Abstract—This study deals with the production of phenolic compounds from decomposition of rice bran biomass under subcritical water condition. Experiments were performed in a batch stainless steel vessel over a wide temperature ranging from 373 to 633 K for 10 min. The main identified and quantified phenolic compounds were caffeic acid, ferulic acid, gallic acid, p-coumaric acid, p-hydroxybenzoic acid, protocatechuic acid, syringic acid, vanillic acid and, vanillin. In addition, the antioxidant activity of aqueous solution were also evaluated. Both phenolic compounds and antioxidants showed increasing with temperature rising, and leveled off at the temperatures higher than 493 and 553 K, respectively. It was also realized that due to acidic properties of produced compounds, autocatalysis may occur during subcritical water treatment of rice bran. Key words: Antioxidants, Phenolic compounds, Rice bran biomass, Subcritical water treatment I. INTRODUCTION Rice is the main staple food in Asian countries like Japan. During rice milling process, rice bran is produced as major by-product. Its production rate only in Japan is about 900 thousand tons per year [1]. This abundant biomass is a natural resource of oil, proteins, carbohydrates and, dietary minerals [2]. Also, it has been discovered that rice bran may contain even 100 different antioxidants like phenolic compounds [3]. The health-promoting properties of phenolic compounds of rice bran reduce the risk of different diseases, and offer beneficial effects against cancers, cardiovascular disease, diabetes, and Alzheimer’s disease [4]. Although rice bran has great nutrition and pharmaceutical benefits; however, is currently underutilized and a large quantity of rice bran (nearly 30% of the produced rice bran) goes to waste in Japan Submission date: July 17, 2009. International Conference on Chemical Engineering (ICCE'09) F. Salak is with Research Institute on Material Cycling Engineering, Research Institutes for 21st Century, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai-shi, Osaka 599-8531, Japan (e-mail: fsalak@21c.osakafu-u.ac.jp). O. Pourali is with the Chemical Engineering Department, College of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai-shi, Osaka 599-8531, Japan (e-mail: opourali@chemeng.osakafu-u.ac.jp). Pourali gratefully acknowledges the Monbukagakusho Scholarship from Japanese government. H. Yoshida is with the Chemical Engineering Department, College of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai-shi, Osaka 599-8531, Japan (corresponding author; phone/fax: +81-72-254-9298; e-mail: yoshida@chemeng.osakafu-u.ac.jp). [5]. It is worth and necessary to extract and recover these valuable materials before discarding this precursor. So far, this purpose has been achieved by utilization of conventional extraction methods using several organic solvents [6]. Conventional methods usually 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/or sometimes toxic organic solvents [7]. To overcome to these drawbacks, alternative methods like supercritical fluid and subcritical water treatment can be used. Subcritical water as green treatment method has wide applications in various fields of green engineering and material cycling [8]-[10]; however, very few research works have been conducted for phenolic compounds production from rice bran using subcritical water. The objective of this study was to investigate the feasibility of decomposition and hydrolysis of rice bran and consequently production of phenolic compounds under subcritical water condition in a short residence time. II. EXPERIMENTAL Japanese rice bran was utilized in this experimental study. The batch reactor used for subcritical treatment was a stainless steel tube. In typical experiment, an accurately weighed amount of rice bran and distilled water were charged into the reactor. Argon gas was utilized to force air out of the reactor, and then it was capped tightly. It was immersed in a preheated oil or salt bath with temperature ranging from 373 to 633 K. After desire residence time, the reactor was then removed from the thermal bath and quickly quenched by soaking in a cold-water bath at room temperature. Reactor content was washed into a test tube, and it was classified and isolated into two phases: water-soluble and solid residue phases. The pH of water-soluble phase was measured using a glass pH electrode. After centrifuging, these two phases were separated with taking out of supernatant (water-soluble phase) to a volumetric flask by Pasteur pipette. Solid residue was washed by ethanol and, the obtained mixture was shaken for 1 min and then centrifuged. The obtained ethanol phase was transferred by Pasteur pipette and added to the collected water-soluble phase. This solution was mixed and made up to 50 cm 3 with ethanol and, filtered with a 0.2 μm filter. Total phenolic compounds (TPC) and antioxidants was assayed by UV-visible (Shimadzu UV-160, Shimadzu Co., Japan) [11]-[12]. Individual concentration of phenolic compounds was determined by an HPLC system (Varian Application of Subcritical Water Treatment for Production of Phenolic Compounds from Rice Bran Biomass F. Salak, O. Pourali, H. Yoshida Proceedings of the World Congress on Engineering and Computer Science 2009 Vol I WCECS 2009, October 20-22, 2009, San Francisco, USA ISBN:978-988-17012-6-8 WCECS 2009