Production of phenolic compounds from rice bran biomass under subcritical water conditions
Introduction
Rice is one of the most important cereals in the world which is the main staple food in many countries, especially Asian countries like Japan. Global production of paddy in 2007 was estimated to be 638 million tons [1]. After harvesting of paddy, dehusking, and milling processes are performed to separate different parts of paddy; i.e. white rice, bran, and hull or husk [2]. During the milling process, rice bran is produced as major by-product which is a brown layer present between rice and the outer husk of the paddy [3], and its weight ratio to milled rice is about 8% [4]. Annually about 50–60 million tons of rice bran is produced in the world [5]; in Japan, it is about 900 thousand tons [6]. This abundant biomass contains oil, proteins, carbohydrates, and dietary minerals [7], [8]. Also, it is well known to be rich in various kinds of phenolic compounds [9].
On the other hand, there has been a considerable increasing demand for natural phenolic compounds in recent years [10]. Natural phenolic compounds are not uniformly distributed in plants: some of them linked with cell walls, while others exist without any chemical bonds within the plant cell vacuoles [11]. Phenolic compounds are important due to their antioxidant activities. They possess aromatic structure along with hydroxyl substituents which enable them to protect human tissues from damages caused by oxygen or free radicals [12], and consequently reduce the risk of different diseases, and offer beneficial effects against cancers, cardiovascular disease, diabetes, and Alzheimer's disease [13]. For instance, ferulic acid (3-(4-hydroxy-3-methoxyphenyl)prop-2-enoic acid) is one of the major phenolic compounds that owing to its high antioxidant properties, has applications in food industries as well as in the health and cosmetic markets [14].
Rice bran as a natural source of phenolic compounds is currently underutilized and a large quantity of rice bran remains unused as agricultural waste or use as animal feed and boiler fuel [15], [16]. In Japan, nearly 30% of the produced rice bran goes to waste [17].
So far, numerous attempts have been conducted for recovery and extraction of phenolic compounds from rice bran using conventional techniques. For this purpose, application of organic solvents such as methanol, ethanol, propanol, acetone, ethyl acetate, dimethylformamide and/or their combinations have been reported [18]. For example, Renuka and Arumughan [5] have studied the extraction of phenolic compounds from rice bran by using organic solvents and utilization of soxhlet technique. Chotimarkorn et al. [19] and Iqbal et al. [9] extracted phenolic compounds with methanol from various kinds of rice bran by application of direct solvent–solid extraction method. Taniguchi et al. [20] have patented a method for hydrolyzing of waste materials of rice bran oil production industries at 100 °C and pH of 10 at residence time from 8 to 10 h, the produced ferulic acid which was extracted by hexane solvent.
Conventional extraction methods have several drawbacks; e.g. they are time-consuming, are of low selectivity, give low extraction yield, and use large amount of expensive, explosive, and sometimes toxic organic solvents [21]. Furthermore, the phenolic compounds in the rice bran are extensively bounded to carbohydrate and lignin in the cell wall, and their solubility in common organic solvent is low, unless rice bran is treated at high temperature and/or under acidic and basic conditions [22]. Therefore, utilization of supercritical carbon dioxide and particularly subcritical water methods (which later one provides both temperature and acidic condition for hydrolysis reactions) has been reported recently to eliminate or reduce the above limitations [23].
Generally, subcritical water has been extensively utilized in various fields of green engineering and material cycling [24], [25], [26], [27], [28], [29], [30], [31], [32]. In fact, its applications are due to the easy manipulation of its dielectric constant, and variable concentration of hydrogen and hydroxide ions with temperature. For instance, its dielectric constant decreases from 80 (at room temperature) to 27 (at 250 °C) almost equaling that of ethanol at ambient temperature [33]. The increase/decrease in hydrogen and hydroxide ions in subcritical water [34] along with the decreasing of its dielectric constant, make it very suitable medium for the extraction and hydrolysis of natural matrices.
So far there have been several academic reports on the applications of supercritical fluid and subcritical water for treatment of rice bran. King and Dunford [35], [36] have reported an efficiently method for extraction of the valuable phytosterol-enriched products from rice bran oil under supercritical fluid conditions. Wiboonsirikul et al. [22], [37] have produced phenolic compounds from defatted rice bran using subcritical water at 50–250 °C and 20–260 °C for 5 min, and also at 200 and 260 °C for 5–120 min; they investigated total phenolic content (TPC) yield and antioxidant activity of the aqueous extract. In another report [34], antioxidant activity, and total soluble sugars yield were evaluated after subcritical water treatment of the defatted rice bran at the limited temperature range of 180–280 °C for 5 min.
To the best of our knowledge, there is no comprehensive report on the study of rice bran hydrolysis into phenolic compounds over the whole temperature range of subcritical water. The objective of this research work was to investigate the possibility of phenolic compounds production by decomposition of rice bran under subcritical water conditions as a green and environmentally friendly treatment technique. The influences of whole subcritical water temperature and residence time as main experimental parameters were studied in detail.
Section snippets
Materials
Japonica-type rice (Oryza sativa) was used in this study. Gallic acid (3,4,5-trihydroxybenzoic acid) was purchased from Tokyo Chemical Industry Co. Ltd. (Japan). Sodium bicarbonate (sodium hydrogen carbonate) and phenol (hydroxybenzene) were obtained from Nacalai Tesque, Inc. (Japan). Folin-Ciocalteu phenol reagent, gentisic acid (2,5-dihydroxybenzoic acid), p-coumaric acid (3-(4-hydroxyphenyl)-2-propenoic acid), sinapic acid (3-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-enoic acid), syringic acid
TPC yield and antioxidant activity of ethanolic solution
In order to realize the application of subcritical water for production of phenolic compounds from rice bran and/or defatted rice barn, a series of experiments were performed over a temperature range of 100–360 °C at residence time of 10 min. Fig. 2 shows the effect of reaction temperature on the yield of TPC obtained both from rice bran and defatted rice bran. Based on previous reports, there are two possibilities for formation of TPC: from decomposition of bounds between lignin, cellulose, and
Conclusions
Decomposition and conversion of rice bran into valuable chemical compounds were successfully conducted using subcritical water. Degradation of the lignin/phenolics–carbohydrates complexes of rice bran were achieved (up to 92% of rice bran) in the water without using organic solvent, acid, base, and/or enzyme. Decomposition of rice bran and defatted rice bran have resulted almost the same amount of phenolic compounds; it was understood that phenolic compounds were mainly produced from
Acknowledgements
The authors gratefully acknowledge the support of a part of this work provided by the ministry of Education, Culture, Sports, Science and Technology of Japan in the form of 21st Century COE program (E19, Science and Engineering for Water Assisted Evolution of Valuable Resources and Energy from Organic Wastes). One of the authors (Omid Pourali) deeply appreciates the Monbukagakusho Scholarship from Japanese government.
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