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Rice straw stem was treated with subcritical water, ethanol, or a mixture at 170℃ to 230℃. The yield, total carbohydrate content, UV-Vis absorption spectra, phenolic content, radical scavenging activity, and HPLC analysis of the extracts were determined. Higher stem extract yield and total carbohydrate content were achieved when the stem was treated using a subcritical ethanol/water mixture with high ethanol content and at the higher temperature. A maximum yield and total carbohydrate content of 0.387 ± 0.004 g/g-stem and 0.219 ± 0.021 g/g-stem, respectively, were achieved when the stem was treated with 50% (v/ v) ethanol at 230℃. The extract obtained using 75% (v/v) ethanol at 230℃ had the highest total phenolic content and radical scavenging activity of 45.2 ± 1.1 mg-gallic acid equiv./g-stem and 0.308 ± 0.022 mmolvitamin C equiv./g-stem, respectively.Keywords: extraction, radical scavenging, rice straw, subcritical fluid *To whom correspondence should be addressed. E-mail: adachi@kais.kyoto-u.ac.jp IntroductionRice straw is very abundant and is considered an agricultural waste (Dominguez-Escriba and Parcar, 2010;Kadam et al., 2000). Many applications of rice straw have been investigated; however, the amount of rice straw utilized remains very low compared to the worldwide annual production. The three major components of rice straw are cellulose, hemicellulose, and lignin, which are considered to contain promising substances (Lawther et al., 1996). Due to the hydrogen bonds between the glucose molecules of cellulose and the covalent bonds in lignin, rice stem is only slightly soluble in water at ambient temperature (Xiao et al., 2001). At high temperatures (100℃ to 374℃), water can maintain its liquid state under pressurized conditions, and is called subcritical water or compressed hot water. The ion product of subcritical water increases to over 1 × 10 3 fold greater than that of water at room temperature (Marshall and Franck, 1981;Wagner and Pruß, 2002). Due to high concentrations of hydroxium and hydroperoxide ions, subcritical water has the ability to hydrolyze hemicellulose, lignin, and the amorphous part of cellulose. Another feature is its low dielectric constant, which can be adjusted by controlling the temperature. The dielectric constant of water decreases from 80 at ambient temperature to 27 at 250℃. The dielectric constant of subcritical water matches that of ethanol or acetone at room temperature. With these features, water under subcritical conditions can liberate phenolic compounds, which are bonded to the carbohydrate backbone (Bobleter, 1994;Kumar et al., 2010). Chiou et al. (2012) reported that a subcritical water/ethanol mixture could more effectively extract protein, carbohydrate, and phenolic compounds from defatted rice bran than subcritical water or ethanol alone. The addition of ethanol enhances extraction efficiency because constituents with different polarities in the extract are obtained by the ethanol and water (Durling et al., 2007;Richter et al., 1996). Other advantages of ethanol...

Rice stem was repeatedly treated 3 times with subcritical water, ethanol, and 75% (v/v) ethanol in different orders at 230ºC for 5 min during each treatment step to extract its constituents. The obtained extracts were subjected to an analysis of their yield, carbohydrate content, phenolic content, radical scavenging ability, color, and UV absorption spectra. The highest total carbohydrate and phenolic contents were obtained by treatment with subcritical water, and with subcritical ethanol after pre-treatment with subcritical water, respectively. However, the extract with the highest radical scavenging ability was obtained by treatment with subcritical 75% (v/v) ethanol as the 1st treatment.

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Rice stem was treated with subcritical water at 260ºC for 0-20 min. The extracts treated for 5 min were subject to re-treatment for 0-60 min. In the further treatment, the total phenolic content, total carbohydrate content, radical scavenging ability, and metal chelating ability decreased with extended treatment time. Tyrosinase inhibition ability, however, did not depend on the treatment time. There were correlations between the total phenolic content and radical scavenging ability and between the total carbohydrate content and metal chelating ability under any treatment conditions.