Article pubs.acs.org/JAFC
Isoflavone Content and Profile Comparisons of Cooked Soybean−Rice Mixtures: Electric Rice Cooker versus Electric Pressure Rice Cooker Ill-Min Chung,† Bo-Ra Yu,† Inmyoung Park,‡ and Seung-Hyun Kim*,† †
Department of Applied Bioscience, College of Life and Environmental Science, Konkuk University, Seoul 143-701, Republic of Korea ‡ Department of Microbiology, College of Natural Sciences, Pusan National University, Pusandaehak-ro 63 beon-gil, Geumjeong-gu, Pusan 609-735, Republic of Korea ABSTRACT: This study examined the effects of heat and pressure on the isoflavone content and profiles of soybeans and rice cooked together using an electric rice cooker (ERC) and an electric pressure rice cooker (EPRC). The total isoflavone content of the soybean−rice mixture after ERC and EPRC cooking relative to that before cooking was ∼90% in soybeans and 14−15% in rice. Malonylglucosides decreased by an additional ∼20% in EPRC-cooked soybeans compared to those cooked using the ERC, whereas glucosides increased by an additional ∼15% in EPRC-cooked soybeans compared to those in ERC-cooked soybeans. In particular, malonylgenistin was highly susceptible to isoflavone conversion during soybean−rice cooking. Total genistein and total glycitein contents decreased in soybeans after ERC and EPRC cooking, whereas total daidzein content increased in EPRCcooked soybeans (p < 0.05). These results may be useful for improving the content of nutraceuticals, such as isoflavones, in soybeans. KEYWORDS: soybean−rice mixture cooking, isoflavone change, heat treatment, pressure treatment, mass balance
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activity.13−16 For instance, heat treatment during tofu and soy milk manufacturing induces the formation of β-glucoside isoflavone conjugates from malonylglucoside isoflavone conjugates. Furthermore, fermentation processes, such as those used in miso or tempeh production, decrease the β-glucoside content of soybeans and increase the aglucon content.17 In addition, malonylglucosides in soybeans and soy foods are converted to glucosides or acetylglucosides during baking at 190 °C.15 In general, high-pressure (HP) treatment during the processing or cooking of certain foods has received increasing attention because it inactivates microbial and enzymatic activity; therefore, traditional thermal processing has been replaced by HP treatment in several food industries to extend the shelf life of food products and eliminate harmful microorganisms. However, HP or heat treatment usually affects the content and functionality of nutraceuticals in foods.18,19 For example, when the heating temperature is increased to 60 °C, the aglucon, malonylglucoside, and acetylglucoside contents in isoflavones decrease in steamed black soybeans, whereas the glucoside content increases.8 Furthermore, pressure treatment with moderate heat promotes the conversion of malonylglucosides to glucosides during milk production.20 Many previous studies have described the effects of cooking methods (including soaking, heat, pressure, enzyme treatment, and fermentation) on the isoflavone composition and content
INTRODUCTION Rice and soybeans are among the five major crops produced in the world. The annual production of rice and soybeans worldwide in 2012−2013 was 463 215 (as a milled rice) and 260 459 kilotons, respectively.1 Approximately half of the world’s population uses rice as a staple food.2,3 However, rice is deficient in essential amino acids, such as lysine, that are plentiful in soybeans. Therefore, rice is often consumed with soybeans, following the practice of cereal−legume complementation to enhance overall protein quality by increasing essential amino acid content.3−5 A previous study determined the ratio of rice and soybeans required to achieve an optimal amino acid balance during soybean−rice cooking.5 Particularly in Korea, rice is cooked with soybeans (soybean−rice mixture cooking) using either an electric rice cooker (ERC) or an electric pressure rice cooker (EPRC) to improve nutritive values. Furthermore, soybean−rice cooking using EPRCs is preferred to ERC cooking because it results in better taste. Soybean and soy foods contain various phytonutrients, such as flavonoids, phenolics, saponins, and oligosaccharides, the consumption of which has many health benefits.6−8 In particular, soy isoflavones known as phytoestrogens reduce the risk of cardiovascular disease and breast and prostate cancers and alleviate menopausal disorder.9−11 However, soy isoflavone consumption has also been associated with some negative health effects, such as cancer and weight gain; in particular, long-term, low-level intake of genistein (≤500 ppm) promotes human breast tumor (MCF-7) growth.12 The composition and content of isoflavones in soybeans is affected by factors, such as crop year, crop location, storage period, processing conditions, processing type, and presence of microorganisms with β-glucosidase © 2014 American Chemical Society
Received: Revised: Accepted: Published: 11862
July 16, 2014 November 11, 2014 November 13, 2014 November 13, 2014 dx.doi.org/10.1021/jf5033944 | J. Agric. Food Chem. 2014, 62, 11862−11868
Journal of Agricultural and Food Chemistry
Article
Figure 1. (A) Representative UPLC chromatograms of 12 isoflavone STDs and the isoflavones in cooked soybeans and rice. (B) Comparison of chromatograms of the soybeans cooked using an EPRC (black solid line) and the same soybean aliquot spiked with isoflavone STDs 5, 10, and 12 (blue dotted line). (C) Comparison of chromatograms of the rice cooked using the EPRC (black solid line) and the same rice aliquot spiked with isoflavone STDs 2, 6, and 9 (red dotted line). Isoflavone STDs: 1, Din; 2, Glin; 3, Gin; 4, MD; 5, MGl; 6, AD; 7, MG; 8, AGl; 9, Dein; 10, Glein; 11, AG; and 12, Gein. “Chung-ja” is classified as the type of soybean cooked with rice in Korea.25 The rice cultivar “Choochung” was purchased from a local market in Seoul, Korea, to be cooked with the soybeans. This rice grain had a moisture content of approximately 14% and was stored at room temperature until it was cooked with the soybeans. Soybean−Rice Mixture Cooking. Two rice cookers were used in the present study: an ERC (LJ-MG0402) and an EPRC (LJP-SA063E), both from Lihom (Seoul, Korea). During the soybean−rice mixture cooking, the ERC was operated at a cooking temperature of ∼100 °C and a cooking pressure of ∼14.7 psi, whereas the EPRC was operated at a cooking temperature of 120−130 °C and a cooking pressure of 25−30 psi. For soybean−rice mixture cooking, 40 and 300 g of soybeans and rice, respectively, were weighed and combined. Tap water (400 mL) was added, and the soybeans and rice were soaked together in water for 30 min at room temperature. Thereafter, the soybeans and rice were cooked using the ERC and EPRC (n = 3). Then, the soybeans and rice were separated and lyophilized at −45 °C for 3 days and pulverized for isoflavone analysis. Isoflavone Extraction. Isoflavones from the soybean and rice samples were extracted using acidic extraction.7 Pulverized samples (2 g each; n = 3) were added to 10 mL of acetonitrile and 2 mL of 0.1 N hydrochloric acid and then extracted for 2 h at room temperature on a shaker at 200 rpm (Green-Sseriker, Vision Scientific Co., Ltd., Gyeonggi-Do, Korea). The crude extract was filtered through No. 42 Whatman filter paper (125 mm diameter, Maidstone, U.K.), and the filtrate was evaporated at a temperature lower than 32 °C with a vacuum evaporator (EYELA, SB-1200, Tokyo Rikakikai Co., Ltd., Gyeonggi-Do, Korea). The dried samples were reconstituted with
of soybeans and soy foods.8,15,21−24 However, to the best of our knowledge, few studies have examined the effects of a combination cooking method involving heat and pressure treatment on isoflavone composition and content of soybean− rice mixtures cooked using rice cookers. Therefore, the present study investigated the effects of heat and pressure processing on the composition and content of 12 isoflavones in soybeans during soybean−rice mixture cooking using an ERC and an EPRC. The present study also examined the transfer of these 12 isoflavones between soybeans and rice after ERC and EPRC cooking of a soybean−rice mixture. The results of the present study describe the effects of cooking on the composition and content of isoflavones in soybeans and provide basic information for the development of functional foods using soybeans in the food industry.
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MATERIALS AND METHODS
Chemicals. All solvents used for isoflavone extraction and analysis were of high-performance liquid chromatography grade. All solvents and reagents were obtained from Fischer Scientific Korea, Ltd. (Seoul, Korea), Merck (Darmstadt, Germany), J.T.Baker (Phillipsburg, NJ), or Daejung Chemical & Materials Co., Ltd. (Gyeonggi-Do, Korea). A total of 12 isoflavone standards (STDs) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). Rice and Soybean Materials. The soybean cultivar “Chung-ja” (black-coated soybean seed) was cultivated, harvested, and obtained from the Rural Development Administration in 2012. In general, 11863
dx.doi.org/10.1021/jf5033944 | J. Agric. Food Chem. 2014, 62, 11862−11868
Journal of Agricultural and Food Chemistry
Article
Table 1. Calibration Curve, Concentration Range, Linearity, LOD, and LOQ of 12 Isoflavone STDs group aglucon
glucoside
acetyl glucoside
malonyl glucoside
STDa
concentration rangeb (μg mL−1)
linearity (r2)
slope (S)
SD of y intercept
LODc (μg mL−1)
LOQc (μg mL−1)
Dein Gein Glein Din Gin Glin AD AG AGl MD MG MGl
0.5−2.5 0.5−2.5 0.5−2.5 0.5−25 0.5−25 0.5−5 0.5−5 0.5−10 0.5−5 0.5−25 0.5−75 0.5−5
1 1 1 0.999 1 1 0.999 0.999 0.999 0.999 0.999 1
7.37 12.14 9.19 8.15 9.55 7.63 5.58 10.11 8.21 4.78 4.40 2.70
0.06 0.05 0.02 0.41 0.51 0.07 0.43 0.44 0.23 0.04 0.07 0.01
0.02 0.01 0.01 0.16 0.16 0.03 0.23 0.13 0.08 0.02 0.05 0.01
0.08 0.04 0.02 0.52 0.53 0.09 0.78 0.44 0.28 0.07 0.15 0.04
a
Dein, daidzein; Gein, genistein; Glein, glycitein; Din, daidzin; Gin, genistin; Glin, glycitin; AD, acetyldaidzin; AG, acetylgenistin; AGl, acetylglycitin; MD, malonyldaidzin; MG, malonylgenistin; and MGl, malonylglycitin. bThe calibration curve was composed of 4−8 different concentrations for each standard solution. cThe LOD and LOQ were calculated as follows: LOD = 3SD/S and LOQ = 10SD/S, where SD is the standard deviation of the y intercept in the calibration curve and S is the slope of each calibration curve.26 10 mL of 80% methanol and then filtered through a 0.2 μm nylon membrane syringe filter (Sun Sri, Rockwood, TN). Isoflavone Analysis with Ultraperformance Liquid Chromatography (UPLC). Isoflavones were analyzed with an Agilent UPLC system coupled with a diode array detector (1290 infinity binary pump, Agilent, Seoul, Korea). A reverse-phase column (Agilent Zorbax Eclipse C18, 100 × 2.1 mm internal diameter, 1.8 μm) was used to separate the 12 isoflavones. The ultraviolet and reference wavelengths were set at 254 and 360 nm, respectively. The mobile phase consisted of 0.1% glacial acetic acid in water (solvent A) and 0.1% glacial acetic acid in acetonitrile (solvent B). The gradient conditions of the mobile phases were slightly modified as previously described7 and optimized for isoflavone analysis using UPLC as follows: initial, 85% A/15% B; 10 min, 75% A/25% B; 13 min, 65% A/35% B; 16 min, 65% A/35% B; and 20 min, 85% A/15% B. The stop time was set at 20 min, and the post time (equilibrium time) was set at 5 min. The injection volume was 1 μL, and the flow rate was 0.5 mL min−1. The isoflavones were identified by comparing the retention times of the authentic STDs and the peaks in the sample aliquots. Furthermore, each isoflavone STD was added to the sample aliquots (sample aliquot + isoflavone STD) to confirm the correct peak assignment in the sample aliquots; we could then properly assign the isoflavone peaks in the samples (Figure 1). The limit of detection (LOD) and limit of quantification (LOQ) were calculated using the calibration curve as follows: LOD = 3SD/S and LOQ = 10SD/S, where SD is the standard deviation of the y intercept of the calibration curve, and S is the slope of the calibration curve.26 The LOD and LOQ of the 12 isoflavones were 0.01−0.23 and 0.02−0.78 μg mL−1, respectively (Table 1). Statistical Analysis. Statistical analysis was performed using the general linear model procedure of the SAS software (SAS Institute, Inc., Cary, NC). The results were analyzed using a variance analysis. Additionally, Fisher’s least significant difference test at the 0.05 probability level (LSD0.05) was used to determine differences among the means of the samples.
daidzein (sum of daidzein, daidzin, acetyldaidzin, and malonyldaidzin), total genistein, and total glycitein in the soybeans before cooking were 69.4 ± 3.1, 220.9 ± 7.8, and 12.1 ± 0.6 μg g−1 (aglucon equivalents), respectively (p < 0.05; Table 3). Among the 12 isoflavones, malonylgenistin was the major isoflavone type (359.1 ± 12.6 μg g−1), accounting for approximately 64% of the total isoflavone content in the soybeans before cooking. Acetylglycitin and glycitein were not detected or found in only trace levels (indicating levels below the limit of detection,
