Author: Hu, Zhuoyan
Title: Microwave assisted extraction : the effects, mechanisms and applications on selected plant materials
Degree: Ph.D.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Plants -- Analysis
Department: Department of Applied Biology and Chemical Technology
Pages: xviii, 157 p. : ill. ; 30 cm.
Language: English
Abstract: The application of solvent free microwave extraction (SFME) and microwave assisted extraction (MAE) of the effective compounds from plant matrices was investigated. The diffusion coefficient of saikosaponins through the solid matrix under MAE was determined. The effect of microwave irradiation on microstructure of plant tissues was observed using scanning electronic microscopy (SEM) technique. SFME was employed to obtain essential oil from pomelo fruit peels, and sequential MAE of pectin from oil extracted peels was also performed. SFME was superior to the conventional hydrodistillation (HD) method in terms of extraction efficiency and the essential oil yield. The chemical composition analysis by GC-MS shows that SFME did not affect the quality of essential oils compared with HD. In extracting pectin from oil extracted pomelo peels, the extraction time of MAE was significantly shorter than that of the conventional method. The sequential extraction of essential oil and pectin from pomelo fruit peels by SFME and MAE was a feasible processing method. In the study of MAE for extraction of saikosaponins from Radix Bupleuri, the individual effects of microwave power, irradiation time, temperature, ethanol concentration, solvent-to-sample ratio, and sample particle size were evaluated. It was found that the extraction of saikosaponins a, c, and d by MAE with 300 to 500 W power level for 5 min at 75 ℃ with 30 - 70 % ethanol in water, 30:1 solvent-to-sample ratio, and 0.30 to 0.45 mm particle size were the favorable extraction conditions. Compared with conventional extraction methods, MAE can significantly reduce the extraction time, resulting in better extraction efficiency. In the optimization of MAE for saikosaponins, microwave power, time, temperature and ethanol concentration were optimized using response surface methodology (RSM) with desirability function approach. The optimum MAE conditions for extracting saikosaponin a, c, and d simultaneously were found to be at the microwave power of 360-400 W, ethanol concentration of 47-50 %, temperature of 73-74 ℃ and time of 5.8-6.0 min. At these conditions, the yields from the verification experiments were 96.18-96.91 % for saikosaponin a, 95.05-95.71 % for saikosaponin c, and 97.05-97.25 % for saikosaponin d, which were in good agreement with the predicted values from the fitted models. In the mechanism studies, the diffusion coefficients of saikosaponins through the solid matrix under MAE were determined using a Fick's second law-based model. It was found that the effective diffusion coefficients ( Deff ) under different microwave heating conditions increased as a result of the increase in microwave power, and were significantly higher than those extracted with the conventional extraction method. SEM results indicated that microwave heating produced distinguishable microstructure changes on pomelo peels and Radix Bupleuri. Microwave irradiation caused the explosion of oil glands of pomelo peels and rupture of parenchymal cells; therefore the target compounds within the cell were rapidly released into the surrounding extraction solvents. As the liquid phase absorbed the microwaves, the kinetic energy of the molecules increased, and consequently, the diffusion rate accelerated. As a result, better extraction efficiency and significantly reduced extraction time for extraction of the effective compounds from two plant matrices were obtained using MAE.
Rights: All rights reserved
Access: open access

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