A Comparative Study on the Physicochemical Characteristics of Black Rice Varieties

Abstract

Physicochemical properties of black rice (Dragon eyeball 100, Heukjinjubyeo, Heukgwangbyeo, Heuknambyeo, and Josaengheukchal) and white rice (Hwayoungbyeo) varieties were investigated. Morphological properties slightly differ among the rice samples analyzed. Black rice varieties showed a higher amount of minerals, faster hydrolysis rate, and lower blue value than the ordinary white rice. High amino acid and sugar contents were found in Heuknambyeo and Heukgwangbyeo varieties, respectively. Lower oleic acid, but higher linoleic acid, was observed in Dragon eyeball 100 and Heukjinjubyeo compared with the other samples. Likewise, the pasting and viscosity values were generally higher in Heuknambyeo and Heukgwangbyeo, while lowest in Josaengheukchal. No substantial difference in X-ray diffraction pattern was observed among the samples. This study illustrates the wide variation in the physicochemical properties of the black rice varieties analyzed. The results could serve as baseline information for food processors in evaluating the quality of black rice suitable for specialty food processing.

Keywords:

• Black rice
• Chemical composition
• Physical property
• Rice endosperm

INTRODUCTION

Black rice (Oryza sativa L.) is a specialty rice variety with black bran covering the endosperm. The rice endosperm, which is translucent with gray to almost black color, turns deep purple when cooked. It is usually higher in gluten, takes longer time to cook, and is stickier than white rice. Due to its unusual color and sweet nutty flavor, black rice has become popular for making sweet snacks and desserts in many Asian countries. Anthocyanins, particularly cyanidin 3-glucosidase and peonidin 3-glucosidase, are responsible for the color of black rice.[1,2] These bioactive compounds were reported to have strong free radical scavenging and antioxidant effects,[3–5] help lower cholesterol levels,[6] and reduce the risks of cardiovascular diseases and cancers.[7]

Because of the unique color, flavor, and nutritional health benefits of black rice, the demand for this grain has increased rapidly in the past years. As a result, the production of black rice has also been intensified. This increase in black rice production created an opportunity for new product development, such as black rice bread, black noodles, and black rice cake.[8] High-quality and specialty rice varieties, including black rice, suitable for food processing is continuously being developed to enhance the competitiveness of processed rice foods.[9] A wide variation in the rice starch characteristics is essential to meet the diverse requirements for specific food processing by consumers and processors. It is, therefore, important to elucidate the physicochemical properties of rice to facilitate the understanding of its potential uses and applications.

Recently, some rice varieties having blackish purple pericarp, such as Heukgwangbyeo, Heukjinjubyeo, and Heuknambyeo, were developed in Korea.[9] However, little is known about the nutritional composition of the endosperm among these black rice cultivars, and there are no reports on their starch and flour properties. Thus, this study was conducted to compare the physicochemical properties of rice endosperm from different black rice varieties. The amino and fatty acids compositions and sugar and mineral contents of rice endosperm, as well as the pasting properties of rice flour, and the iodine absorption and hydrolysis rate of starch were analyzed.

MATERIALS AND METHODS

Rice Sample Preparation

Six rice cultivars were obtained from the Korea Rural Development Administration. Five were black rice (Heukgwangbyeo, Dragon eyeball 100, Heuknambyeo, Heukjinjubyeo, and Josaengheukchal) and one was a traditional white rice cultivar (Hwayoungbyeo). The rice samples were milled using a testing rice miller (MC-90A, Toyo Co., Japan) and stored at −20°C in brown glass bottles until used. Rice starch was prepared from the rice grains using the alkali method described by Yamamoto et al.[10] Briefly, milled rice was steeped in 3–4 volumes of 50 mM lithium hydroxide solution at 4°C for 12 h to soften the endosperms. The liquid was drained and the endosperms were ground lightly with mortar and pestle. Isoamyl alcohol, acetone, and ethyl alcohol were added to eliminate protein and lipid of endosperm and the mixture was washed with distilled water. The precipitate was collected and dried in a cabinet dryer at 30°C. The starch was stored in a desiccator until analyzed.

Scanning Electron Microscopy

The rice starch samples were mounted on a circular aluminum specimen stub and gold-coated in a vacuum using a sputter coater. The starch granular structures were examined using an environmental scanning electron microscope (S-570, Hirachi, Japan) operating at 15 kV.[11]

Free Amino Acid Composition

The free amino acid composition was analyzed according to the method described by Steenson and Sathe.[12] Rice endosperm samples were powderized using a blender (J-World Tech., Ansan, Korea), passed through a 100-mesh sieve, added to pyrolyzed borosilicate vial, and dried under vacuum. Each vial was placed in a vacuum hydrolysis vessel with constant boiling 6 N HCl. After the vapor hydrolysis, samples were dried, derivatized with methanol:H₂O:triethylamine:phenylisothiocyanate (7:1:1:1, v/v/v/v) and subjected to HPLC analysis. The HPLC (SMART/HPLC 1100, Amersham Pharmacia Biotech Inc., USA) was equipped with a variable wavelength detector (HP 1100 Series, 254 nm) and waters symmetry C₁₈ column (4.6 × 250 mm, 5 μm). 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 standards.

Mineral Content Analysis

Rice powder (1.2 g) was placed in a Teflon container and digested with 5 mL concentrated nitric acid at 100–150°C. The residual acid was vaporized and 30 mL distilled water was added. The samples were then analyzed using an inductively coupled plasma optical emission spectrophotometer (ICP-OES, Optima 3200 RL, Perkin-Elmer Inc., Shelton, CT, USA). The amount of each trace element was measured based on the standard curve of standard minerals.[13]

Fatty Acid Composition

Ground rice grains (1.0 g) were mixed with chloroform:methanol (2:1, v/v) to obtain the lipid extracts.[14] The fatty acid composition of the lipid extracts was determined according to the method described by Chung.[15] Briefly, the lipid extracts were saponified with 0.1 N potassium hydroxide in methanol at 70–75°C and followed by methanolysis with 1.2 M hydrochloric acid in methanol at the same temperature. The methyl esters of fatty acids were extracted with n-hexane prior to GC-MS analysis. The gas chromatograph (6890 plus, Hewlett Packard, Co., USA) was equipped with a DB-225 capillary column (30 m × 0.25 mm × 0.25 μm) and coupled with a mass spectrometer (JMS700, Jeol, Japan). The column temperature was held at 140°C for 1 min, then increased up to 200°C at a rate of 1°C/min, and finally increased to 230°C at a rate of 10°C/min and held for 10 min. Helium was used as the carrier gas and the flow rate was 1 mL/min. The amount of each fatty acid was calculated based on the peak area compared with that of the standards.

Analysis of Non-Starch Polysaccharide

The sugar content of non-starch polysaccharide was determined using the method described by Englyst et al.[16] with some modifications. Powdered rice samples (200 mg) were mixed with 2 mL dimethyl sulfoxide and vortexed 2 or 3 times during a 30-min period at boiling water bath. Termamyl:sodium acetate (0.6:100.0, v/v) solution (8 mL) was added and the mixture was transferred to a 50°C water bath after 10 min standing. Pancreatin:H₂O:pullulanase solution (1.2:12.0:2.5, w/v/v) was added, followed by 40 mL absolute ethanol after 30 min. The mixture was transferred in ice-water for 30 min and centrifuged at 1500 g for 10 min. The clear supernatant liquid was added with 40 mL ethanol (85%) and 20 mL acetone, placed in a water bath (80°C), and mixed until dry. Sulfuric acid (5 mL, 12 M) was added to the residue and the mixture was kept at 35°C for 1 h. Distilled water (25 mL) was rapidly added to the mixture and placed in boiling water bath for 1 h. After cooling, allose internal standard (1 mg allose/mL) was added to 3 mL of the cool hydrolysates and to 3 mL of standard sugar mixture. A 1-mL amount of ammonia solution (12.5 M) was added, followed by 5 μL of antifoaming agent octan-2-ol and 0.2 mL of ammonia-sodium tetrahydroborate solution. The mixture was placed in a water bath (40°C) for 30 min and added with 0.4 mL glacial acetic acid. A 0.5-mL amount 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 potassium hydroxide (7.5 M). 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 a DB-225 capillary column and coupled to a mass spectrometer. The column was held at 150°C for 2 min, then increased up to 230°C at a rate of 4°C/min and held for 10 min. Helium was used as the carrier gas and the flow rate was 1 mL/min. The sugars were quantified based on the peak area compared with that of the standards.

Iodine Absorption and Blue Value Analysis

The blue value is a crude index of the amylose content of starch. It is determined by a colorimetric assay in which iodine binds with amylose to produce a blue-colored complex.[17] This technique has long been used as an indicator of the amount of amylose present in starch. In the present study, the blue value was determined based from the method of Gilbert and Spragg[18] with some modifications. Briefly, rice starch samples were added with 2 N sodium hydroxide solution and neutralized with 0.1 N acetic acid. Iodine potassium iodide reagent (1% I₂ in 10% KI) was added and the absorbance of the sample mixture was scanned from 500 to 700 nm using UV/visible spectrometer (DU 800 series, Beckman Coulter, USA) to identify the wavelength with maximum absorbance. The blue value of rice starch was determined by obtaining the absorbance of the sample at 680 nm.

Enzymatic Hydrolysis of Rice Starch

Starch samples (100 mg) were placed in 15-mL screw-cap tubes. Acetic acid (0.4%, pH 4.8) and 59.9 units of 5% amyloglucosidase (Fluka, Switzerland) were added. The mixture was incubated with constant shaking at 37°C for 3 h. After the enzyme hydrolysis, 100 μL of aliquot was transferred to 15-mL clean tubes and was mixed with distilled water. The sample was boiled for 10 min. The total concentration of carbohydrates in the samples was determined using the phenol-sulfuric acid method described by Dubois et al.[19] The absorbance was measured at 490 nm against a glucose standard. In addition, the concentration of the liberated glucose as a result of enzymatic hydrolysis was determined by measuring the absorbance at 525 nm following the glucose-oxidase peroxidase method.[20]

X-Ray Diffraction Analysis

The X-ray patterns of starches were obtained with an X-ray diffractometer (X'pert APD, Philips, Amsterdam, Netherlands). The starch powder was tightly packed in small holders. Each sample was exposed to X-ray beams with generator running at 20 mA and 30 kV. The scanning region of the diffraction angle (2θ) ranged from 5° to 40°. The overall degree of crystallinity was quantified as the ratio of the area of crystalline reflections to the overall diffraction area.[21]

Analysis of Pasting Properties of Rice Flour

The pasting properties of rice flour samples were measured according to the AACC-approved method 61-02[22] using a Rapid Visco Analyzer (RVA, Newport, Australia). Three grams of flour slurry (13% dry basis, 2.5 mL 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 1°C/min, held for 2 min, cooled to 50°C at a rate 12°C/min and held for 1.5 min. The temperature and time corresponding to the initial increase in viscosity was designated as the pasting temperature and pasting time, respectively. Viscosity parameters (peak, trough, final, breakdown, and set back viscosity) were expressed in centipoises.

Statistical Analysis

All experiments were done in triplicate (n = 3) and the data obtained were analyzed statistically using the Statistical Analysis System for Windows V8. Analysis of variance and Duncan's multiple range test were employed.[23]

RESULTS AND DISCUSSION

Morphological Properties

Scanning electron micrographs of rice starch granules from different black rice cultivars are shown in Fig. 1. The starch granules are polyhedral and irregular in shape, which is typical of many rice starches. Starch from Josaengheukchal was mainly composed of small size irregular granules while the rest of the samples mostly consisted of large polyhedral granules with a few small and irregular granules. The starch granules in Hwayoungbyeo, Dragon eyeball 100, and Heukjinjubyeo appeared to be loosely packed with larger air spaces between granules than that of the Heukgwangbyeo, Heuknambyeo, and Josaengheukal. The variations in starch granule morphology may be due to the biological origin and physiology of rice. This may also be due to the variations in the amylose and amylopectin contents of rice samples, which play an important role in the control of the starch granule size and shape.[24]

Figure 1 Scanning electron micrographs of rice starches (5000×): (A) Hwayoungbyeo, (B) Dragon eyeball 100, (C) Heukjinjubyeo, (D) Heukgwangbyeo, (E) Heuknambyeo, and (F) Josaengheukchal.

Free Amino Acid Composition

Protein is the most abundant component in rice grain next to starch[25] and black rice was reported to contain higher protein and total essential amino acids than regular rice.[26] The amino acid composition of the black rice samples is presented in Table 1. Aspartic (68.07–165.51 ng/mg rice), glutamic (138.91–232.29 ng/mg rice), and asparagines (73.55–242.89 ng/mg rice) were the most abundant amino acids in all the samples while cysteine and tryptophan were the limiting amino acids. This is in agreement with the findings of Muzafarov and Mazhidov.[27] The amino acid content of the rice samples varied significantly among cultivars. Higher amounts of aspartic, glutamic, alanine, and tyrosine were found in regular rice Hwayoungbyeo than the black rice varieties. On the other hand, most of the essential amino acids were significantly higher in Heukjinjubyeo and Josaengheukchal compared with the other samples. Heuknambyeo exhibited the highest total amino acid content while Dragon eyeball 100 contained the least amount of amino acids. These findings suggest that Heuknambyeo rice is superior to other black rice varieties in terms of amino acid composition.

Table 1 Free amino acid composition (ng/mg rice) of black rice

Amino acid	Hwayoungbyeo ^1	Dragon eyeball 100	Heukjinjubyeo	Heukgwangbyeo	Heuknambyeo	Josaengheukchal
CYS	0.00	0.00	0.00	0.00	0.00	0.00
ASP	165.51 ± 0.19^f	68.07 ± 0.08^a	81.19 ± 0.01^b	147.36 ± 0.09^e	107.53 ± 0.14^c	113.49 ± 0.12^d
GLU	232.29 ± 0.09^f	185.44 ± 0.15^d	151.41 ± 0.20^b	160.30 ± 0.12^c	193.81 ± 0.09^e	138.91 ± 0.94^a
ASN	107.17 ± 0.07^c	73.55 ± 0.02^a	194.79 ± 0.15^e	90.61 ± 0.10^b	242.89 ± 0.16^f	114.99 ± 0.20^d
SER	35.65 ± 0.04^e	25.62 ± 0.05^b	51.09 ± 0.09^f	20.12 ± 0.21^a	29.27 ± 0.07^c	32.77 ± 0.33^d
GLN	15.38 ± 0.00^d	17.14 ± 0.01^e	24.27 ± 0.08^f	10.08 ± 0.07^a	14.90 ± 0.09^c	13.48 ± 0.04^b
GLY	23.58 ± 0.00^c	22.24 ± 0.04^b	31.84 ± 0.08^f	25.38 ± 0.04^e	21.72 ± 0.03^a	24.74 ± 0.23^d
HIS	23.17 ± 0.00^e	8.26 ± 0.04^a	23.92 ± 0.04^f	15.89 ± 0.04^b	16.29 ± 0.01^c	18.61 ± 0.07^d
ARG	49.72 ± 0.07^c	27.40 ± 0.06^a	48.19 ± 0.05^b	61.33 ± 0.07^e	55.76 ± 0.30^d	49.51 ± 0.65^c
THR	14.68 ± 0.01^d	8.77 ± 0.02^a	19.92 ± 0.14^e	11.72 ± 0.07^b	11.61 ± 0.07^b	12.64 ± 0.19^c
ALA	60.64 ± 0.02^f	31.67 ± 0.02^a	43.56 ± 0.09^c	41.81 ± 0.06^b	53.45 ± 0.34^e	47.50 ± 0.54^d
PRO	24.67±0.01^e	15.84±0.04^b	20.44±0.15^c	15.60±0.03^a	28.57±0.07^f	20.70±0.02^d
TYR	13.30 ± 0.00^e	5.84 ± 0.04^a	12.14 ± 0.09^c	11.63 ± 0.03^b	11.72 ± 0.01^b	12.59 ± 0.06^d
VAL	15.35 ± 0.01^b	10.90 ± 0.02^a	18.77 ± 0.05^e	16.30 ± 0.11^d	15.79 ± 0.02^c	21.50 ± 0.06^f
MET	6.83 ± 0.00^d	4.22 ± 0.02^a	5.28 ± 0.02^b	6.93 ± 0.02^e	6.70 ± 0.00^c	7.63 ± 0.07^f
ILE	7.15 ± 0.00^e	5.14 ± 0.00^a	8.49 ± 0.02^f	5.22 ± 0.02^b	5.27 ± 0.01^c	5.56 ± 0.02^d
LEU	5.85 ± 0.01^e	4.52 ± 0.00^a	9.29 ± 0.01^f	5.11 ± 0.01^b	5.72 ± 0.01^d	5.17 ± 0.01^c
PHE	5.11 ± 0.00^c	4.64 ± 0.01^a	8.28 ± 0.01^f	6.23 ± 0.02^d	6.97 ± 0.00^e	4.79 ± 0.01^b
TRP	0.00	0.00	0.00	0.00	0.00	0.00
LYS	9.23 ± 0.00^c	4.21 ± 0.00^a	6.02 ± 0.00^b	10.58 ± 0.09^e	10.39 ± 0.03^d	10.82 ± 0.03^f
Total	815.34 ± 0.45^e	523.51 ± 0.12^a	758.91 ± 0.16^d	662.24 ± 0.39^c	838.41 ± 0.42^f	655.44 ± 0.09^b
Values are means of three replications ± standard deviation. Means followed by different superscripts within the column are significantly different (p < 0.05).
^1A kind of white rice.

Mineral Content

The most abundant minerals found in rice are potassium (K), magnesium (Mg), and calcium (Ca).[28] The white rice, Hwayoungbyeo, exhibited a considerably lower amount of these minerals compared with the black rice varieties (Table 2). Heukjinjubyeo and Heukgwangbyeo contained the highest Ca and K contents, respectively. On the other hand, the highest amount of Mg was observed in Heuknambyeo variety. These results confirmed that higher mineral contents can be found in black rice varieties than ordinary white rice. Similarly, Zhang et al.[29] reported that a higher amount of minerals, such as iron, zinc, manganese, and phosphorus, were found in black rice than common white rice varieties.

Table 2 Mineral content of black rice

	Mineral content (ppm of rice)
Rice variety	K	Ca	Mg
Hwayoungbyeo ^1	82.96 ± 0.00^a	2565.50 ± 3.53^a	1147.50 ± 0.70^a
Dragon eyeball 100	90.10 ± 0.01^b	3541.50 ± 0.70^d	1202.00 ± 1.41^b
Heukjinjubyeo	143.15 ± 0.07^c	4034.00 ± 1.41^f	1545.00 ± 1.41^d
Heukgwangbyeo	191.30 ± 0.14^f	3215.50 ± 2.12^b	1714.50 ± 0.70^e
Heuknambyeo	171.45 ± 0.07^d	3748.50 ± 0.70^e	1847.50 ± 2.12^f
Josaengheukchal	185.75 ± 0.21^e	3260.00 ± 2.82^c	1404.50 ± 0.70^c
Values are means of three replications ± standard deviation. Means followed by different superscripts within the column are significantly different (p < 0.05).
^1A kind of white rice.

Fatty Acid Composition

The major fatty acids found in black rice samples were oleic (18:1), linoleic (18:2), and palmitic (16:0) acids (Table 3), which accounted for more than 90% of the total fatty acid contents in the samples. These three fatty acids were also found to be the major fatty acids in pigmented rice varieties.[30] Dragon eyeball 100 and Heukjinjubyeo showed relatively lower oleic acid content, but significantly higher linoleic acid content than the other samples. Josaengheukchal variety, on the other hand, exhibited a high amount of palmitic acid.

Table 3 Fatty acid composition of black rice

	Fatty acid composition (g/100 g rice)
Rice variety	C14:0	C16:0	C16:1	C18:0	C18:1	C18:2	C18:3	C20:0	C20:1
Hwayoungbyeo ^1	0.28^a	15.70^a	0.16^a	1.94^a	44.37^b	35.14^b	1.18^ab	0.65^ab	0.55^c
Dragon eyeball 100	0.23^a	18.45^ab	0.11^a	1.79^a	37.10^a	40.14^c	1.41^ab	0.40^ab	0.33^ab
Heukjinjubyeo	0.42^b	16.17^a	0.15^a	2.27^a	37.29^a	40.93^c	1.64^b	0.74^ab	0.36^ab
Heukgwangbyeo	0.25^a	16.74^a	0.14^a	1.93^a	45.09^b	34.08^b	1.10^a	0.37^a	0.27^a
Heuknambyeo	0.48^b	16.43^a	0.15^a	2.20^a	45.52^b	32.69^b	1.30^ab	0.76^b	0.45^bc
Josaengheukchal	0.60^c	23.03^b	0.13^a	2.56^a	41.49^b	29.67^a	1.47^ab	0.58^ab	0.46^bc
Values are means of three replications. Means followed by different superscripts within the column are significantly different (p < 0.05).
^1A kind of white rice.

Sugar Content of Non-Starch Polysaccharide

The sugar content of the non-starch polysaccharide composition of the rice samples are shown in Table 4. Significant differences were observed among varieties of all sugars analyzed. Arabinose has the highest sugar concentration in all the samples, except in Dragon eyeball 100. Heukgwangbyeo variety showed substantial amounts of rhamnose, fucose, ribose, and arabinose, while the Dragon eyeball 100 contained the least amount of these sugars. On the other hand, significantly higher xylose content was found in Hwayoungbyeo and Dragon eyeball 100 compared with the other varieties analyzed.

Table 4 Sugar content of non-starch polysaccharide in black rice

	Sugar content (μg/g rice)
Rice variety	Rhamnose	Fucose	Ribose	Arabinose	Xylose
Hwayoungbyeo ^1	1.91 ± 0.04^e	1.38 ± 0.05^e	0.09 ± 0.01^a	3.01 ± 0.13^c	0.63 ± 0.02^e
Dragon eyeball 100	0.32 ± 0.03^a	0.28 ± 0.02^a	0.09 ± 0.01^a	0.56 ± 0.02^a	0.65 ± 0.04^e
Heukjinjubyeo	1.77 ± 0.10^d	0.90 ± 0.05^c	0.21 ± 0.01^b	2.55 ± 0.05^b	0.23 ± 0.01^a
Heukgwangbyeo	2.77 ± 0.11^f	2.22 ± 0.04^f	1.06 ± 0.04^d	6.20 ± 0.06^d	0.28 ± 0.02^b
Heuknambyeo	1.08 ± 0.03^b	1.09 ± 0.04^d	0.27 ± 0.02^c	2.97 ± 0.12^c	0.39 ± 0.03^d
Josaengheukchal	1.12 ± 0.00^c	0.85 ± 0.00^b	0.22 ± 0.01^b	2.51 ± 0.05^b	0.32 ± 0.01^c
Values are means of three replications ± standard deviation. Means followed by different superscripts within the column are significantly different (p < 0.05).
^1A kind of white rice.

Iodine Absorption and Blue Value

Significant differences on the blue value and wavelength of the maximum absorbance among the samples analyzed are presented in Table 5. Hwayoungbyeo and Dragon eyeball 100 varieties showed the highest blue values, maximum wavelength, and absorbance values. On the other hand, a very low blue value was obtained in Josaengheukchal rice. Consequently, it also exhibited the lowest maximum wavelength and absorbance value among the rice samples analyzed. Amylose in starch, which is one of the most important factors affecting the cooking and processing behavior of rice, is responsible for the formation of a deep blue color in the presence of iodine.[31] The value of the maximum wavelength and absorbance for the starch-iodine complex is related to the chain length of the starch molecules. As the chain length increases, the maximum wavelength value also increases.[32] Hence, the results of this study suggest that, in general, black rice varieties have lower amylose content and shorter chain starch molecules than the common white rice. Among the black rice analyzed, Josaengheukchal contained the lowest amount of amylose and shortest chain of starch molecules.

Table 5 Maximum absorbance of iodine absorption and blue value of black rice

	Maximum absorbance (λmax)
Rice variety	Blue value (680 nm)	Wave length (nm)	Absorbance (OD)
Hwayoungbyeo ^1	0.21 ± 0.00^d	569.66 ± 1.15^c	0.33 ± 0.00^d
Dragon eyeball 100	0.22 ± 0.02^d	570.00 ± 2.00^c	0.32 ± 0.03^d
Heukjinjubyeo	0.14 ± 0.01^b	560.33 ± 0.57^b	0.23 ± 0.02^b
Heukgwangbyeo	0.17 ± 0.00^c	562.33 ± 0.57^b	0.29 ± 0.01^c
Heuknambyeo	0.18 ± 0.00^c	563.66 ± 1.15^b	0.29 ± 0.01^c
Josaengheukchal	0.05 ± 0.00^a	526.66 ± 1.52^a	0.20 ± 0.00^a
Values are means of three replications ± standard deviation. Means followed by different superscripts within the column are significantly different (p < 0.05).
^1A kind of white rice.

Hydrolysis Rate

The hydrolysis rate of starch increased with time and significantly differs between the white rice and black rice varieties (Table 6). The rate of hydrolysis was considerably lower in Hwayoungbyeo than in black rice varieties, indicating that the ordinary white rice has lower digestibility compared with the black rice. Heukjinjubyeo and Josaengheukchal showed the highest hydrolysis rate after 3 and 6 h, respectively. The differences in the hydroysis rate could be due to the differences in the amylose content among the rice samples. It has been reported that the starch digestibility is dependent on the amylose content of rice starch.[33] In general, starch with low amylose content, such as that of the black rice, is more susceptible to amylase digestion.[34]

Table 6 Hydrolysis rate of black rice starches using glucoamylase

	Hydrolysis rate (%)
Rice variety	3 h	6 h
Hwayoungbyeo ^1	10.58 ± 1.55^a	30.12 ± 0.56^a
Dragon eyeball 100	27.76 ± 1.38^c	43.83 ± 0.15^b
Heukjinjubyeo	43.11 ± 1.46^f	61.98 ± 0.57^e
Heukgwangbyeo	31.16 ± 2.08^d	54.04 ± 2.04^d
Heuknambyeo	22.64 ± 1.06^b	47.49 ± 1.82^c
Josaengheukchal	35.42 ± 2.21^e	68.94 ± 0.89^f
Values are means of three replications ± standard deviation. Means followed by different superscripts within the column are significantly different (p < 0.05).
^1A kind of white rice.

X-Ray Diffractometry

X-ray diffractometry is widely used to characterize the crystalline structure of starch.[35] The relative crystallinity of starch is obtained from the intensities of the distinctive diffraction lines. The presence and characteristics of the crystalline structures of starch granules are shown in Fig. 2. No considerable difference in the X-ray diffraction patterns was observed among varieties. All samples exhibited an A-type pattern, which is typical to most cereal starches.[36] Choi et al.[37] also found an A-type diffraction pattern in waxy black rice starch.

Figure 2 X-ray diffractograms of rice starches from different black rice cultivars: (A) Hwayoungbyeo, (B) Dragon eyeball 100, (C) Heukjinjubyeo, (D) Heukgwangbyeo, (E) Heuknambyeo, and (F) Josaengheukchal.

Pasting Properties

Pasting properties are important indicators in determining the application values of flours and starches. In this study, Heuknambyeo exhibited the highest pasting temperature and time, while Heukgwangbyeo had the highest viscosity values (Table 7). Josaengheukchal variety showed the lowest pasting and viscosity values. The other black rice samples, on the other hand, showed a higher pasting temperature and time than the white rice cultivar. These differences in the pasting and viscosity properties among the samples can be attributed to the differences in the amount of amylose present in rice starch. Kang et al.[38] reported that flour from low amylose or waxy rice varieties exhibited a significantly lower pasting temperature compared to that of the non-waxy ones. During heating in water, starch granules swell and the amylose leaches out. The swollen granules cause an increase in viscosity in RVA and the breakdown viscosity results from the breakdown of the gelatinized starch granules.[39] The high peak and breakdown viscosities of starch granules from Heukgwangbyeo variety demonstrated the ease of these granules to be broken upon heating after the maximum swelling at the peak viscosity. Low amylose rice possesses this property, which results in the stickiness of the paste.

Table 7 Pasting properties of black rice flours using rapid visco analyzer

			Viscosity (cP)
Rice variety	Pasting temperature (°C)	Pasting time (min)	Peak	Trough	Final	Breakdown
Hwayoungbyeo ^1	74.35 ± 0.07^b	3.07 ± 0.00^b	1975.0 ± 1.41^e	991.50 ± 0.70^d	2274.50 ± 2.12^e	983.50 ± 2.12^d
Dragon eyeball 100	86.91 ± 0.01^d	4.12 ± 0.00^d	1295.50 ± 0.70^b	646.00 ± 1.41^b	1480.50 ± 0.70^b	649.50 ± 2.12^b
Heukjinjubyeo	89.87 ± 0.04^e	4.39 ± 0.01^e	1453.00 ± 1.41^c	799.50 ± 0.70^c	1733.50 ± 2.12^c	653.50 ± 0.70^b
Heukgwangbyeo	75.87 ± 0.03^c	3.20 ± 0.00^c	2750.50 ± 3.53^f	1452.50 ± 0.70^f	2564.00 ± 1.41^f	1298.00 ± 2.82^e
Heuknambyeo	91.63 ± 0.02^f	4.52 ± 0.00^f	1615.00 ± 1.41^d	1021.50 ± 2.12^e	2244.50 ± 2.12^d	593.50 ± 3.53^a
Josaengheukchal	72.80 ± 0.06^a	2.92 ± 0.00^a	1151.50 ± 0.70^a	457.00 ± 1.41^a	608.50 ± 0.70^a	694.50 ± 2.12^c
Values are means of three replications ± standard deviation. Means followed by different superscripts within the column are significantly different (p < 0.05).
^1A kind of white rice.

CONCLUSION

The present study demonstrates that the physicochemical properties significantly differ among the black rice varieties analyzed. Screening of the physicochemical properties of rice is necessary for quality evaluation and possible food industry applications. Results of this study could lead to a better appreciation of black rice and assist food processors in selecting the cultivar with unique characteristics for specialty food processing.