A microwave-assisted extraction method is proposed as an alternative to the conventional method for determining the hot water solubility of wood. In this alternative method, microwave heating substitutes for the boiling water to extract part of the extraneous components as well as starches in wood tissues. Experimental results indicate that 100 mL water can be heated to boiling in only 75 s under the microwave radiation. Hence, only 15-20 min are required to complete the extraction procedure for Liquidambar formosana Hance and Swietenia mahagoni Jacq., 10 min and 5 min for Taiwania cryptomerioides and Cunninghamia lanceolata (Lamb) Hook, respectively. In general, a 15-min microwave heating has the potential to be an alternative to the conventional method, which requires 3 h for the hot water solubility determination

Cheng, Yin-li

Hwang, Wen-shuang

Jen, Jen-fon

Kuo, Lan-sheng

Lee, Maw-rong

English

Wood and Fiber Science (E-Journal)

A MICROWAVE-ASSISTED EXTRACTION METHOD FOR DETERMINING HOT WATER SOLUBILITY OF WOOD Lan-sheng  K u o  Professor Department of Forestry Jen-fon Jen and Maw-rong  Lee Professors Department of Chemistry Wen-shuang Hwang Graduate Student and Yin-li Cheng Senior Researcher Department of Forestry National Chung Hsing University Taichung 402, Taiwan (Received November 2000) ABSTRACT A microwave-assisted extraction method is proposed as an alternative to the conventional method for determining the hot water solubility of wood. In this alternative method, microwave heating sub- stitutes for the boiling water to extract part of the extraneous components as well as starches in wood tissues. Experimental results indicate that 100 mL water can be heated to boiling in only 75 s under the microwave radiation. Hence, only 15-20 min are required to complete the extraction procedure for Liquidambur formosana Hance and Swietenia mahagoni Jacq., 10 min and 5 min for Taiwania cryptomerioides and Cunninghamia lunceolata (Lamb) Hook, respectively. In general, a 15-min mi- crowave heating has the potential to be an alternative to the conventional method, which requires 3 h for the hot water solubility determination. Keyword.?: Microwave-assisted extraction, hot water solubility, wood meal, irradiation, heat. INTRODUCTION The water-soluble materials in wood in- clude inorganic salts, sugars, polysaccharides, cycloses and cyclitols, and some phenolics (Browning 1967). In the conventional method for determining cold water solubility, wood sawdust is extracted with water at 23 -t 2°C with stirring for 48 h to remove part of the extraneous components such as inorganic compounds, tannins, gums, sugars, and col- oring matter present in wood (Browning 1967). For the determination of hot water sol- ubility, wood meal is extracted with water un- der reflux in a boiling water bath for 3 h to remove the above-mentioned components plus starches. These procedures were revised in 1999 (TAPPI Standard T207, 1999). It takes 3 h to perform the extraction. It is tedious and time-consuming. In modern analytical prac- tice, saving time is an essential target for most chemists. However, a new method should be proven in its applicability, including precision and accuracy, in addition to time-saving. Therefore, a time-saving procedure applicable to determine hot water solubility of wood is worthy of being investigated. Wr~od ~rnd Fzber Science. 34(1), 2002, pp. 72-76 0 2002 by the Society of Wood Science and Technology Kuo et 01.-MICROWAVE-ASSISTED EXTRACTION FOR HOT WATER SOLUBILITY 73 In past decades, the application of micro- wave-assisted extraction was studied in a wide range of materials such as soils, plant and an- imal tissues, and a variety of manufactured products (Camel 2000; Carro et al. 1997; Kingston and Jassie 1998; Llompart et al. 1997; Pensado et al. 2000). The application is increasing. In general, the extractions assisted by microwave-irradiation are more selective and rapid with similar or better recoveries in comparison with the conventional extraction processes (Zuloaga et al. 1998). In addition, microwave-assisted extraction has the advan- tage of using less solvent and energy than con- ventional techniques. Thus, an application of microwave-assisted extraction has the poten- tial to be an alternative to the conventional method of determining the hot water solubility of wood. The objective of the present work was to compare hot water extraction for four Taiwan wood species (two hardwoods and two soft- woods) with water under reflux in a micro- wave oven to the conventional method. MATERIALS AND METHODS Apparatus A 720-watt power home-use microwave oven (ME-66M, National Brand, Taiwan) was obtained from a local supplier, modified, and assembled as a microwave-assisted apparatus as shown in Fig. 1. An electronic balance (AND HR-200, Japan, precision: 0.1 mg) was used to measure the weight of wood dust. Materials Four 20-30-yr-old trees were obtained from plantations near Taichung. The trees included two hardwoods-Formosan sweet gum (Liq- uidambar formosana Hance) and mahogany (Swietenia mahagoni Jacq.) and two soft- woods-Taiwania (Taiwania cryptomerioides Hayata) and china fir (Cunninghamia lanceo- lata (Lamb) Hook). Using a power-driven saw, a sample 1,000 mm long was cut from each log. All bark and knots, decayed portion, and other abnormalities were separated and discarded. The specimens were stored in a dry, cool room for wood analysis. Logs were debarked and ground in a labo- ratory grinder. The finer material was separat- ed by sifting on a 40-mesh screen. Accepts (size: 40-60 mesh) were used for hot water extraction. Microwave extraction method A 2-g sample was suspended in a 250-mL round bottom flask refluxed with 100 mL dis- tilled water. The condenser was connected to the flask. The suspensions were irradiated for 5 to 20 min in a modified microwave oven (Fig. 1). The suspensions were heated to a boil. After irradiation at fixed times, the sam- ples were transferred to a tared 1G3 glass filter for filtering. The sample was washed with 200 mL hot water and then dried to a constant weight at 105 2 3°C. Conventional extraction method Two g of air-dry wood meal was transferred to a 250-mL round bottom flask attached to the reflux condenser, 100 mL hot distilled wa- ter was added, and the flask was placed in a boiling water bath with hot plate heating and digested for 3 h. The water level of the bath was held above the stock level in the flask. The contents of the flask were transferred to a tared 163  glass fil- ter, which had been previously dried to a con- stant weight at 105 + 3°C in an air-circulating oven. The hot water solubility was calculated as: where S% is the hot water solubility A is the initial weight (oven-dry) of wood specimen B is the weight (oven-dry) of wood specimen after extraction The reported hot water solubility is the av- erage of 5 determinations, to the nearest of 0.01 %. The relative standard deviation (RSD) is also given. WOOD AND FIBER SCIENCE, JANUARY 2002, V.  34(1) Microwave heatin r Sample-water mixture (absorbs microwave energy) Water outlet ==I+- Water inlet Vessel wall (transparent to microwave energy) - FIG. 1. A schematic diagram for illustrating the mechanism of microwave heating. RESULTS A N D  DISCUSSION Microwave heating With conventional conductive heating, it usually takes 1 to 3 h or longer to complete a wet digestion for a wood sample in an open vessel. However, with microwave heating, it can be completed in 5-20 min. The difference is due to the sample heating method. Because vessels used in conductive heating are usually poor conductors of heat, it takes time to heat the vessel and transfer that heat by convection to the solution. On the other hand, it is easily understood that microwave energy is trans- ferred directly to all of the molecules of water solution nearly simultaneously without heating the vessel. Therefore, when using microwave heating, the solution reaches its boiling point very quickly. Figure 2 demonstrates the tem- perature profile of the 100-mL distilled water solution by employing microwave heating at 720-watt power. The temperature rises rapidly, and reaches the boiling point (100°C) of water after only 75 seconds. Kuo rt a/.-MICROWAVE-ASSISTED EXTRACTION FOR HOT WATER SOLUBILITY 75 8 I 0 10 20 30 40 50 60 70 80 Heating time (sec) FIG. 2. The corresponding temperature of 100 mL wa- ter in microwave oven vs. heating time. Determination qf the hot water solubility by microwave-assisted method wood species. Whereas the detection of quan- tity of Taiwania and China fir by microwave heating is approximate to that of the conven- tional method in 10- and 5-min irradiation, re- spectively, after the optimum heating time, Taiwania increases to 3.92% and China fir de- creases to 8.10% at 20-min heating time. The difference of hot water solubility between the optimum time and 20-min microwave heating for test wood species depicts the difference in their intrinsic variations of the chemical com- positions in the more dense hardwoods as compared with that of the softwoods. Addi- tionally, by statistical analysis, the lower val- ues of the RSD for each extraction also sug- order to examine the applicability of mi- gest that the method of microwave-assisted crOwave heating in the determination of hot hot water solubility can likely be used as an water solubility, wood meal of four over 20- alternative method to determine the hot water yr-old trees was extracted with the microwave- solubility of wood. Nonetheless, the standard- assisted method. ~ h ~ i ~  hot water solubility ization of the microwave-assisted method for was to those Obtained by the con- determining the hot water solubility of wood ventional method. Table 1 lists the hot water needs further solubility of the four wood specimens under varying microwave times. As shown in Table 1, the quantity of detec- tion increases with microwave irradiation time from 5 to 20 min, and approximates that ob- tained by the conventional method for For- mosan sweet gum, mahogany and Taiwania wood species. This shows that the determina- tions of hot water solubility by conventional and microwave methods agree well for these CONCLUSIONS From the results of the hot water solubility determinations, the microwave-assisted heat- ing extraction method has proven to be effec- tive in time-saving (15 min vs. 3 h). It indi- cates that the proposed microwave heating method has the potential to be an alternative to the conventional conductive heating method in the hot water solubility determination of TABLE I. Cornpuri.ron of'rnic.rowuve rind conventionul extraction methods f o r  determining hot water .solubility offour Tuiwat~ wood species. Hot water solubility (%) Liyuidumhrrr ,fi~rnio.sa~zu Hance S~ietenicr trtuhugotii Jacq. Tuiwuniu cr~ptontc.rioide.c Hayata Curntinghumiu Iut~crolutu (I>amb) Hook I Convcnttonal \anrple\ wcl-c healed In hoiling water bath for 3 h ' Nurnhcr\ In ( 1 are the rrlatlvr 5l;indard deviation\ ( % I  for 5 cxtractlons 76 WOOD AND FIBER SCIENCE, JANUARY 2002, V. 34(1) wood. The standardization of this new method needs to be further investigated. REFERENCES BROWNIN(;, B. L. 1967. Method of wood chemistry, Vol. 1, Appleton, W1. Pp. 81-82. CAMEL, V. 2000. Microwave-assisted solvent extraction of environmental samples. Trends Anal. Chem. 19(4):229- 248. CARRO, N., C. M. GARCIA, AND R. CELAR. 1997. Micro- wave-assisted extraction of monoterpenols in mush samples. Analyst 122:325. KINGSTON, H. M., AND L. B. JASSIE. 1988. Introduction to microwave sample preparation, American Chemical So- ciety, Washington, DC. LLOMPART, M. P., R. A. LOKENZO, R. CELA, AND J. R. J .  PARE. 1997. Optimization of a microwave-assisted ex- traction method for phenol and methylphenol isomers in soil samples using a central composite design. Ana- lyst 122: 133. PENSADO, L., C. CAAIS, C. MEJJIJTO, AND R. C. CELA. 2000. Optimization of the extraction of polycyclic aromatic hydrocarbons from wood samples by the use of micro- wave energy. J. Chromatography a, Vol. 1 1 :505-5 13. TAPPI STANDARD. 1999. Water solubility of wood and pulp. T 207 om-93 Atlanta, GA. ZULOAGA, O., N. ETXEBARRIA, A N D  A. LUIS. 1998. Com- parison of accelerated solvent extraction with micro- wave-assisted extraction and Soxhlet for the extraction of chlorinated biphenyls in soil samples. Trends Anal. Chem. 17(10):642-647. 

A Microwave-Assisted Extraction Method for Determining Hot Water Solubility of Wood

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A Microwave-Assisted Extraction Method for Determining Hot Water Solubility of Wood

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