Physicochemical Properties and Eating Qualities of Milled Rice from Different Korean Elite Rice Varieties

Abstract

Physicochemical properties and palatability of rice from six elite varieties in Korea (Chucheongbyeo, Saechucheongbyeo, Mihyangbyeo, Hitomebore, Nampyeongbyeo, and Ilpumbyeo) were analyzed. All samples, which contained 17–18 g/100 g rice starch amylose, belong to low-amylose rice group. Hitomebore variety showed abundant amount of essential amino acids, highest palatability score (82.9), and lowest mineral content. The rice samples contained relatively similar concentrations of saturated (21–24 g/100 g rice) and unsaturated (75–78 g/100 g rice) fatty acids. Mihyangbyeo variety exhibited the highest amount of protein (8.10 g/100 g rice), sugar content, and pasting temperature (82.75°C) and time (3.78 min), but lowest viscosity values. Ribose, rhamnose, and potassium were found to have negative correlations with palatability and breakdown viscosity, indicating that gelatinization characteristics could also be used in evaluating the eating quality of rice. Results of this study could assist plant breeders in developing rice varieties with improved genetic traits and high eating quality.

Keywords:

• Rice
• Physicochemical property
• Palatability
• Correlation coefficient

INTRODUCTION

Rice (Oryza sativa L.) is the second most widely grown cereal crop and the staple food for more than half of the world's population, providing 27% of dietary energy supply and 20% of dietary protein worldwide.[1] Thousands of rice varieties are being grown throughout the world and different countries have different preference for the type of rice. For instance, Indica rice varieties, which are hard but non-sticky when cooked, are generally preferred in India, Pakistan, and Indonesia, while Japonica rice varieties are favored in Japan and Korea because of their moderate elasticity and stickiness.[2] Genetic and environmental factors are mainly responsible for the variation in the composition and cooking quality of rice. Rice quality is a multi-faceted trait composed of several components such as the physical appearance, cooking and eating qualities, and nutritional value. Each component consists of many attributes whose values are determined not only by their physicochemical properties but also by the historical and cultural traditions of the people who consume the rice.[3]

Grain quality is the major concern in the rice production in Korea and many other rice-producing areas in the world. The eating quality of rice, also known as rice palatability, is a very important factor that determines the commercial value of rice. For Asian markets, the hardness and stickiness have been reported as the two most important parameters for determining the palatability of cooked rice.[4] Knowledge accumulated in the past decades indicates that the rice eating quality is directly related to the physicochemical properties of endosperm components, such as amylose, protein, water contents, and gelatinization characteristics.[3 ,5 ,6] Quality improvement of rice is now increasingly being emphasized in Korea breeding programs to increase rice consumption and international competitiveness.[7]

Assessment of the quality of cooked rice was reported to be more precisely measured by a combination of physical, chemical, and sensory properties. Enhanced knowledge on the relevant components of the grain that affect quality traits is of great importance, as this would likely assist the plant breeders on how to continually refine and improve the genetic traits of new varieties with the most desirable eating characteristics.[8] Earlier researches have indicated that rice of high eating quality possesses low amylose contents.[5] In general, cooked rice with low amylose content was soft and sticky, while rice with high amylose content was relatively firm and fluffy.[9] However, many cultivars with similar amylose contents exhibited different pasting and textural properties, suggesting that components other than amylose contribute to the cooking properties and palatability of rice.[10] Yu et al.[11] found that the eating quality of Yunnan japonica rice was negatively correlated with protein content, thus, they suggested that reducing the protein content should be the major target in improving the palatability of japonica rice cultivars. While extensive studies have been conducted with regards to the physicochemical properties of rice, it is still not entirely clear what specific components affect the eating quality of rice. Thus, this study was conducted to determine the chemical composition of the endosperm and texture properties of cooked rice from six elite varieties grown in Korea. Specifically, it aims to characterize and compare the composition of endosperm components of these varieties and establish correlations between the components and textural characteristics of rice.

MATERIALS AND METHODS

Materials

Six Korean elite rice varieties (Chucheongbyeo, Saechucheongbyeo, Mihyangbyeo, Hitomebore, Nampyeongbyeo, and Ilpumbyeo) were obtained from the National Institute of Crop Science (Suwon, Korea) and milled using a testing rice miller (MC-90A, Toyo Co., Japan). All rice samples were stored at −20°C in brown glass bottles until analysis.

Determination of Physicochemical Properties and Palatability

The moisture, protein, and amylose contents of the milled grains were measured using a whole grain analyzer (Foss Infratec 1241 Grain Analyzer, Sweden), while the palatability was determined using a Toyo taste meter (Toyo MB-90A, Japan).

Determination of Amino Acids

The milled rice samples were transformed into powder and dissolved in 0.5 mL distilled water. The sample was completely dried, derivatized with phenylisothiocyanate and analyzed using HPLC (SMART/HPLC 1100, Amersham Pharmacia Biotech Inc., Piscataway, NJ, USA), equipped with a variable wavelength detector (HP 1100 Series, 254 nm; Hewlett-Packard, Waldbron, Germany) and Waters symmetry C₁₈ column (4.6 × 250 mm, 5 μm; Waters Co., Milford, MA, USA). Samples were eluted with linear gradients using acetonitrile: H₂O (60:40, v/v) with a flow rate of 1 mL/min. The amino acids were quantified from HPLC chromatograms based on the peak area compared with that of the 20 amino acid standards.

Mineral Content Analysis

Rice powder (1.2 g) was placed in Teflon container and digested with 5 mL of HNO₃ at 100−150°C. The residual acid was vaporized and 30 mL distilled water was added. Samples were analyzed using ICP (Optima 3200 RL, Perkin-Elmer Inc., Shelton, CT, USA). The amount of each trace element was measured based from the standard curve of standard minerals.

Determination of Fatty Acids

Ground rice grains (1.0 g) were mixed with chloroform:methanol (2:1, v/v) to obtain the lipid extracts.[12] The fatty acids were converted into methyl ester and analyzed using GC-MS (HP 6890 series, Hewlett Packard Co., Waldbron, Germany) according to the method described by Chung.[13] The GC-MS was equipped with DB-225 capillary column (30m × 0.25mm × 0.25 μm) and Helium was used as carrier gas.

Determination of Non-Starch Polysaccharide Composition

The sugar content of non-starch polysaccharide was determined using the method described by Englyst et al.[14] with some modifications. Powdered rice sample (200 mg) was added with 2 mL dimethyl sulfoxide and vortexed 2 or 3 times during 30-min period in a boiling water bath. The mixture was added with 8 mL of enzyme solution 1 (kept at 50°C), vortexed, and kept in the boiling water bath for 10 min. The sample was transferred to a water bath maintained at 50°C and added with 0.5 mL of the enzyme solution 2 after 3 min and 40 mL absolute ethanol after 30 min along with mixing the contents. After standing for 30 min, 40 mL of absolute ethanol was added and mixed. The mixture was then placed in an ice-water for 30 min and centrifuged at 1500 × g for 10 min. The clear supernatant liquid was removed by aspiration, without disturbing the residue, and discarded. Appropriate volume of 85% ethanol was added to the residue to make up to 50 mL. The solution was mixed by inversion and a suspension of the residue was formed using a magnetic stirrer. The sample mixture was centrifuged at 1500 × g for 10 min and the supernatant was removed as described above. The tube was kept in a beaker having water at 80°C and placed on a hot-plate stirrer and the residue was mixed until dry. Sulfuric acid (12 M, 5 mL) was added to the dry residue and the mixture was kept at 35°C for 1 h with occasional or continuous mixing to disperse the cellulose. Distilled water (25 ml) was rapidly added and the tube was placed in a boiling water bath for 1 h with continuous stirring. The tube was cooled in tap water at room temperature. An allose internal standard (1 mg allose/mL) was added to 3 mL of the cooled hydrolysates and to 3 mL of the standard sugar mixture and vortexed. The tubes were placed in ice-water and added with 1 mL of 12.5 M ammonia solution, followed by 5 μL of the antifoam agent octan-2-ol and 0.2 mL of the ammonia-sodium tetrahydroborate solution. The uncapped tubes were placed in a water bath at 40°C for 30 min and added with 0.4 mL glacial acetic acid. A 0.5 mL of the sample mixture was placed into a 30 mL glass tube and mixed with 0.5 mL 1-methyl imidazole and 5 mL acetic anhydride. After 10 min, 0.9 mL of absolute ethanol and 0.5 mL bromophenol blue solution were added and mixed. The tubes were placed in ice-water and added with 5 mL of 7.5 M potassium hydroxide. After 2 min, another 5 mL of potassium hydroxide was added and the solution was left standing until the separation into 2 phases was complete. The supernatant was analyzed using gas chromatography equipped with DB-225 capillary column and coupled to a mass spectrometer. The column was held at 150°C for 2 min and temperature was increased up to 230°C at a rate of 4°C/min and held for 10 min. Helium was used as the carrier gas with a flow rate of 1 mL/min.

Analysis of Pasting Properties

Pasting properties of starch were measured using a Rapid Visco Analyzer, (RVA, Newport, Australia). Three grams of starch slurry (13% dry basis, 25 mL of deionized water) was placed in a disposable aluminum canister. The slurry was first held at 50°C for 1.5 min, heated to 95°C at a rate of 12°C/12 min and held for 2.0 min, and cooled to 50°C at a rate of 12°C/min and held for 1.5 min. The temperature corresponding to the initial increase in viscosity was designated as pasting temperature. Viscosity parameters (peak, trough, final, breakdown, and set back viscosity) were expressed in centipoises.

Statistical Analysis

Data were analyzed statistically using the Statistical Analysis System for Windows V8 (SAS institute, Inc., Cary, NC, USA). Analysis of variance and Duncan's multiple range tests were employed.

RESULTS AND DISCUSSION

Chemical Compositions and Palatability

The palatability, protein, moisture, and amylose values of the six Korean elite rice varieties are presented in Table 1. Hitomebore variety showed the highest palatability score (82.9), followed by Chucheongbyeo and Ilpumbyeo. Mihyangbyeo exhibited highest protein level with 8 g/100 g rice, but lowest amylose content (17.6 g/100 g rice) and palatability score (67.1). On the other hand, lowest protein content with 6.7 g/100 g rice was observed in Ilpumbyeo variety. Saechucheongbyeo and Chucheongbyeo contained higher amylose contents than that of the other varieties. Protein and amylose contents are two of the most important determinants of grain quality in rice.[15] Protein in rice is of particular importance for health especially for those whose main staple food is rice. Among the common cereal grains, rice has the highest net protein utilization.[16] However, protein content has been reported to have a negative correlation with the rice eating quality.[17] In a study conducted by Chikubu et al.,[18] it was found that rice protein content had negative correlation coefficients with appearance, aroma, taste, and stickiness, while it had positive correlation with hardness. Ohtsubo et al.[19] also reported that cooked rice with high protein content tends to be hard and non-sticky.

Table 1 Chemical composition and palatability of elite rice in Korea

Rice variety	Palatability	Protein (g/100 g rice)	Moisture (g/100 g rice)	Amylose (g/100 g starch)
Chucheongbyeo	76.93 ± 1.08^b	7.03 ± 0.05^c	14.20 ± 0.00^b	18.07 ± 0.05^a
Saechucheongbyeo	71.90 ± 2.44^d	6.93 ± 0.08^cd	12.77 ± 0.47^d	18.25 ± 0.08^a
Mihyangbyeo	67.13 ± 0.83^e	8.10 ± 0.00^a	13.27 ± 0.05^cd	17.57 ± 0.05^c
Hitomebore	82.92 ± 1.26^a	6.92 ± 0.09^cd	15.70 ± 0.55^a	17.68 ± 0.38^c
Nampyeongbyeo	73.37 ± 0.14^cd	7.37 ± 0.05^b	13.47 ± 0.05^c	17.63 ± 0.43^c
Ilpumbyeo	75.55 ± 2.17^bc	6.70 ± 0.06^d	12.93 ± 0.36^cd	17.77 ± 0.35^c
Values are means of three replications ± standard deviation. Means followed by different superscripts within the column are significantly different (p < 0.05).

Cooking characteristics, texture, water absorption, stickiness, volume expansion, and hardness, as well as whiteness and gloss, of cooked rice are affected by amylose content.[5] Low amylose-containing rice shows little expansion when cooked, has glossy and sticky appearance, and is relatively firm when cooked.[20] In general, amylose content is affected by temperature,[21] variety, degree of milling,[22] and nitrogen fertilization.[23] On the basis of amylose content, milled rice is classified as waxy (0–2% amylose, dry basis), low (10–20%), intermediate (20–25%) and high (>25%).[24] The amylose values (17.57–18.25 g/100 g rice starch) obtained in this study suggest that all the rice samples analyzed belong to the low amylose group.

With regards to the moisture content, Hitomebore rice showed the highest value (15.7 g/100 g rice starch), followed by Chucheongbyeo (14.2 g/100 g rice). Moisture content is important in maintaining the quality of grain. High moisture content is associated with loss of viability, high incidence of pests and diseases, and reduction in eating quality. For best grain quality, a 14% moisture content is recommended for rice grain.[3]

Amino Acids

The amino acids of the rice samples are presented in Table 2. Consistent with the findings of Muzafarov and Mazhidov,[25] glutamic acid showed the highest amount, with 17.71–24.51 ng/mg rice, in all the rice samples analyzed, whereas cysteine exhibited the lowest concentration (0.31–0.39 ng/mg rice). Abundant amount of essential amino acids was observed in Hitomebore and Ilpumbyeo varieties. Among the non-essential amino acids, glycine and alanine contents were also highest in Hitomebore, while aspartic acid, glutamic acid, and glutamine were abundant in Saechucheongbyeo rice. Asparagine and arginine contents were highest in Chucheongbyeo and Mihyangbyeo samples, respectively. No significant differences were found in the concentrations of cysteine, serine, histidine, proline, and tyrosine among the varieties analyzed. These findings indicate that in terms of non-essential amino acid content, no particular variety was superior. Superiority of a particular variety will greatly depend on the type of amino acid being considered.

Table 2 Amino acid content (ng/mg rice) of elite rice in Korea

Amino acid	Chucheongbyeo	Saechucheongbyeo	Mihyangbyeo	Hitomebore	Nampyeongbyeo	Ilpumbyeo
Cysteine	0.33 ± 0.03^a	0.31 ± 0.00^a	0.39 ± 0.04^a	0.39 ± 0.06^a	0.33 ± 0.03^a	0.34 ± 0.09^a
Aspartic acid	15.57 ± 0.29^ab	17.18 ± 0.27^a	15.23 ± 1.76^ab	14.83 ± 0.81^b	15.60 ± 0.38^ab	16.83 ± 0.86^ab
Glutamic acid	21.09 ± 1.32^b	24.51 ± 0.27^a	22.81 ± 0.26^ab	17.71 ± 1.58^c	21.46 ± 0.46^ab	20.86 ± 0.58^b
Asparagine	16.88 ± 0.56^a	13.88 ± 1.98^b	14.23 ± 0.32^b	14.36 ± 0.95^b	13.45 ± 0.06^b	13.47 ± 0.47^b
Serine	3.69 ± 0.99^a	4.07 ± 0.32^a	4.29 ± 0.31^a	4.28 ± 0.09^a	4.54 ± 0.01^a	4.35 ± 0.20^a
Glutamine	3.11 ± 0.15^de	4.95 ± 0.02^a	3.54 ± 0.16^cd	2.62 ± 0.38^e	4.47 ± 0.36^ab	4.09 ± 0.16^bc
Glycine	3.82 ± 0.26^b	3.48 ± 0.24^b	3.63 ± 0.03^b	4.74 ± 0.18^a	3.42 ± 0.28^b	3.57 ± 0.43^b
Histidine	1.14 ± 0.12^a	1.11 ± 0.01^a	1.22 ± 0.04^a	1.14 ± 0.15^a	1.22 ± 0.12^a	1.26 ± 0.04^a
Arginine	3.79 ± 2.20^bc	3.58 ± 0.04^c	5.11 ± 0.14^a	2.73 ± 0.12^d	4.07 ± 0.14^b	5.08 ± 0.16^a
Thereonine	1.59 ± 0.01^b	1.50 ± 0.06^b	1.61 ± 0.09^b	1.65 ± 0.02^ab	1.69 ± 0.07^ab	1.90 ± 0.19^a
Alanine	12.90 ± 1.12^b	8.64 ± 0.41^d	10.77 ± 0.91^c	15.27 ± 1.30^a	6.77 ± 0.39^d	7.63 ± 0.07^d
Proline	2.60 ± 0.57^a	2.32 ± 0.04^a	3.11 ± 0.18^a	2.48 ± 0.73^a	2.52 ± 0.27^a	2.27 ± 0.22^a
Tyrosine	0.91 ± 0.21^a	0.94 ± 0.02^a	1.13 ± 0.12^a	0.93 ± 0.08^a	1.19 ± 0.19^a	1.14 ± 0.15^a
Valine	1.63 ± 0.08^ab	1.35 ± 0.16^b	1.86 ± 0.24^a	1.47 ± 0.2^ab	1.53 ± 0.02^ab	1.43 ± 0.22^ab
Methionine	0.86 ± 0.17^ab	0.76 ± 0.01^b	0.88 ± 0.07^ab	0.96 ± 0.08^ab	0.90 ± 0.01^ab	1.11 ± 0.16^a
Isoleucine	0.80 ± 0.04^cb	1.07 ± 0.07^ab	0.71 ± 0.17^c	1.19 ± 0.06^a	1.18 ± 0.24^a	1.01 ± 0.02^abc
Leucine	0.85 ± 0.05^c	1.04 ± 0.01^bc	1.18 ± 0.01^ab	1.28 ± 0.10^a	1.03 ± 0.05^bc	0.99 ± 0.14^bc
Phenylalanine	0.68 ± 0.16^c	0.63 ± 0.05^c	0.64 ± 0.01^c	1.84 ± 0.05^a	1.09 ± 0.21^b	0.88 ± 0.14^ab
Tryptophan	6.79 ± 0.21^c	7.30 ± 0.24^c	6.56 ± 0.04^c	9.41 ± 0.26^b	10.49 ± 0.69^a	9.17 ± 0.14^b
Lysine	0.69 ± 0.02^b	0.64 ± 0.02^b	0.74 ± 0.06^b	0.67 ± 0.11^b	0.75 ± 0.00^b	1.31 ± 0.33^a
Values are means of three replications ± standard deviation. Means followed by different superscripts within the row are significantly different (p < 0.05).

Mineral Contents

Magnesium (Mg), calcium (Ca), and potassium (K), are the most abundant minerals found in rice.[26] While these minerals are important for human health, they could negatively affect the palatability of rice. It was previously reported that Ca and K have negative correlation to the overall palatability of cooked rice.[27] In this study, Mihyangbyeo variety obtained the highest Mg (359.71 ppm of rice) and K (1057.99 ppm of rice) contents, whereas Hitomebore showed the lowest values (Table 3). On the other hand, Chucheongbyeo, Saechucheongbyeo, Nampyeongbyeo, and Ilpumbyeo varieties exhibited relatively higher Ca content with 53.00-56.33 ppm than Mihyangbyeo (44.17 ppm of rice) and Hitomebore (42.65 ppm of rice) samples. Since Ca and K have negative correlations to the rice eating quality, the high palatability score of Hitomebore variety (Table 1) could partly be due to the low amounts of Ca and K found in the rice sample.

Table 3 Mineral content of elite rice in Korea

	Mineral content (ppm of rice)
Rice variety	Mg	Ca	K
Chucheongbyeo	254.06 ± 1.36^b	54.34 ± 2.76^a	762.65 ± 11.38^d
Saechucheongbyeo	224.00 ± 12.72^c	53.67 ± 0.01^a	826.55 ± 6.15^c
Mihyangbyeo	359.71 ± 3.69^a	44.17 ± 1.18^b	1057.99 ± 2.83^a
Hitomebore	203.00 ± 0.14^d	42.65 ± 0.78^b	669.75 ± 8.69^e
Nampyeongbyeo	51.55 ± 6.29^b	56.33 ± 0.49^a	841.60 ± 0.98^bc
Ilpumbyeo	253.06 ± 2.77^b	53.00 ± 0.46^a	877.20 ± 3.96^b
Values are means of three replications ± standard deviation. Means followed by different superscripts within the column are significantly different (p < 0.05).

Fatty Acids

The major fatty acids found in the rice samples analyzed were linoleic (C18:2), oleic (C18:1), and palmitic (C16:0) acids (Table 4). These three fatty acids accounted for more than 95% of the total fatty acids in samples as also reported by previous researchers in other varieties of rice.[28–30] A slight variation was found in the fatty acid compositions among samples. The amounts of oleic and linoleic acids did not significantly differ (p < 0.05) among the rice varieties, while palmitic acid content was relatively lower in Mihyangbyeo rice than that of the other samples analyzed. On the other hand, Mihyangbyeo exhibited highest amount of linolenic (C18:3) and arachidic (C20:0) acids. No significant varietal differences were found in the myristic (C14:0), palmitoleic (C16:1), stearic (C18:0), and gadoleic (C20:1) contents. In general, all the rice samples exhibited relatively similar concentrations of saturated (21–24 g/100 g rice) and unsaturated (75–78 g/100 g rice) fatty acids.

Table 4 Fatty acid content of elite rice in Korea

	Fatty acids (g/100 g rice)
Rice variety	C14:0	C16:0	C16:1	C18:0	C18:1	C18:2	C18:3	C20:0	C20:1
Chucheongbyeo	0.36 ± 0.14^a	21.61 ± 1.51^a	0.16 ± 0.09^a	1.90 ± 0.28^a	35.04 ± 2.33^a	39.12 ± 0.20^a	0.96 ± 0.02^c	0.43 ± 0.02^b	0.33 ± 0.01^a
Saechucheongbyeo	0.23 ± 0.04^a	20.07 ± 0.36^ab	0.18 ± 0.01^a	1.15 ± 0.32^a	36.76 ± 0.19^a	39.32 ± 1.04^a	1.05 ± 0.09^bc	0.46 ± 0.01^b	0.35 ± 0.01^a
Mihyangbyeo	0.33 ± 0.09^a	18.95 ± 0.77^b	0.18 ± 0.10^a	1.60 ± 0.32^a	38.02 ± 2.81^a	38.59 ± 2.26^a	1.26 ± 1.00^a	0.58 ± 0.03^a	0.39 ± 0.04^a
Hitomebore	0.29 ± 0.01^a	20.09 ± 0.20^ab	0.24 ± 0.15^a	1.84 ± 0.14^a	38.51 ± 0.84^a	37.05 ± 1.29^a	1.06 ± 0.04^bc	0.47 ± 0.06^b	0.38 ± 0.09^a
Nampyeongbyeo	0.35 ± 0.05^a	20.96 ± 0.52^a	0.17 ± 0.09^a	1.78 ± 0.03^a	34.66 ± 0.57^a	39.99 ± 0.34^a	1.16 ± 0.08^ab	0.47 ± 0.00^b	0.34 ± 0.02^a
Ilpumbyeo	0.29 ± 0.01^a	19.95 ± 0.20^ab	0.15 ± 0.06^a	1.54 ± 0.03^a	37.28 ± 1.21^a	38.81 ± 1.32^a	1.05 ± 0.02^bc	0.48 ± 0.02^b	0.36 ± 0.07^a
Values are means of three replications ± standard deviation. Means followed by different superscripts within the column are significantly different (p < 0.05).

Sugar Contents of Non-Starch Polysaccharides

Significant variations on the sugar content of non-starch polysaccharides were observed within the rice varieties (Table 5). Arabinose had the highest sugar concentration, with 2.69–14.27 μg/g rice, in all the samples, followed by fucose (1.15–4.75 μg/g rice), and rhamnose (0.36–4.07 μg/g rice). Mihyangbyeo variety contained the highest amounts of all the sugars analyzed. On the other hand, Hitomebore rice exhibited low concentrations of rhamnose and ribose, whereas Nampyeongbyeo and Ilpumbyeo samples showed the lowest fucose and xylose contents. Least amount of arabinose was observed in Nampyeongbyeo rice.

Table 5 Sugar content of elite rice in Korea

	Sugar content (μg/g rice)
Rice variety	Rhamnose	Fucose	Ribose	Arabinose	Xylose
Chucheongbyeo	0.96 ± 0.11^c	1.67 ± 0.03^cd	0.32 ± 0.05^cd	4.86 ± 0.52^b	0.40 ± 0.16^c
Saechucheongbyeo	1.26 ± 0.13^bc	2.47 ± 0.25^b	0.84 ± 0.03^b	6.37 ± 0.19^b	0.28 ± 0.08^bc
Mihyangbyeo	4.07 ± 0.25^a	4.75 ± 0.27^a	2.22 ± 0.11^a	14.27 ± 0.40^a	0.63 ± 0.07^a
Hitomebore	0.36 ± 0.15^d	2.13 ± 0.24^bc	0.22 ± 0.11^d	5.17 ± 1.21^b	0.11 ± 0.01^cd
Nampyeongbyeo	1.28 ± 1.27^bc	1.15 ± 0.15^d	0.50 ± 0.22^c	2.69 ± 0.66^c	0.06 ± 0.00^d
Ilpumbyeo	1.68 ± 0.09^b	1.44 ± 0.30^d	0.90 ± 0.13^b	5.45 ± 0.77^b	0.02 ± 0.23^d
Values are means of three replications ± standard deviation. Means followed by different superscripts within the column are significantly different (p < 0.05).

Pasting Properties

The pasting properties are indicators of the processing quality of rice. They are assessed based from the pasting curves obtained using a rapid visco analyzer. Genotype, growing season, and growing area affect the pasting behavior of rice flour.[31] In addition, molecular structure and composition of starch have been demonstrated to have relationships on its pasting properties.[32 ,33] The pasting time, temperature, and viscosity values differed significantly among the elite rice samples analyzed (Table 6). Mihyangbyeo and Nampyeongbyeo varieties exhibited significantly higher pasting temperature (82.75°C) and time (3.78 min) than that of the other rice samples. On the other hand, lowest peak (2522 cP), trough (1617 cP), and breakdown (905 cP) viscosities were observed in Mihyangbyeo. Hitomebore showed the highest peak and breakdown viscosities with 3285 cP and 1575 cP, respectively, but lowest final viscosity (3208 cP). The higher breakdown value of Hitomebore demonstrates the ease with which starch granules are broken upon heating after the maximum swelling at the peak viscosity. Low-amylose containing rice possesses this property, which results in the stickiness of the paste. A considerably higher trough viscosity was obtained in Chucheonbyeo and Ilpumbyeo samples, whereas the highest final viscosity value was found in Chucheongbyeo and Nampyeongbyeo varieties. The observed variations in the pasting properties among the samples could be attributed to the differences in the chemical compositions of the rice kernels of the varieties examined.

Table 6 Pasting properties of elite rice in Korea

			Viscosity (cP)
Rice variety	Pasting temperature (°C)	Pasting time (min)	Peak	Trough	Final	Breakdown
Chucheongbyeo	70.20 ± 0.86^b	2.71 ± 0.07^b	3047 ± 81.24^b	1774 ± 61.25^a	3698 ± 178.84^a	1273 ± 23.00^bc
Saechucheongbyeo	69.68 ± 0.75^b	2.67 ± 0.06^b	3046 ± 51.48^b	1720 ± 40.20^ab	3451 ± 107.03^b	1326 ± 91.32^b
Mihyangbyeo	82.75 ± 1.00^a	3.78 ± 0.84^a	2522 ± 24.55^d	1617 ± 39.15^c	3473 ± 33.08^b	905 ± 45.82^d
Hitomebore	70.45 ± 0.08^b	2.73 ± 0.00^b	3285 ± 59.75^a	1710 ± 66.55^ab	3208 ± 90.20^c	1575 ± 31.57^a
Nampyeongbyeo	81.65 ± 8.36^a	3.69 ± 0.71^a	2900 ± 67.84^c	1660 ± 33.60^bc	3710 ± 125.92^a	1240 ± 37.58^bc
Ilpumbyeo	69.70 ± 0.05^b	2.67 ± 0.00^b	2961 ± 51.40^bc	1757 ± 37.82^a	3466 ± 100.89^b	1203 ± 24.54^c
Values are means of three replications ± standard deviation. Means followed by different superscripts within the column are significantly different (p < 0.05).

Correlations Among Various Physicochemical Properties and Palatability

Results of the correlation analysis revealed that the protein content was positively correlated with rhamnose, linoleic acid, and arachidic acid contents (Table 7). The amylose was positively correlated with breakdown viscosity (0.892), but negatively correlated with threonine content (−0.817). The palatability, on the other hand, showed negative correlations with ribose (−0.846), rhamnose (−0.854), and potassium (−0.922). The correlation coefficients of amino acids in relation to other chemical properties are shown in Table 8. Serine, arginine, and proline showed negative correlations with breakdown viscosity. Glutamine and alanine exhibited a positive correlation with linoleic and K content, respectively. Proline, on the other hand, was positively correlated with linolenic acid, Mg, and rhamnose.

Table 7 Correlation coefficient of protein, amylose, and palatability in relation to other chemical components in Korean elite rice

	Chemical component	Correlation coefficient
Protein	Rhamnose	0.874*
	Linoleic acid	0.859*
	Arachidic acid	0.818*
Amylose	Threonine	−0.817*
	Breakdown	0.892*
Palatability	Ribose	−0.846*
	Rhamnose	−0.854*
	K	−0.922**
*Significant at p < 0.05 level; **Significant at p < 0.01 level.

Table 8 Correlation coefficient of amino acids in relation to other chemical components in Korean elite rice

Amino acid	Chemical component	Correlation coefficients
Cysteine	Palmitoleic acid	0.909*
Aspartic acid	Stearic acid	−0.934**
Serine	Breakdown	−0.860*
Glutamine	Linoleic acid	0.840*
Glycine	Xylose	−0.851*
	Palmitoleic acid	0.917**
	Xylose	0.830*
Arginine	Breakdown	−0.856*
Alanine	Xylose	0.888*
	K	0.916*
Proline	Linolenic acid	0.850*
	Mg	0.838*
	Rhamnose	0.835*
	Breakdown	−0.905*
Leucine	Gadoleic acid	0.851*
Phenylalanine	Palmitoleic acid	0.876*
*Significant at p < 0.05 level; **Significant at p < 0.01 level.

Correlation analysis between the mineral content and other chemical properties showed that Mg was positively correlated with rhamnose, K, and ribose, whereas it was negatively correlated with breakdown viscosity (Table 9). Ca showed positive and negative correlations with linoleic and oleic acids, respectively. On the other hand, K was positively correlated with rhamnose and ribose, while negatively correlated with breakdown viscosity. The relationship among fatty acids, sugars, and pasting properties are presented in Table 10. Both rhamnose and ribose showed negative correlations with breakdown viscosity.

Table 9 Correlation coefficient of minerals in relation to other chemical components in Korean elite rice

Mineral	Chemical component	Correlation coefficient
Mg	Rhamnose	0.931**
	K	0.904*
	Ribose	0.873*
	Arabinose	0.855*
	Arachidic acid	0.841*
	Breakdown	−0.935**
Ca	Linoleic acid	0.848*
	Oleic acid	−0.900*
	Gadoleic acid	−0.949**
K	Rhamnose	0.943**
	Ribose	0.943**
	Arachidic acid	0.835*
	Arabinose	0.831*
	Breakdown	−0.967**
*Significant at p < 0.05 level; **Significant at p < 0.01 level.

Table 10 Correlation coefficient of fatty acids in relation to other chemical components in Korean elite rice

Fatty acid	Chemical component	Correlation coefficient
Palmitic acid	Oleic acid	−0.823*
Palmitoleic acid	Xylose	0.837*
	Fucose	0.855*
Arachidic acid	Ribose	0.925**
	Rhamnose	0.922**
	Arabinose	0.902*
Rhamnose	Breakdown	−0.892*
Ribose	Breakdown	−0.859*
*Significant at p < 0.05 level; **Significant at p < 0.01 level.

Previous studies have shown that rice palatability is negatively correlated with protein content and positively correlated with breakdown viscosity values.[34 ,35] While no direct correlations were found between palatability and protein content or pasting property in this study, the chemical components (ribose, rhamnose, and potassium) that were negatively correlated to palatability, also exhibited positive correlations with breakdown viscosity. This suggests that viscosity properties could also be good indicators for evaluating the palatability of rice.

CONCLUSION

The present study demonstrates that the elite rice varieties in Korea, with superior eating qualities, significantly differ in their physicochemical properties. Correlations among the physicochemical properties and palatability of rice were established. Biochemical indices such as cell wall components and gelatinization characteristics could be used in evaluating the eating quality of rice. High palatability is an important factor in determining the quality of rice and elucidating the correlations between palatability and physicochemical components in rice grain could help the plant breeders in developing highly palatable cultivars.

ACKNOWLEDGMENTS

This study was supported by a grant from the second stage of Brain Korea 21 Project of the Ministry of Education and Human Resources Development, Korea.