Modeling and prediction of extraction profile for microwave-assisted extraction based on absorbed microwave energy
Highlights
► AED modeling method enables direct determination of MAE kinetic coefficients. ► The developed model can capture extraction profile of MAE based on absorbed power. ► The deviation of predicted yield by the developed model is less than 10%. ► Absorbed microwave energy required for equilibrium extraction is 100 to 300 J/ml. ► Extraction time of MAE estimated from 100 to 300 J/ml ensures high extraction yield.
Introduction
Microwave-assisted extraction (MAE) has been widely employed in the extraction of active compounds from plants due to its superior performance in terms of its extraction yields, solvent consumptions and extraction time (Chan et al., 2011, Wang and Weller, 2006). Hence, MAE has the potential to replace conventional extraction techniques. Various extraction models for MAE have been developed to study the effects of operating parameters on the extraction kinetics. For instance, derivation from Fick’s law (Gujar, Wagh, & Gaikar, 2010), chemical kinetic equations (Spigno and De Faveri, 2009, Xiao et al., 2012) and other empirical models (Amarni & Kadi, 2010) were employed and their extraction constants were reported for extraction of various active compounds from plants. However, the predictive capacity of such models is restricted to the extraction constants obtained using specific instrumental setup. Indirectly, this implies that the employment of such model to predict MAE profile (extraction yield vs. extraction time) of different instrumental setup would result in lack of fit. Moreover, the extraction constants were limited to certain operating conditions as the acquisition of experimental data for determining the constants of extraction models is time-consuming. Currently, the application of these kinetic models in the MAE is limited as the reported extraction constants, e.g. diffusivity, can only be used to indicate the kinetic of the extraction for comparison purpose such as at different operating conditions and extraction techniques (Amarni and Kadi, 2010, Gujar et al., 2010). To alleviate the shortcomings of the existing models, a new approach based on microwave energy absorbed in the extraction system is proposed in this work to model and predict the extraction profiles of MAE at various extraction conditions.
The microwave irradiation power for a given unit of extraction volume, known as “energy density”, is claimed to be more applicable for MAE study as compared to microwave power level (Alfaro, Belanger, Padilla, & Pare, 2003). It has been proven to be the most significant parameter in optimisation studies (Li et al., 2012). However, the amount of power supply for the microwave heating does not reflect the actual power absorbed in the extraction system. Only some of the power will be absorbed and this depends on dielectric constant of the system (Mandal, Mohan, & Hemalatha, 2007). The energy absorbed in the extraction system is important as it provides localised heating to disrupt the cells and elute the active compounds (Sparr Eskilsson & Björklund, 2000). In this study, the microwave energy absorbed during extraction has been used to model MAE process and the crucial parameters such as solvent to feed ratio, applied microwave power and solvent loading have been included.
The model was employed to study MAE of antioxidant compounds from cocoa leaves. The theory on the development of the extraction model was presented and the developed model was employed to predict extraction curves of MAE at various extraction conditions. Predictive capability of the model was evaluated with experimental data. Furthermore, a technique for estimating suitable extraction time of MAE at equilibrium extraction was established. The feasibility of the estimation was evaluated by comparing its yield with the conventional extraction.
Section snippets
Materials and reagents
Standards of antioxidant compounds, i.e. isoquercitrin (IQ), (−)-epicatechin (EC) and rutin (RT) were purchased from Sigma–Aldrich Co. (Malaysia). Acetonitrile and ethanol for chromatography analysis (HPLC grade) were purchased from Merck co. (Malaysia). The extraction solvent, denatured alcohol (EtOH) was purchased from LGC Scientific co. (Malaysia).
Extraction procedures
Fresh cocoa leaves were collected during pruning from local cocoa plantation in Pahang, Malaysia. The leaves were washed and dried in air drying
AED extraction model
The extraction constants (b′ and k′) in Eq. (6) under influence of AED are strongly influenced by other parameters such as sample preparation techniques, e.g. sample drying and grinding (So & Macdonald, 1986), solvent concentration and solvent to feed ratio. In this study, the effect of solvent to feed ratio (20–80 ml/g) on the extraction constants was investigated. The extraction constants at these solvent to feed ratios were determined by fitting Eq. (6) with the extraction profile as
Conclusion
A modeling technique based on absorbed microwave energy is proposed. The developed AED-model exhibits high predictive capability in capturing the trend of the extraction profile of MAE. The development of extraction model based on the technique mentioned involves less experimental data as the coefficients of the model can be determined from the APD of the extraction system according to the extraction conditions. In other words, the APD values can be used to characterise the MAE profile. Similar
Acknowledgements
This work was carried out under the Centre for Separation Science and Technology (CSST), University of Malaya and financially supported through UMRG (RG008/09 AET) and PPP (PV062/2011B) grants.
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