Extraction of Functional Substances from Agricultural Products or By-products by Subcritical Water Treatment

Abstract

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...

Chemical Pretreatments of Wood Chips Prior to Alkaline Pulping - A Review of Pretreatment Alternatives, Chemical Aspects of the Resulting Liquors, and Pulping Outcomes

“…However, the properties of water are dramatically altered as the temperature and pressure are raised and the hydrogen-bonded lattice is disrupted due to the increasing thermal motion of the water molecules (Yu et al 2008). Between 100 °C and 374 °C, heated water can have a permittivity very similar to that of typical organic solvents, making the organic non-polar compounds more soluble in water (Wiboonsirikul and Adachi 2008). The increased diffusitivity characteristics, the reduced viscosity, polarity, relative permittivity, and surface tension together with enhanced solvent properties towards organic solutes allow more efficient mass-transfer reactions from solid samples (such as wood) compared to the extractions conducted under ambient conditions.…”

“…Several factors, such as temperature, pressure, extraction time, and the analyte characteristics affect the efficiency of PHWE (Wiboonsirikul and Adachi 2008;Teo et al 2010;Kleen et al 2011;Hanim et al 2012;Borrega et al 2013a,b;. Other parameters include the flow rate, the type of reaction vessel, and the use of varying modifiers and additives.…”

Degradation of N-Acetyl-D-glucosamine and D-Glucosamine in Subcritical Water and Properties of the Degradation Products

“…The relative dielectric constant of water decreases as the temperature increases (Archer and Wang, 1990). Because subcritical water is non-toxic and non-flammable, it holds promise as an extractant for recovering useful substances from agricultural resources or waste (Hartone et al., 1997;Herrero et al, 2006;Hata et al, 2008;Wiboonsirikul and Adachi, 2008).…”

“…The relative dielectric constant of water decreases as the temperature increases (Archer and Wang, 1990). Because subcritical water is non-toxic and non-flammable, it holds promise as an extractant for recovering useful substances from agricultural resources or waste (Hartone et al., 1997;Herrero et al, 2006;Hata et al, 2008;Wiboonsirikul and Adachi, 2008).The ion product of subcritical water is higher by three orders of magnitude than that of ambient water (Marshal and Franck, 1981). This feature indicates that subcritical water can act as an acid or base catalyst, and indeed, the hydrolyses of biological substances such as cellulose (Sasaki et al, 1998), proteins (Yoshida et al, 1999 and fatty acid esters (Khuwijitjaru et al, 2004) have been reported.…”

Sulfur-free pulping of hot-water-extracted spruce sawdust

“…One of the most studied pre-treatment methods includes pressurized hot-water extraction (PHWE) (Smith 2002;Amidon, Liu 2009). PHWE (often also known as sub-critical water extraction, superheated water extraction, extraction with water at elevated temperatures and pressures, or near critical water extraction) has become a potential green extraction method for several compounds present in many different biological matrices (Smith 2002;Wiboonsirikul, Adachi 2008;Amidon, Liu 2009). PHWE is typically performed at the temperatures above 100°C and below 374°C (the critical temperature of water).…”