Defatted rice bran was extracted with water and subcritical water at 50-250 degrees C for 5 min. The highest extract yield was achieved at 200 degrees C, at which the maximum amounts of protein and carbohydrate were also obtained. The total phenolic and furfural contents, radical scavenging activity, and antioxidative activity for the autoxidation of linoleic acid increased with increasing treatment temperature. The bran extracts exhibited emulsifying activity except for the extract prepared at 250 degrees C, which was concomitant with the disappearance of its high-molecular-mass substances. The extract prepared at 200 degrees C also had the highest emulsion-stabilizing activity.

Water that maintains its liquid state in the temperature range from 100℃ to 374℃ is called subcritical water, compressed hot water or pressurized hot water. This type of water has unique properties compared to ambient water. One is a low relative dielectric constant and another is a high ion product. Due to these properties, this water can be used to extract functional substances from natural resources. In this article, the application of subcritical water for the extraction of substances from agricultural products or their wastes is reviewed.Keywords: subcritical water, agro-waste, extraction, functional substance *To whom correspondence should be addressed. Email: adachi@kais.kyoto-u.ac.jp IntroductionAgricultural products, such as rosemary, grapes, tea leaves, and grain bran, contain bioactive compounds that can function as health-promoting, antioxidative, and radical scavenging substances. Among them, phenolic substances, catechin, epicatechin, anthraquinones, and essential oils are of interest (Deng et al., 2004;Piñeiro et al., 2004;Ibáñez et al., 2003;Shotipruk et al., 2004). Several approaches have recently been used to recover these substances involving their extraction with organic solvents, such as methanol, ethanol and acetone. However, these techniques are timeconsuming and require a large amount of organic solvents, which are harmful to human health and cause environmental stress. Thus, an alternative method that is the effective, economical, environmentally friendly, safe and fast, is required to alleviate these drawbacks. The most common techniques, which have recently been discussed, include supercritical fl uid extraction (e.g., carbon dioxide), pressurized liquid extraction or accelerated solvent extraction, and subcritical water extraction. Of these techniques, subcritical water extraction using water as the extractant is one of the most interesting methods because water is non-fl ammable, non-toxic, cheap, and environmentally safe. The subcritical water, also called pressurized hot water, compressed hot water or superheated water, is hot water that maintains its liquid state at temperatures between 100℃ and 374℃ (the critical temperature and pressure of water are 374℃ and 22.4 MPa, respectively) under pressurized conditions. When the temperature of water increases, its physicochemical properties, in particular its relative dielectric constant and ion product, change. The ion product of water is the product of the concentrations of hydrogen and hydroxyl ions. The relative dielectric constant, ε, or polarity of water can signifi cantly decrease with increasing temperature from approximately 80 at 25℃ to 27 at 250℃ (Fig. 1), which is close to that of methanol (ε = 33) and ethanol (ε = 24) at 25℃ (Wagner and Pruß, 2002). Therefore, subcritical water has the ability to recover or dissolve both polar and apolar substances from natural products, such as phenolic, polycyclic aromatic compounds and oils (Deng et al., 2004;García-Marino et al., 2006; MoralesMuños et al., 2002). In addition, the dissoci...

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