9626
Ind. Eng. Chem. Res. 2005, 44, 9626-9630
Decomposition Reaction of Alginic Acid Using Subcritical and Supercritical Water Keiichiro Matsushima* and Hironori Minoshima Department of Environmental Process Engineering, Hokkaido Industrial Research Institute, Sapporo, Hokkaido 060-0819, Japan
Hajime Kawanami and Yutaka Ikushima Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology, Miyagi 983-8551, Japan
Makoto Nishizawa Department of Food Sciences and Technology, Tokyo University of Agriculture, Hokkaido 099-2493, Japan
Atsushi Kawamukai and Kozo Hara Kyosei Pharmaceutical Co., Ltd, Hokkaido 047-0013, Japan
Alginic acid is a liner hetero-polysaccharide consists of D-mannuronic acid (M) and L-guluronic acid (G). The alginate, whose molecular weight and the mannuronic acid/guluronic acid (M/G) ratio are adjusted, are used extensively in food, pharmaceutical industries, and so on. In this study, we developed the rapid decomposition of sodium alginate (Alg-Na) using subcritical and supercritical water (abbreviated as subH2O and scH2O, respectively) microreaction systems, and investigated the controllability of the molecular weight and the M/G ratio by varying temperature and pressure. In scH2O, glucosidic linkages and hexose rings of Alg-Na were completely cleaved by hydrolysis to form unknown complex mixture. Whereas in the subH2O (523 K, 25 MPa), G-M and M-G linkages were hydrolyzed selectively. As the result, we successfully obtained the almostpure guluronic acid homo-polymer and the mannuronic acid-rich water-soluble hetero-polymer selectively. Introduction Seaweed is one of the important materials that contain useful saccharides in the food practices in Japan. Alginic acid is a liner unbranched heteropolysaccharide widely distributed in the cell-wall and cytoplasmic membrane of brown algae such as kombu and wakame seaweed. It has been utilized as gelatinizers, thickeners, wound coatings, and gastric parietal protectives in various industries (e.g., food and pharmaceutical).1,2 Recently, alginic acid has become a more important material in the health food market, because it has many bioactivities, such as toxic metal absorption inhibitory effect, hypocholesterolemic activity and elevated blood pressure inhibitory effect.3-7 Alginic acid is a liner copolymer that consists of D-mannuronic acid (M) and L-guluronic acid (G), as shown in Figure 1. It is composed of homo-polymeric G block, homo-polymeric M block, and alternating heteropolymer GM block, and random combinations thereof.1,8 The physical and chemical properties of alginic acid (e.g., viscosity, solubility, interaction with metal cations) are directly attributable to the molecular weight and mannuronic acid/guluronic acid (M/G) ratio.1,9-13 To use alginic acid industrially, it must be prepared by adjusting the molecular weight and M/G ratio, because the natural alginic acid is slightly soluble in water and the * To whom correspondence should be addressed. Tel: (+81)11-747-2950. Fax: (+81)-11-726-4057. E-mail: matusima@ hokkaido-iri.go.jp.
viscosity of its aqueous solution is too high to allow the solution to be used. Recently, to produce the appropriate alginic acid, the depolymerization of alginate in the presence of enzyme reactions has attracted great interest and has been widely investigated as a selective decomposition method by many researchers, and the many effective enzymes for the reaction were observed.14-21 Although they are useful, these methods have many problems in regard to industrialization, from the viewpoints of cost and long reaction time. To overcome these defects, especially reaction time, a method of autoclaving the decomposition has been developed for industrial production; it can control the molecular weight of alginate, but the selectivity of the M/G ratio is extremely very low.22 On the other hand, from the standpoint of environmentally friendly processes for material production, supercritical water technology has been attracting much attention from not only industrial chemists but also biochemists. Because the physicochemical properties of supercritical water can be widely changed with only variation of the pressure and temperature, for example, in the supercritical region, the dielectric constant of water remarkably decreases and organic substances can be dissolved into the water.23 Furthermore, we have previously reported the possibility of using supercritical water in acid- or base-catalyzed organic synthesis as the catalyst as well as the medium, even in the absence of a catalyst.24 Utilizing these specific characters effectively, a microreaction system has been developed.
10.1021/ie0502640 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/01/2005
Ind. Eng. Chem. Res., Vol. 44, No. 25, 2005 9627
Figure 2. Scheme of the microreaction system.
Figure 1. Structure of alginic acid.
The system is a continuous-flow-type reactor that can control the reaction temperature strictly by quickly heating the reaction species to the supercritical state and then rapidly quenching them to room temperature after the reaction. This quick-heating-quick-cooling method, using a microreaction system, can exclude the influence of the hot-water region on the way to the supercritical or subcritical state, and it eliminates unwanted side reactions and promotes the desired reactions.25 For the aforementioned reasons, the system is useful for reactions that require precise control of the conditions, such as the selective decomposition of alginate. In this study, we applied this subcritical and supercritical water (abbreviated as subH2O and scH2O, respectively) microreaction system to the selective decomposition of alginic acid for the production of the desired alginic acid (having a suitable molecular weight and M/G ratio within a very short time), and we report the successful attainment of the almost-pure guluronic acid homopolymer and the mannuronic acid-rich watersoluble heteropolymer. Experimental Section 1. Decomposition of Sodium Alginate: Preparation of the Aqueous Solution of Sodium Alginate. The 2.0 wt % aqueous solution of sodium alginate (AlgNa) was prepared with 6.00 g of Alg-Na (commercially available Solgin, from Kyosei Pharmaceutical Co., Ltd.) dissolved in 300 g of distilled water. The scheme of the microreaction system is shown in Figure 2, and the reaction procedures were as follows.
The 2.0 wt % Alg-Na aqueous solution was introduced continuously, using the high-pressure pump A (model PU-2086, JASCO Co., Ltd.), into a quick-heating reactor that was composed of a Hastelloy-C tube (0.5 mm × 30 mm, 5.9 mm3). Superheated water (or supercritical water) then was passed into the quick-heating-reaction portion of the apparatus and mixed with the Alg-Na solution. The flow rate of superheated water was ∼3 times than that of the Alg-Na solution for the stable temperature of flow. The sample temperature was measured at the end of the quick-heating-reaction portion of the apparatus, and the pressure was controlled by the back-pressure regulator (model SCF-Bpg/ M, JASCO Co., Ltd.). The products were preserved by freeze-drying. 2. Structure Analysis of Products. 2.1 1H NMR. The 1H NMR spectra were measured with a 270 MHz spectrometer (JEOL model GX-270). The sample (1517 mg) was dissolved in 0.6 mL of D2O containing 0.1% acetonitrile as an internal standard (δ 2.06 for the methyl group). 2.2. Measurement of Weight-Average Molecular Weight by Gel Permeation Chromatography. The weight-average molecular weight (WAMW) was measured using a gel permeation chromatography (GPC) system that was equipped with an Ashahipak GS-520 column (6 mm inner diameter × 250 mm, Showa Denko, Tokyo). NaNO3 aqueous solution (0.3 M) was used as an eluent at a flow rate of 1.0 mL/min at 60 °C. The WAMW was calculated using pullulan as a standard, and acetone was used for calibration of the retention time. Results and Discussion 1. Weight-Average Molecular Weight and 1H NMR Spectroscopy Analyses of Decomposed Products. The decomposition of Alg-Na was studied under subcritical and supercritical conditions, and the results are shown in Table 1. The decomposition proceeds under supercritical and subcritical conditions, and, in the supercritical region, the WAMW of Alg-Na decreases to
