikari) was cultivated at the Niigata Prefectural Agricultural Research Institute in 2014. Japonica rice cultivars (Tsuyahime, Yumepirika, Sagabiyori, and Kinumusume) were purchased from a local market. Chinese rice cultivars (Kenjing 5, Shendao 529, Jinyuan 45, Changyou 5, Lianjing 7, Longjing 31, Nanjing 9108, Jinongda 878, Shennong 265, Daohuaxiang, and Jinchuan 1) were provided by Professor J. Cui of Tenjian Agricultural University, China. Eleven Chinese rice cultivars, including low, medium, and high amylose-containing, aromatic and japonica rice varieties, were obtained from the Heilongjiang, Liaoning, Jilin, Jiangsu, and Tianjin rice-production regions (Supplemental Table 1).

Preparation of polished white rice samples

Brown rice was polished using an experimental friction-type rice milling machine (Yamamotoseisakusyo Co., Ltd, Tendoh, Japan) to obtain a milling yield (yield after polishing) of 90–91%. White rice flour was prepared using a cyclone mill (SFC-S1; Udy, Fort Collins, CO, USA) with a screen containing 1-mm diameter pores.

Measurement of the moisture content of rice flour

The moisture content of the milled rice grains was measured using an oven-drying method by drying 2 g flour samples for 1 h at 135 °C.

Protein content

Nitrogen was determined using a nitrogen analyzer (Leco FP-528, LECO, USA) based on the combustion method (modified Dumas method). The protein content was obtained based on the nitrogen by multiplying a nitrogen-protein conversion factor of 5.95.

Measurement of iodine absorption spectra

The iodine absorption spectrum of milled rice was measured using a Shimadzu UV-1800 spectrophotometer. The apparent amylose content (AAC) of milled rice was estimated using the iodine colorimetric method (as described by Juliano’s).⁸⁾

The iodine absorption spectrum⁵⁾ was analyzed from 200 to 900 nm using a square cell of which inner dimension was 1 × 1 cm. The control was distilled water in the same cell. The absorbance was measured at λmax, i.e. peak wavelength, in the visible light range, of iodine staining of starch, which has a high correlation with the length of the glucan chain, the molecular size of amylose, and the super-long chain (SLC), as well as determining the absorbance at λmax (A_λmax), λmax/A_λmax ratios, and “New λmax”, as follows.

New λmax = (73.307 × A_λmax + 0.111 × λmax – 73.016)/(λmax of various rice starches – λmax of glutinous starch).

Measurement of pasting properties of rice flours

The pasting properties of starch rice flours were measured using RVA (model Super 4; Newport Scientific Pty Ltd, Warriewood, Australia). A programmed heating and cooling cycle was followed, as described by Toyoshima et al.⁹⁾ Novel indices, such as the setback/consistency (SB/Con) and maximum viscosity/minimum viscosity (Max/Min) ratios, have very strong correlations with the proportion of intermediate and long chains of amylopectin: Fb1 + 2 + 3 (DP≧13).⁶⁾

Measurement of glucose content

We added 1 mL of 60% ethyl alcohol to the rice flour sample (0.1 g), and the mixture was subjected to the glucose extraction by rotation at 20 °C for 1 h. For the boiled rice samples, flours samples were prepared by pulverizating after lyophilization. The solution was centrifuged (1500 × g for 15 min), and the supernatant was used as the sample solution to obtain measurements. The glucose content of the sample solution was measured by the NADPH enzyme assay method using a glucose assay kit (Roche, Darmstadt, Germany).

Measurement of L-glutamic acid

The l-glutamic acid content was measured using an F-kit (Roche Diagnostics K.K. Japan). The absorbance was measured at 510 nm. Each cooked sample (1 g) was extracted by shaking with distilled water (1 mL) for 30 min at room temperature. The l-glutamic acid content of the sample was measured according to the manufacturer’s instructions.

α-Glucosidase activity

The α-glucosidase activity of milled rice was determined using a kit (Kikkoman Biochemicals Corp.). We added 1 mL of 0.01 M acetate buffer solution (pH 5.0, including 0.5% NaCl) to the rice flour sample (0.2 g), and the mixture was extracted at 5 °C for 16 h and then centrifuged for 5 min at 3000 × g. p - nitrophenyl-alpha-D-glucopyranoside substrate solution (2.0 mL) was pre-warmed at 37 °C for 5 min and then mixed with 0.1 mL of the extraction solution, before heating at 37 °C for 10 min, followed by the addition of 1.0 mL of stopping solution (0.2 M Na₂CO₃) and stirring. The absorbance of the sample solution was measured at 400 nm.

α-Amylase activity

The α-amylase activity of milled rice flour samples was determined using an enzyme kit (Megazyme Ltd, Wicklow, Ireland). To obtain α-amylase activity measurements, rice flour (0.5 g) was extracted with 1 mL of extraction buffer, (pH 5.4) at 40 °C for 20 min, before centrifugation for 10 min at 1000 × g. The extraction solution (0.1 mL) and substrate (0.1 mL) were preincubated at 40 °C for 5 min. Next, each solution was incubated at 40 °C for exactly 20 min before adding the stopping reagent (1.5 mL). The absorbance was measured at 400 nm.

Measurement of RS

The RS of the starch in rice flour was measured according to the AOAC method using an RS assay kit (Megazyme, Ltd, Wicklow, Ireland). Each sample (100 mg) was digested with pancreatin and amyloglucosidase at 37 °C for 6 h, and the glucose content was measured using a spectrophotometer at 510 nm.

Physical properties of boiled rice grains

For standard samples, the milled rice (10 g) was added with 14 g (1.4 times; w/w, standard moisture content; 13.5%, coefficient (gross water volume/dry matter weight): 1.77, calculated for each sample) of distilled water in an aluminum cup. After soaking for 1 h, the samples were boiled in an electric rice cooker (National SR-SW182). The boiled rice samples were kept in the vessel at 25 °C for 2 h and then used to obtain measurements. The hardness and stickiness of the boiled rice grains were measured using a Tensipresser (My Boy System, Taketomo Electric Co., Tokyo, Japan) with the individual grain method in low compression (25%) and high compression (90%) tests.¹⁰⁾ The average of each parameter was calculated by measuring 20 individual grains.

As a staling test for the cooked rice, the cooked samples were stored at 10 °C for 16 h and measured again with a Tensipresser according to the previously described method in low compression (25%) and high compression (90%) tests.¹⁰⁾

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS–PAGE)

Protein were extracted from milled rice flour samples (0.5 g) by shaking with 2 mL of buffer A (50 mM Tris-HCl, pH 6.8, 2% SDS, 5% 2-mercaptoethanol) at 37 °C for 30 min and then centrifuged for 5 min at 3000 × g. The supernatant (1 mL) was diluted with an equal volume of sample buffer (0.125 M Tris-HCl pH6.8, 10% 2-mercaptoethanol, 4% SDS, 10% sucrose, 0.004% bromophenol blue) and mixed well, before heating for 2 min at 100 °C. In total 10 μg of extracted protein was loaded into each lane. SDS–PAGE was conducted with a 12% polyacrylamide gel according to the modified method described by Laemmli.¹¹⁾

The values were calculated based on the intensities of various spots on the gel after SDS–PAGE analysis using the ATTO densitograph software library (CS Analyzer ver 3.0).

Statistical analyses

All results, including the significance of regression coefficients, were statistically analyzed using the Student’s t test, one-way ANOVA, and the Tukey test with Excel Statistics (ver. 2006, Microsoft Corp., Tokyo, Japan).

Results and discussion

Main chemical components

The quality of cooked rice is affected greatly by its moisture content. As shown in Table 1, the moisture contents of Chinese rice cultivars (7.9–11.5%; mean = 10.0%) were significantly lower than those of Japanese rice cultivars (10.6–13.8%; mean = 13.0%) at p < 0.01.

Table 1. Analysis of iodine absorption curve, amylose content, and main chemical components of milled rice.

No.	Sample	Moisture		Protein		Amylose		λmax		Aλmax		New λmax		λmax/Aλmax
		%	SD	%	SD	%	SD	nm	SD		SD		SD		SD
1	Koshihikari	13.2d	0.0	6.5c	0.0	12.1c	0.4	558.0b	0.0	0.28b	0.0	0.26a	0.0	1982.2b	0.0
2	Tsuyahime	13.8d	0.1	6.2c	0.0	12.9c	0.4	561.0b	2.8	0.29b	0.0	0.25a	0.0	1954.7b	0.0
3	Yumepirika	13.6d	0.0	6.4c	0.2	9.7b	0.2	550.5b	4.9	0.26a	0.0	0.23b	0.0	2142.0a	0.0
4	Sagabiyori	13.8d	0.1	6.4c	0.1	14.6d	0.3	572.5c	0.7	0.30b	0.0	0.24b	0.0	1924.4b	0.0
5	Kinumusume	10.6a	0.1	6.8c	0.1	14.4d	0.4	568.0b	0.0	0.30b	0.0	0.25a	0.0	1896.5c	0.0
6	Kenjing 5	10.8a	0.0	9.0a	0.0	14.7d	0.3	574.0c	1.4	0.30b	0.0	0.23b	0.0	1932.7b	0.0
7	Shendao 529	9.9a	0.0	7.9b	0.5	15.2e	0.2	576.5c	3.5	0.30b	0.0	0.23b	0.0	1908.9b	0.0
8	Jinyuan 45	10.0a	0.1	8.6a	0.4	14.5d	0.9	576.0c	0.0	0.29b	0.0	0.22c	0.0	1982.8b	0.0
9	Changyou 5	8.6b	0.1	8.8a	0.1	15.7e	0.2	575.5c	2.1	0.31b	0.0	0.24b	0.0	1871.5c	0.0
10	Lianjing 7	11.5c	0.0	7.4b	0.1	17.2f	0.4	578.5c	2.1	0.33b	0.0	0.26a	0.0	1771.8d	0.0
11	Longjing 31	9.9a	0.0	7.5b	0.2	14.2d	0.2	573.5c	0.7	0.29b	0.0	0.23b	0.0	1964.0b	0.0
12	Nanjing 9108	9.8a	0.0	6.8c	0.1	6.6a	0.4	537.0a	4.2	0.21a	0.0	0.11d	0.0	2553.2a	0.0
13	Jinongda 878	9.6a	0.1	7.7b	0.1	15.7e	0.2	569.5b	0.7	0.31b	0.0	0.27a	0.0	1813.7c	0.0
14	Shennong 265	7.9b	0.1	7.8b	0.4	14.1d	0.2	569.0b	1.4	0.29b	0.0	0.24b	0.0	1942.0b	0.0
15	Daohuaxiang	11.4c	0.1	7.0c	0.4	16.3f	0.3	576.5c	0.7	0.32b	0.0	0.25a	0.0	1830.2c	0.0
16	Jinchuan1	10.5a	0.0	7.5b	0.0	13.4c	0.1	564.5b	2.1	0.29b	0.0	0.24b	0.0	1963.5b	0.0

Protein is the second most abundant component of milled rice after starch, and it affects the physical properties of cooked rice grains. A higher protein content makes the rice grains harder and less sticky.¹²⁾

The protein content of milled japonica rice is around 6.8%.¹³⁾ Table 1 shows that the protein contents of Chinese rice cultivars (6.8–9.0%; mean = 7.8%) were significantly higher than those of Japanese rice varieties (6.2–6.8%; mean = 6.5%) at p < 0.01. Supplemental Table 2 shows that the intensities of the spots 13-kDa prolamins were correlated positively with λmax/A_λmax (r = 0.66) and “New λmax” (r = 0.65) at p < 0.05 in Chinese rice varieties, and with the glutamic acid (r = 0.85) at p < 0.01, whereas with λmax (r = −0.65) had negative correlation at p < 0.05.

AAC has been used as a good parameter for estimating the cooking or eating quality of rice grains, and the iodine colorimetric method for AAC measurement at 620 nm was developed by Juliano.⁸⁾ AAC comprises a large amount of amylose and a small amount of SLC in amylopectin. In general, low amylose rice becomes soft and sticky after cooking, whereas high amylose rice becomes hard and separated.¹⁰⁾ Table 1 shows that the AAC values for Chinese rice cultivars (6.6–17.2%; mean = 14.3%) were higher than those of Japanese rice cultivars (9.7–14.6%; mean = 12.7%). Lianjing 7 (17.2%) and Daohuaxiang (16.3%) were characterized as having high amylose contents. Yumepirika low-amylose rice (9.7%), and Nanjing 9108 (6.6%) was characterized as having exceptionally low amylose contents. In a previous study,⁵⁾ we showed that the iodine absorption curve differed among various samples of rice cultivars, and we developed a novel formula for estimating AAC. AAC = 73.31 × A λmax + 0.11 × λmax – 73.02). Fig. 1(A) shows that a multiple coefficient of determination of 0.996 was obtained when we used this formula to estimate AAC for Chinese rice cultivars. Thus, the calibration curve (as standard amylose and standard amylopectin) was unnecessary when using the AAC measurements, even in the case of Chinese rice cultivars. Supplemental Table 2 shows that AAC had negative correlations with the λmax/A_λmax ratio (r = −0.99), maximum viscosity/final viscosity (Max/Fin) ratio (r = −0.75), glutamic acid content (r = −0.81), and α-glucosidase content (r = −0.80) at p < 0.01, and with “New λmax” (r = −0.71) at p < 0.05 in Chinese rice cultivars, but positive correlations with the λmax (r = 0.96), A_λmax (r = 1.00), Set/Con ratio (r = 0.82) and RS (r = 0.83) at p < 0.01, and with SB (r = 0.68) and Cons (r = 0.69) at p < 0.05. The Japanese rice cultivars obtained almost the same results as the Chinese rice cultivars.

Fig. 1. Validation test of the formula for estimating AAC in Chinese rice cultivars.

Notes: (A) Estimation formula: AAC = 73.31 × A_λmax + 0.11 × λmax – 73.02. (B) Estimation formula: AAC = −0.84 × Pt + 37.76 × SB/Con – 13.09 × Max/Min + 103.92 × Max/Fin – 8.02. 1, Kenjing 5; 2, Shendao 529; 3, Jinyuan 45; 4, Changyou 5; 5,Lianjing 7; 6, Longjing 31; 7, Nanjing 9108; 8, Jinongda 878; 9, Shennong 265; 10, Daohuaxiang; 11, Jinchuan 1.

The molecular structures of many starches, including the molecular sizes of amylose and the amylopectin branch chain lengths, have been reported previously.^14–17) The high molecular weight amyloses tend to have a longer wavelength for λmax. We found that the glutinous rice cultivars had very low λmax values, and indica rice, japonica-indica hybrid rice cultivars, and a high-amylose japonica rice cultivar had higher λmax values. The λmax values of the japonica rice cultivars were intermediate.⁵⁾ Table 1 shows that the λmax values of the Chinese rice cultivars (537.0–578.5 nm; mean = 570.0 nm) were higher than those of the Japanese rice cultivars (550.5–572.5 nm; mean = 562.0 nm). Lianjing 7 (578.5 nm) and Daohuaxiang (576.5 nm) had high values, whereas Nanjing 9108 (537.0 nm) and Yumepirika (550.5 nm) had very low values. Supplemental Table 2 shows that λmax had a higher correlation with SB (final viscosity (Final.vis) – maximum viscosity (Max.vis), Cons (Final.vis – (Min.vis), Set/Cons ratios, and Max/Fin ratios than AAC in the Chinese rice varieties, and the Japanese rice cultivars had almost the same values as those of Chinese rice cultivars.

The A_λmax values of the amylose extender mutant rice (ae) samples were higher than those of the japonica rice, japonica-indica hybrid rice, and the glutinous rice samples.⁵⁾ Table 1 shows that the A_λmax values of the Chinese rice cultivars (0.212–0.327; mean = 0.294) were higher than those of the Japanese rice cultivars (0.257–0.300; mean = 0.285). Lianjing 7 (0.327), Daohuaxiang (0.315), and Jinongda 878 (0.314) had high values, whereas Nanjing 9108 (0.212) and Yumepiruka (0.257) had very low values.

The “New λmax” values are assumed to be related to the SLC content of amylopectin.⁵⁾ Table 1 shows that the “New λmax” values of the Chinese rice cultivars (0.11–0.27; mean = 0.23) were lower than those of the Japanese rice cultivars (0.23–0.26; mean = 0.25). Lianjing 7 (0.26) and Jinongda 878 (0.27) had high values, whereas Nanjing 9108 (0.11) had a very low value. Table 1 shows that λmax/A_λmax ratios of the Chinese rice cultivars (177.1–2553.2; mean = 1957.7) were lower than those of the Japanese rice cultivars (1896.5–2142.0; mean = 1980.0). Lianjing 7 (1771.8) had a low value, whereas Nanjing 9108 (2553.2) had a very high value.

The starches in the rice cultivars grown under low temperatures had a significantly higher amylose contents than those of the rice cultivars grown under at high temperature, whereas the SLC contents of the amylopectin were lower.^18–22) Thus, we consider that the starch properties of Japanese rice cultivars were influenced by the ambient temperatures during the development of the grain, which yielded high “New λmax” values.

Pasting properties of milled rice flours

The pasting properties also influence the rice eating quality; therefore, it is useful to test the gelatinization properties as a quality assay for rice. Breakdown (BD) indicates the degree of ease with which the starch granules are disintegrated. High-amylose rice cultivars had higher final viscosities than low-amylose cultivars, where the Final.vis was related to the degree of starch retrogradation during cooling.²³⁾

In a previous study,⁶⁾ we developed a novel index for the ratios of SB/Con, and Max/Fin, which have higher negative correlations with the RS content than the conventional indexes obtained using RVA. Moreover, for polished japonica rice, we developed a formula for estimating AAC using the pasting properties of the novel indexes obtained using RVA. [AAC (%) = −0.84 × pasting temperature (Pt) + 37.76 × SB/Con – 13.09 × Max/Min + 103.92 × Max/Fin – 8.02)].

Fig. 1(B) shows that a multiple coefficient of determination of 0.720 was obtained by employing the formula for estimating AAC of Chinese rice cultivars.

Thus, the iodine absorption curve had a higher correlation with AAC than the pasting property values obtained using RVA.

Table 2 shows that the Max.vis values for Chinese rice cultivars were 265.4–405.7 RVU, the Mini.vis values were 97.5–162.5 RVU, the BD values were 168.0–270.1 RVU, the Final.vis values were 214.7–310.5 RVU, the SB values were −159.3 to 42.8 RVU, the Pt values were 63.1–69.3 °C, and the Cons were 88.7–151.3 RVU. The novel SB/Con ratios were −1.80 to −0.30, the Max/Min ratios were 2.19–3.07, and the Max/Fin ratios were 1.15–1.71. Table 2 shows that the Japanese rice cultivars had similar values to the Chinese rice varieties for the Max.vis, BD, Pt, Cons, SB/Con ratio, and Max/Fin ratio, whereas the Final.vis values were higher than those of the Chinese rice cultivars.

Table 2. Pasting properties of Chines and Japanese rice cultivars.

No.	Sample	Maximum.vis.		Min.vis.		Break.d		Final.vis		Setback		Pasting.t		Consistency		Setback/Cons		Max/Min		Max/Final
		RVU	SD	RVU	SD	RVU	SD	RVU	SD	RVU	SD	°C	SD		SD		SD		SD		SD
1	Koshihikari	377.3d	1.4	160.4f	2.2	216.8d	3.5	277.6d	3.6	–99.7e	2.2	68.6e	1.0	117.2c	5.8	–0.85d	0.07	2.35b	0.04	1.36c	0.01
2	Tsuyahime	378.6d	0.3	183.9g	5.9	194.8b	5.7	311.0e	2.1	–67.6c	1.8	66.1c	0.5	127.1d	3.8	–0.53b	0.00	2.06a	0.07	1.22b	0.01
3	Yumepirika	366.0d	0.7	145.5d	1.6	220.5d	0.9	244.7b	2.9	–121.3f	2.2	66.8c	0.5	99.1b	1.4	–1.22f	0.04	2.51c	0.02	1.50d	0.02
4	Sagabiyori	322.5a	2.6	153.6e	1.3	168.9a	1.3	287.2d	1.2	–35.3a	1.4	64.2a	0.0	133.6e	0.1	–0.26a	0.01	2.10a	0.00	1.12a	0.00
5	Kinumusume	355.3c	2.4	152.7e	0.4	202.6c	2.7	281.5d	0.9	–73.7c	1.5	64.6a	0.6	128.9d	1.2	–0.57b	0.01	2.33b	0.02	1.26b	0.00
6	Kenjing 5	320.3a	1.1	133.6c	0.1	186.7b	1.2	277.5d	1.4	–42.8a	0.3	64.0a	0.0	143.9f	1.5	–0.30a	0.01	2.40b	0.01	1.15a	0.00
7	Shendao 529	355.2c	1.0	129.9b	1.4	225.3d	0.4	259.0c	1.7	–96.2e	0.7	67.2d	0.0	129.1d	0.3	–0.74c	0.01	2.73d	0.02	1.37c	0.01
8	Jinyuan 45	265.4b	2.0	97.5a	1.0	168.0a	1.0	214.7a	1.2	–50.7b	0.8	65.7b	0.0	117.2c	0.2	–0.43b	0.01	2.72d	0.01	1.24b	0.00
9	Changyou 5	405.7e	0.1	159.3e	0.1	246.4e	0.2	310.5e	0.4	–95.1e	0.3	67.9d	0.0	151.3g	0.5	–0.63c	0.00	2.55c	0.00	1.31c	0.00
10	Lianjing 7	320.0a	2.2	121.7b	5.4	198.3b	3.2	236.5b	4.9	–83.4d	2.7	69.3f	0.0	114.8c	0.5	–0.73c	0.02	2.63d	0.10	1.35c	0.02
11	Longjing 31	347.6c	5.9	137.5c	0.8	210.1c	6.7	263.6c	0.6	–84.0d	6.5	66.5c	0.1	126.2d	0.1	–0.67c	0.05	2.53c	0.06	1.32c	0.03
12	Nanjing 9108	383.3d	0.5	135.4c	1.6	248.0e	1.1	224.1a	1.9	–159.3g	1.4	65.8b	0.1	88.7a	0.3	–1.80g	0.02	2.83e	0.03	1.71e	0.01
13	Jinongda 878	327.3a	1.0	132.3c	0.1	195.0b	0.9	253.0c	0.0	–74.3c	1.0	65.7b	0.1	120.7d	0.1	–0.62c	0.01	2.47c	0.01	1.29b	0.00
14	Shennong 265	372.5d	4.9	138.6c	2.4	233.9d	2.5	254.3c	0.7	–118.3f	4.2	67.5d	0.5	115.6c	1.7	–1.02e	0.05	2.69d	0.01	1.47d	0.02
15	Daohuaxiang	355.9c	6.4	162.5f	2.7	193.4b	3.7	305.5e	3.1	–50.4b	3.3	63.1a	0.5	143.0f	0.4	–0.35b	0.02	2.19a	0.00	1.17b	0.01
16	Jinchuan1	400.3e	2.9	130.2c	1.1	270.1f	1.8	249.1c	0.8	–151.2 g	2.1	67.5d	0.5	118.9c	0.3	–1.27f	0.02	3.07e	0.00	1.61e	0.01

Glucose content

α-Amylase and α-glucosidase decompose starch and generate glucose during the early stage of cooking. The gelatinization onset temperature of rice flour was 60–64 °C for the japonica rice cultivars. The key temperature for the substantial production of glucose by the endogenous enzymes is around 60 °C in milled rice, and the amount of low-molecular-weight sugars in cooked rice depends mainly on the cooking conditions.²⁴⁾

The glucose contents of the Chinese rice cultivars had significant correlation with α-amylase activities at p < 0.01, although that of α-glucosidase activity had no significant difference with the glucose contents. On the other hand, the glucose contents of the Japanese rice cultivars had significant correlation with α-amylase activities at p < 0.01, and α-glucosidase activity at p < 0.05, respectively.

L-glutamic acid

The taste of rice is strongly related to its protein content and free amino acid content. Thus, the taste of rice has a high positive correlation with the l-glutamic acid content. Reducing sugars and amino acids accumulate more greatly with lower milling yields than higher yields and different milling yields may affect the accumulation of chemical components during rice cooking.²⁵⁾ Matsuzaki et al. showed that the l-glutamic acid and aspartic acid contents might help to improve eating quality of rice kernels as well as the amylose and nitrogen contents.²⁶⁾ As a result, the l-glutamic acid contents of the Japanese rice and Chinese rice cultivars had no significant difference with physical parameters of cooked rice as shown in Table 4. Moreover, Japanese rice cultivars of iodine analysis, RS content, pasting properties, and main chemical components had no significant difference with l-glutamic acid contents as shown in supplemental Table 2. On the other hand, that of Chinese rice cultivars had significant correlation with a negative correlation with AAC and Cons (indicator for retrogradation). The results of measurements are assumed to variations in grain polishing or to varietal characteristics in Chinese rice cultivars

α-Glucosidase activity

As shown in Supplemental Table 3, the Chinese rice cultivars had an average α-glucosidase activity of 0.025 U/mL, which was lower than that of the Japanese rice cultivars, with an average of 0.028 U/mL.

Nanjing 9108 and Koshihikari (0.04 U/mL) had very high values. Iwata reported that the α-glucosidase activity has a positive correlation with the fresh weight, GBSS activity, and amylose content.²⁷⁾ As a result, the α-glucosidase activity of the Japanese rice and Chinese rice cultivars had no significant correlation with physical parameters of cooked rice as shown in Table 4. Moreover, Japanese rice cultivars of iodine analysis, RS content, pasting properties, and main chemical components had no significant correlation with α-glucosidase activity as shown in supplemental Table 2. On the other hand, that of Chinese rice cultivars had a significant negative correlation with AAC (r = −0.80), A_λmax (r = −0.82) at p < 0.01, and with the Set/Con ratio (r = −0.62) at p < 0.05, whereas λmax/A_λmax ratio (r = 0.81) had a positive correlation at p < 0.01.

On the other hand, that of Chinese rice cultivars had significant negative correlation with AAC (r = −0.80), A_λmax (r = −0.82) at p < 0.01, and with the Set/Con ratio (r = −0.62) at p < 0.05, whereas λmax/A_λmax ratio (r = 0.81) had a positive correlation at p < 0.01.

α-Amylase activity

As shown in Supplemental Table 3, the α-amylase activities of the Chinese rice cultivars (0.25–0.79 U/g; mean = 0.43 U/g) were lower than those of the Japanese rice cultivars (0.40–0.53 U/g; mean = 0.45U/g). The α-glucosidase enzyme is predominantly localized in the inner endosperm, whereas α-amylase localized mainly in the outer layers.²⁸⁾ As a result, the α-amylase activity of the Japanese rice had significant correlations with balance of −H1/H1 (r = 0.97), A3/A1 (r = 0.96), and A6/A4 (r = 0.93) in physical parameters of cooked rice as shown in Table 4.

Supplemental Table 2 shows that the α-amylase activity had a negative correlation with Cons (r = −0.94) at p < 0.05 in Japanese rice cultivars, whereas A_λmax (r = 0.97) and λmax/A_λmax ratio (r = 0.97) had positive correlations with the α-amylase activity at p < 0.01, and that of AAC (r = −0.95) had a negative correlation with α-amylase activity at p < 0.01. On the other hand, that of Chinese rice cultivars had no significant difference with iodine analysis, RS content, pasting properties, and main chemical components as shown in Supplemental Table 2.

RS contents

As shown in Supplemental Table 3, the RS contents of Chinese rice cultivars (0.41–1.77%; mean = 0.86%) were almost the same those of the Japanese rice cultivars (0.58–1.25%; mean = 0.87%). Yang et al.²⁹⁾ reported that mutant rice are rich in RS. Thus, the japonica rice cultivars had significantly lower RS contents than the indica rice and japonica-indica hybrid rice cultivars with similar amylose contents.⁵⁾ In general, starches rich in amylose are naturally more resistant to digestion and more susceptible to retrograde, where the SLC in amylopectin behaves in a similar manner to amylose by restricting starch swelling.³⁰⁾

Physical properties of cooked rice grains

Table 3 shows the physical properties of the cooked rice grains obtained using the low-compression (25%) and high-compression (90%) methods¹⁰⁾ with the Tensipresser.

Table 3. The physical properties of the cooked rice grains.

Sample	Surface layer	SD	Overall	SD	Surface layer	SD	Overall	SD	Surface layer	SD	Surface layer	SD	Overall	SD	Surface layer	SD	Overall	SD
	Hardness (H1) × 10^5 (N/cm^2)		Hardness (H2) × 10^5 (N/cm^2)		Stickiness (–H1) × 10^5 (N/cm^2)		Stickiness (–H2) × 10^5 (N/cm^2)		Adhered (L3) (mm)		Balance degree (–H1/H1)		Balance degree (–H2/H2)		Balance degree (A3/A1)		Balance degree (A6/A4)
Koshihikari	654.9c	15.8	16985b	131.2	–166.8f	5.2	–3603c	38.6	3.2b	0.3	0.26d	0.05	0.21c	0.03	0.47c	0.11	0.14c	0.03
Tsuyahime	600.6b	19.2	15681a	191.9	–133.2d	4.3	–2716a	63.9	3.2b	0.1	0.23c	0.06	0.17a	0.03	0.44c	0.13	0.13c	0.03
Yumepirika	632.3c	16.5	17475b	130.1	–176.8g	5.1	–4040e	38.5	3.2b	0.2	0.28d	0.06	0.23d	0.03	0.57d	0.16	0.16d	0.04
Sagabiyori	789.5e	19.9	18790d	170.7	–179.4g	5.1	–3630c	42.6	3.2b	0.2	0.23c	0.04	0.20b	0.03	0.36b	0.10	0.11b	0.03
Kinumusume	803.6e	19.4	18319c	158.7	–145.5e	3.6	–3869d	33.2	3.2b	0.1	0.19b	0.06	0.21c	0.03	0.29b	0.11	0.10b	0.03
Kenjing 5	1016.9f	23.7	21084e	205.4	–151.6e	4.9	–3897d	48.0	3.2b	0.0	0.15a	0.04	0.19b	0.04	0.20a	0.08	0.07a	0.02
Shendao 529	816.5e	17.2	18721d	125.7	–179.2g	6.0	–4059e	47.7	3.1b	0.4	0.22v	0.06	0.22d	0.02	0.31b	0.07	0.11b	0.02
Jinyuan 45	707.7d	15.9	16769b	143.6	–120.7c	3.3	–3050b	46.6	3.2b	0.0	0.18b	0.05	0.18a	0.03	0.28b	0.11	0.10b	0.02
Changyou 5	802.4e	23.7	17907c	175.0	–183.2g	4.8	–3361b	26.1	3.1b	0.4	0.23c	0.05	0.19b	0.02	0.36b	0.12	0.12b	0.03
Lianjing 7	753.0d	21.0	20770e	132.3	–88.9a	3.4	–4144f	39.5	2.9a	0.7	0.12a	0.04	0.20b	0.02	0.19a	0.09	0.06a	0.02
Longjing 31	710.3d	13.5	17672c	95.8	–114.7c	2.9	–3678c	41.4	3.2b	0.0	0.16a	0.02	0.21c	0.03	0.26b	0.06	0.09b	0.01
Nanjing 9108	592.4b	15.6	16946b	161.1	–106.6b	4.9	–4239f	47.3	3.2b	0.2	0.18b	0.05	0.25e	0.03	0.34b	0.12	0.11b	0.03
Jinongda 878	509.5a	9.3	14808a	134.4	–90.5a	2.4	–3099b	22.9	3.0a	0.6	0.18b	0.04	0.21c	0.02	0.36b	0.12	0.13c	0.03
Shennong 265	603.5b	24.2	16397b	196.7	–105.5b	2.9	–3782d	39.7	3.1b	0.3	0.19b	0.06	0.23d	0.03	0.39c	0.16	0.13 c	0.04
Daohuaxiang	785.8e	15.0	18083c	159.1	–137.6d	3.7	–4230f	39.9	3.2b	0.1	0.18b	0.04	0.24e	0.03	0.24b	0.06	0.10b	0.03
Jinchuan1	636.1c	12.1	16750b	159.8	–102.1b	3.6	–4097e	44.4	3.0a	0.5	0.16a	0.05	0.25e	0.03	0.27b	0.09	0.11b	0.04

There were differences in the values of H1, H2, −H1, −H2, L3, A1, A3, A4, A6, “balance degree of surface layer (ratio of height); −H1/H1,” “balance degree of overall layer (ratio of height); −H2/H2,” “balance degree of surface layer (ratio of area); A3/A1,”and “balance degree of overall layer (ratio of area); A6/A4” between the cooked rice grains.

The balance of −H1/H1, −H2/H2, A3/A1, and A6/A4 are important indices when evaluating the palatability of rice.¹⁰⁾

As shown in Table 3, the hardness of the surface layer (H1) was higher in the Chinese rice cultivars (509.9–1016.9 × 10⁻⁵ N/cm²; mean = 720.8 × 10⁻⁵ N/cm²) than the Japanese rice varieties (600.2–803.2 × 10⁻⁵ N/cm²; mean = 696.3 × 10⁻⁵ N/cm²), and the hardness of the overall layer (H2) of Chinese rice cultivars (14808.0–21084.3 × 10⁻⁵ N/cm²; mean = 17809.8 × 10⁻⁵ N/cm²) was higher than that of the Japanese rice varieties (15680.8–18789.5 × 10⁻⁵ N/cm²; mean = 17450.0 × 10⁻⁵ N/cm²). The stickiness of the surface layer (–H1) was significantly lower in the Chinese rice cultivars (−183.4 to −89.2 × 10⁻⁵ N/cm²; mean = −125.5 × 10⁻⁵ N/cm²) than the Japanese rice cultivars (−133.4 to −179.5 × 10⁻⁵ N/cm²; mean = −160.8 × 10⁻⁵ N/cm²) at p < 0.05, whereas the stickiness of overall layer (−H2) was higher in the Chinese rice varieties (−4239.4 to −3049.9 × 10⁻⁵ N/cm²; mean = −3785.4 × 10⁻⁵ N/cm²) than the Japanese rice cultivars (−2716.4 to −4040.3 × 10⁻⁵ N/cm²; mean = −3571.6 × 10⁻⁵ N/cm²). The balance degree of the surface layer (−H1/H1) was significantly lower in the Chinese rice varieties (0.12–0.23; mean = 0.18) than the Japanese rice varieties (0.19–0.28; mean = 0.20 at p < 0.01, and that of the surface layer (A3/A1) was significantly lower in the Chinese rice varieties (0.19–0.39; mean = 0.29) than the Japanese rice varieties (0.29–0.57; mean = 0.40) at p < 0.01.

The low amylose cultivars Yumepirika and Nanjing 9108 had good eating qualities due to their low hardness and high stickiness, whereas Changyou 5 had a high amylose content with a hard texture high hardness, and low stickiness. Daohuaxiang, a soft type of high amylose cultivar, had a soft texture after cooking, with medium surface hardness and high overall stickiness.

Low amylose rice cultivars are stale-resistant according to Takami et al., who previously reported the staling characteristics of cooked low amylose rice.³¹⁾ In the staling test, we stored the cooked rice at 10 °C for 16 h. As shown in Supplemental Fig. 1, the staling rate for starch based on the surface layer hardness (H1) was significantly higher in the Chinese rice cultivars (1.02–2.26 times; mean = 1.46 times) than the Japanese rice cultivars (0.93–1.12 times; mean = 1.06 times) at p < 0.01. In particular, Jinongda 878 (2.3 times), Daohuaxiang (1.9 times), and Nanjing 9108 (1.9 times) had very high values. The staling rates for starch based on the hardness of the overall layer (H2) were almost the same in the Chinese and Japanese rice cultivars. The staling rate for starch based on the hardness of the surface layer (H1) in Chinese rice varieties had a positive correlation with the intensities of the 13-kDa prolamin spots and the protein content at p < 0.01(data not shown).

Correlations among the physical parameters of cooked rice and the results of the iodine analysis, amylose contents, RS content, pasting properties, and the main chemical components of 16 Chinese and Japanese rice cultivars

For Japanese rice cultivars, Table 4(A) shows that the values of AAC, λmax, and A_λmax had negative correlations with A3/A1 (r = −0.96, r = −0.92, and r = −0.96, respectively) and A6/A4 (r = −0.96, r = −0.95 and r = −0.94, respectively) at p < 0.05, whereas λmax/A_λmax ratios had positive correlations with −H1/H1 (r = 0.90), with A3/A1 (r = 0.93) and with A6/A4 (r = 0.89) at p < 0.05. Moreover, “New λmax” had a positive correlation with A6/A4 (r = 0.89) at p < 0.05. The values of AAC, λmax, and A_λmax had negative correlations with the balance degree (ratio of stickiness to hardness), but positive correlations with the λmax/A_λmax ratio and “New λmax.” In addition, above-mentioned iodine absorption curve parameters, such as A_λmax and λmax/A_λmax, had stronger correlations with −H1/H1 than AAC.

Table 4. Correlation between physical parameters of cooked rice with analysis of iodine absorption curve, amylose contents, RS contents, pasting properties, and main chemical components of 16 Chinese and Japanese rice varieties.

	AAC	λmax	Aλmax	λmax/Aλmax	New λmax	Min.vis	Past. T	Fin. vis	Max/Min	Cons	α-amylase activity
(A)
H1
H2
–H1
–H2						0.99**
L3							–0.88*
–H1/H1			–0.90*	0.90*							0.97**
–H2/H2								–0.95**	0.96**
A3/A1	–0.96*	–0.92*	–0.96*	0.93*						–0.88*	0.96**
A6/A4	–0.96*	–0.95*	–0.94*	0.89*	0.89*						0.93*
	Moisture content	Protein content	λmax	Protein 13 kDa prolamin	Cons	Fin.vis	Max. vis	BD	SB	SB/Cons	Max/Final	Glucose contents	RS
(B)
H1					0.68*
H2
–H1					–0.70*	–0.62*
–H2				–0.69*
L3
–H1/H1	–0.68*
–H2/H2		–0.80*	–0.65*				0.60*	0.62*	–0.73**	–0.74**	0.72*	0.80**	–0.77**
A3/A1	–0.89**
A6/A4	–0.77**

Notes: A; Japanese rice cultivars, B; Chinese ricew cultivars, Correlation is significant at 5% (*) or 1% (**) by the method of t test.

Previously, λmax was used as an index for AAC because the correlation between λmax and AAC is very high.¹⁸⁾ As shown in Fig. 2, with similar values of λmax in various rice cultivars (Nos. 7, 8, 9, and 15 and Nos. 5, 13, and 14), the difference in the “New λmax” value tended to reflect the amylopectin SLC. Thus, the length of the glucan chain (molecular size of amylose and SLC) could be expressed as a linear function between λmax and “New λmax.”

Fig. 2. The starch structure was expressed as a linear function based on the relationship between λmax and “New λmax.”

Notes: The vertical axis represents λmax and the horizontal axis is “New λmax.” ▲: Chinese rice cultivars, ●: Japanese rice cultivars 1, Koshihikari; 2, Tsuyahime; 3, Yumepirika; 4, Sagabiyori; 5, Kinumusume; 6, Kenjing 5; 7, Shendao 529; 8, Jinyuan 45; 9, Changyou 5; 10, Lianjing 7; 11, Longjing 31; 12, Nanjing 9108; 13, Jinongda 878; 14, Shennong 265; 15, Daohuaxiang; 16, Jinchuan 1. Fig. 2 shows that a coefficient of determination of 0.827 was obtained due to the high correlation between λmax and “New λmax.”

As shown in Fig. 3, we could estimate the texture (A3/A1) of the cooked rice grains based on the iodine absorption curve obtained for milled rice. The equation had a multiple coefficient of determination of 0.628 based on the calibration curve. We obtained the following formula for estimating the balance degree of the surface layer (A3/A1) using 16 Chinese and Japanese rice varieties.

Fig. 3. Formula for estimating the balance degree of the surface layer (A3/A1) based on the iodine absorption curve of milled rice.

Notes: Estimation formula: A3/A1 = –0.002 × λmax/A_λmax – 16.335 × A_λmax + 9.428. ▲: Chinese rice cultivars, ●: Japanese rice cultivars. 1, Koshihikari; 2, Tsuyahime; 3, Yumepirika; 4, Sagabiyori; 5, Kinumusume; 6, Kenjing 5; 7, Shendao 529; 8, Jinyuan 45; 9, Changyou 5; 10,Lianjing 7; 11, Longjing 31; 12, Nanjing 9108; 13, Jinongda 878; 14, Shennong 265; 15, Daohuaxiang; 16, Jinchuan 1. The equation had a multiple coefficient of determination of 0.628 based on the calibration. Thus, we could estimate the texture (A3/A1) of the cooked rice grains based on the iodine absorption curve obtained for the milled rice.

For various rice cultivars (Nos. 7, 8, 9, and 15 and Nos. 5, 13, and 14) with similar λmax values, it was possible to differentiate them using λmax/A_λmax and A_λmax.

Supplemental Fig. 2 shows that a multiple coefficient of determination of 0.595 was obtained by applying the formula given above to seven unknown samples. Thus, the validation test showed that the equation can be applied to unknown samples. This allows us to evaluate the characteristics physical properties of various types of cooked rice using a simple, low cost spectroscopic method.

We found that the iodine absorption curve differed among the various samples of rice cultivars. The glutinous rice cultivars had very low λmax values, whereas the indica rice, japonica-indica hybrid rice, and high-amylose japonica rice cultivars had higher λmax values.⁵⁾ Thus, it is necessary to produce a calibration curve based on the iodine absorption curve for specific varieties of rice (japonica rice, indica rice, and japonica-indica hybrid rice).

The pasting properties also correlated with the texture of the cooked rice grains.

AAC is higher than the actual amylose contents because the long-chain amylopectin binds with iodine. The SLC in amylopectin appears to have a beneficial effect on the consistency of starch.¹⁸⁾

Table 4(B) shows that the protein contents of Chinese rice cultivars had a negative correlation with −H2/H2 (r = −0.80) at p < 0.01; moreover, that of intensities of the 13-kDa prolamin had a negative correlation with −H2 (r = −0.69) at p < 0.05.

We concluded that Chinese rice varieties are characterized by a high protein and the grain texture after cooking has high hardness and low stickiness.

The value of λmax had a negative correlation with −H2/H2 (r = −0.65) at p < 0.05. As a result, the iodine absorption curve parameter (λmax) had a higher correlation with physical parameters of cooked rice (−H2/H2) than the AAC in Chinese rice varieties.

Table 4(B) shows that Cons of Chinese rice cultivars had a positive correlation with H1 (r = 0.68), but a negative correlation with −H1 (r = −0.70) at p < 0.05. Moreover, Fin.vis had a negative correlation with −H1 (r = −0.62) at p < 0.05. The Max.vis, BD, and Max/Fin ratio had positive correlations with −H2/H2 (r = 0.60, r = 0.62, and r = 0.72, respectively) at p < 0.05, whereas SB and SB/Con ratio had negative correlations with −H2/H2 (r = −0.73, and r = −0.74, respectively) at p < 0.01. We found that the novel indexes comprising the SB/Con and Max/Fin ratios had higher correlations with −H2/H2 than the conventional indexes (Max.vis and BD) obtained using RVA. The glucose contents had negative correlations with H1 (r = −0.70) and H2 (r = −0.68) at p < 0.05, but a positive correlation with −H2/H2 (r = 0.80) at p < 0.01. The RS contents had a negative correlation with −H2/H2 (r = −0.77) at p < 0.01.

It is well known that rice with a high protein content has inferior palatability.³²⁾ The viscosity of rice flour increases dramatically after storage for several months as milled rice, where this change depends on the storage temperature and duration.^33,34) The Max.vis, Final.vis, and BD values of all the samples increased noticeably with the increased storage time. Tsujii et al.³⁵⁾reported that the α-amylase activity and debranching enzyme activities are correlated positively with the amount of maltooligomer leached during rice cooking and the adhesiveness of cooked rice. We found that the japonica rice cultivars had significantly lower RS contents than the indica rice and japonica-indica hybrid rice cultivars with similar amylose contents.⁵⁾ In general, starches that are rich in amylose are naturally more resistant to digestion and more susceptible to retrogradation, where the SLC in amylopectin behaves in a similar manner to amylose by restricting starch swelling,³⁶⁾ and amylopectin retrogradation can significantly increase the amount of RS.³⁷⁾ The RS contents seem to be important because they can yield foods with greater nutritional quality.

SDS–PAGE

The rice seed storage proteins mainly comprise glutelins and prolamins.³⁸⁾ PB-I is highly enriched with prolamin and it constitutes approximately 20% of the milled rice protein contents. Prolamin comprises three polypeptide subunits with molecular masses of 10, 13, and 16 KDa. PB-II mainly contains glutelins and it constitutes 60–65% of the milled rice protein.³⁹⁾ The protein content of rice grains is influenced by the weather conditions, as it is increased by high air temperature or high water temperature, but decreased by low water temperature or sun shading during the ripening stage.⁴⁰⁾ Matsui et al.⁴¹⁾ showed that the Final.vis and Cons values of near-isogenic line pairs for the low glutelin gene locus were significantly higher in low glutelin lines, and the surface stickiness was also significantly lower in the low glutelin lines. Protein production also tended to increase with higher levels of nitrogenous fertilizer at any planting density.⁴²⁾ That is to say, glutelin has been reported to be a major protein in rice grains to affect quality and nutrition. The protein bands from raw milled rice are shown in Fig. 4(A) and (B).

Fig. 4. SDS–PAGE analysis of proteins extracted from raw milled rice grains.

Notes: (a) polypeptide; (b) glutelin α-subunit; (c) α-globulin; (d) glutelin β-subunit, (e), (f), (g) prolamin. Shunyo is a low gluterin rice, and Koshihikari is a high-quality rice from Japan. a, Shunyo; b, Koshihikari; 1, Kenjing 5; 2, Shendao 529; 3, Jinyuan 45; 4, Changyou 5; 5, Lianjing 7; 6, Longjing 31; 7, Nanjing 9108; 8, Jinongda 878; 9, Shennong 265; 10, Daohuaxiang; 11, Jinchuan 1. Chinese rice cultivars were characterized by high-intensity 13-kDa prolamin spots, where Kenjing 5, Lianjing 7, and Nanjing 9108 had very high values.

Conclusions

In this study, we evaluated 16 Chinese and Japanese rice cultivars in terms of their main chemical components, iodine absorption curve, AAC, pasting property, RS content, physical properties, SDS–PAGE analysis, and enzyme activity. In a previous study,⁶⁾ we developed a novel formula for estimating AAC based on the iodine absorption curve. The validation test showed a multiple coefficient of determination of 0.996 was obtained formula estimate AAC for Chinese rice varieties. Thus, the calibration curve is unnecessary for estimating the AAC measurements, even in the case of Chinese rice cultivars. This allows us to estimate the characteristics of various type of Chinese rice cultivars based on the iodine curve or RVA analysis.

Based on these quality evaluations, we can conclude that Chinese rice cultivars are characterized by their high protein content (1.2 times more than Japanese rice cultivars). The hardness of the surface layer (H1) and overall layer (H2) were higher in the Chinese rice cultivars than the Japanese rice cultivars (H1: 1.04 times, H2: 1.02 times), whereas the stickiness of the surface layer (−H1: 0.78 times), and the balance degree of the surface layer (−H1/H1, A3/A1) were lower in the Chinese rice cultivars than the Japanese rice cultivars (−H1/H1: 0.75 times; A3/A1: 0.67 times), although the stickiness of the overall layer (−H2) was higher in the Chinese rice cultivars than the Japanese rice cultivars (−H2: 1.06 times). In addition, the texture of cooked rice was strongly correlated with the iodine absorption factors.

We also developed a novel formula for estimating the balance degree of the surface layer (A3/A1) based on the iodine absorption curve obtained for milled rice.

We consider that the starch properties of Japanese rice cultivars are influenced by the ambient temperature during the development of the grain, which leads to a high “New λmax.” The “New λmax” value is increased by high during ripening. In addition, this index reflects the high amylose content, high SLC of starch, and the high hardness of cooked rice grains. However, the physical properties of Chinese cooked rice are influenced mainly by their protein content.

Author contributions

Nakamura S, Cui J, and Ohtsubo K designed the experiments; Nakamura S, Zhang X, Yang F, Xu X, Sheng H performed experiments; Nakamura S, and Ohtsubo K wrote the paper. Cui J commented on the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was supported by the Ministry of Education, Grant in aid for research (B) [15H02891]; Japanese Cabinet Office (SIP).

Supplemental material

Supplemental material for this article can be accessed at http://dx.doi.org/10.1080/09168451.2016.1220823.

Acknowledgment

We thank NSP Perten Co. Ltd. for leasing the RVA device. Part of this research was supported by a Grant in Aid for Scientific Research B and Strategic Innovation Program (SIP, Cabinet Office Government of Japan).

Notes

Abbreviations: AAC, apparent amylose content; RS, resistant starch; SLC, super-long chain; CD, chain length distribution; RVA, Rapid Visco Analyzer; SB, setback; BD, breakdown; Max.vis, maximum viscosity; Min.vis, minimum viscosity; Pt, pasting temperature; Cons, consistency; Final.vis, Final viscosity; SB/Con, setback/consistency; Max/Min, maximum viscosity/minimum viscosity; Max/Fin, maximum viscosity/final viscosity.