Chan, Chung Hung (2013) Optimization and modeling of microwave-assisted extraction of active compounds from cocoa leaves / Chan Chung Hung. PhD thesis, University of Malaya.
Abstract
Microwave-assisted extraction (MAE) is a promising technique for extraction of active compounds from plants and it has the potential to be commercialized. However, due to limited significant parameters to describe the MAE process, optimization and modeling of MAE for scaling up are challenging and restricted. To resolve the problem, two intensive energy-related parameters, i.e. absorbed power density (APD) and absorbed energy density (AED) were introduced and they are respectively defined as the amount of microwave power (W/ml) and energy (J/ml) absorbed in the solvent during the extraction. Following that, three methods namely APD predictive method, AED modeling method and combined APD-AED optimization method were developed to model and optimize MAE at various extraction scales. The methods developed in this work are based on the extraction of anti diabetic compounds, i.e. isoquercitrin (0.13-3.51 mg/g), epicatechin (0.23-2.91 mg/g) and rutin (0.30-7.07 mg/g) from cocoa (Theobroma cacao L.) leaves. Prior to the evaluation of the developed methods, the optimization and modeling of MAE were performed conventionally using response surface methodology (RSM) and Patricelli model, respectively. The optimum MAE conditions were determined to be 85% (v/v) aqueous ethanol at 50 ml/g (2g), 156 W, and 18 min, and its performance was similar to that obtained in Soxhlet extraction but with lesser solvent (50 ml/g vs. 100 ml/g) and shorter extraction time (18 min vs. 6 hr). From the modeling study, the washing step of MAE is strongly affected by the size of sample while the diffusion step is influenced by both the solvent to feed ratio (S/F) and microwave power (P). The findings obtained from the proposed methods suggest that the APD predictive method is able to predict the optimum extraction time for large scale MAE between 100-300 ml under various microwave power based on the correlation established between the optimum extraction time region and the APD of the extraction system. By conducting MAE at the predicted optimum extraction time region, more than 85% of equilibrium extraction yields can be achieved and the prediction is valid at solvent to feed ratio varying from 20 to 80 ml/g. Besides that, AED modeling method enables the prediction of overall extraction profiles of MAE. By adapting suitable extraction model i.e. film theory model at AED basis, a predictive model can be developed. The AED extraction model is accurate in capturing the experimental extraction profile of MAE at various microwave power (200-600 W) and solvent loading (100-300 ml) with R-square value > 0.87. In addition, APD-AED optimization method standardizes the optimization of MAE based on its extraction mechanisms. According to this method, the optimization can be performed using sequential single factor experiments based on APD and AED and the result obtained was similar to those obtained from the optimization using RSM. Most important, the intensive optimum MAE conditions (S/F = 50 ml/g, APD = 0.3 W/ml, AED = 300 J/ml) determined from this method can be used to determine the optimum operating parameters (S/F, Power, Time) of MAE at varying extraction scale (100-300 ml).
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