Optimization of microwave-assisted extraction based on absorbed microwave power and energy

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1 This is an author generated postprint of the article: Chan, C.-H., Yusoff, R., & Ngoh, G.-C. (2014). Optimization of microwave-assisted extraction based on absorbed microwave power and energy. Chemical Engineering Science, 111, doi: /j.ces Optimization of microwave-assisted extraction based on absorbed microwave power and energy Chung-Hung Chan a, *, Rozita Yusoff a, Gek-Cheng Ngoh a a University of Malaya, Department of Chemical Engineering, Kuala Lumpur, Malaysia. ABSTRACT: The intrinsic microwave power and energy required for optimizing MAE was investigated for the extraction of active compounds from cocoa (Theobroma cacao L.) leaves at various extraction scales. To carry out this investigation, an optimization method based on absorbed power density (APD) and absorbed energy density (AED) was developed based on the extraction mechanism of MAE using sequential single factor experiments. The optimized results are consistent with those obtained by the conventional optimization method using response surface methodology (RSM) and also comparable with that of the optimized Soxhlet extraction. The main advantage of the method is that the intensive optimum conditions obtained at solvent to feed (S/F) ratio of 50 ml/g, APD of 0.3 W/ml and AED of 300 J/ml are capable of determining the specific optimum operating conditions (S/F ratio, power, time) of the MAE at various scales of 100 to 300 ml with great accuracy. Keywords: optimization; extraction; energy; absorbed power density; absorbed energy density *Corresponding author. Tel: ; Fax: ; address: ch_chan@um.edu.my The published version is available on

2 1. Introduction MAE is a widely employed extraction technique for the extraction of active compounds from plants. Its influencing operating parameters such as solvent to feed (S/F) ratio, microwave irradiation power and extraction time are often optimized using statistical optimization method (Ballard et al., 2010; Chen et al., 2012; Prakash Maran et al., 2013). However, the optimum extraction conditions reported in literature are restricted as they can only be applied to specific microwave systems. Different microwave system when applied with the same operating conditions gives different heating efficiency and extraction performance. Moreover, the reported optimum conditions of MAE can hardly be applied to a larger extraction scale as the reported optimum microwave power is catered only for specific scale of extraction. Under this constraint, the optimum MAE conditions in literature can only be used as a reference for microwave extraction system. These operational issues are required to be resolved. Consequently, the possible solution is to investigate the optimum conditions of MAE involving the optimization of the intensive parameters. The parameters concerned are energy related such as energy density (Alfaro et al., 2003). This parameter represents microwave irradiation power for a given unit of extraction volume and it is comparatively more significant than the microwave power level in the optimization of MAE (Alfaro et al., 2003; Li et al., 2012). However, the irradiation power for the microwave heating (power density) does not reflect the actual power absorbed in the extraction system. To ensure that the energy absorbed in the extraction system that provides localized heating to disrupt the cells (Sparr Eskilsson and Björklund, 2000) is taken into account, two intensive MAE parameters namely absorbed power density (APD) and absorbed energy density (AED) have been introduced (Chan et al., 2013). APD and AED address the absorbed microwave power and heating energy of the extraction system respectively. These parameters do not have interactive effects with each other and they have been employed to replace microwave irradiation power and extraction time in the modeling of MAE (Chan et al., 2013). This has further encouraged the research in developing an optimization method incorporating APD and AED for MAE. In this study, a standardized procedure to optimize MAE at various extraction scale was proposed. It involves the extraction of antioxidants from cocoa (Theobroma cacao L.) leaves under various operating parameters such as S/F ratio, microwave power, extraction time and solvent loading. The optimization method employs sequential single factor experiments, which was developed based on the extraction mechanisms of MAE.

3 The optimum operating parameters of the MAE at various extraction scales were determined according to the proposed method and their extraction performance was evaluated. For verification, the optimum operating conditions were compared with the optimum conditions obtained from the conventional optimization method and their extraction results were compared with the Soxhlet extraction technique. 2. Materials and Methods Materials and reagents Flavonoid standards of isoquercitrin, (-)-epicatechin and rutin were purchased from Sigma- Aldrich co. (USA). Acetonitrile and ethanol for chromatography analysis were purchased from Merck co. (Germany). The extraction solvent, denatured alcohol (EtOH) was obtained from LGC Scientific co. (Malaysia). Extraction procedures Fresh cocoa leaves were collected during pruning from the local cocoa plantation in Jengka, Pahang, Malaysia (Malaysian Cocoa Board). The collected leaves were washed and then dried in a forced convection oven at 40 o C for 24 hours. The moisture content of the resulted dried leaves was about 5-6%. The dried leaves were powdered to mm and stored in an airtight container at 4 o C until further use for the experiment. MAE was conducted in a domestic microwave oven with adjustable nominal power output of ( W) equipped with fiber optic Luxtron I652 thermometer. Cocoa leaves powder (1-6 g) was weighted and mixed with 85% (v/v) EtOH to a desirable predetermined solvent to feed (S/F) ratio (constant volume) in a 500 ml Duran bottle. The Duran bottle was capped tightly and put in the microwave cavity. The mixture in the bottle was irradiated in the microwave oven of a predetermined power and time without stirring. Upon completion of the extraction process, the closed bottle was cooled down to room temperature in a water bath. The extract in the closed bottle was filtered using fine cloth to remove the plant residues and it was subsequently filtered through 0.2 µm RC (Regenerated Cellulose) syringe filter before HPLC analysis.

4 In the Soxhlet extraction, 2 g of the dried cocoa leaves sample was extracted with 200 ml pure ethanol for 6 hours. This optimized conditions were obtained from preliminary trials whereby single factor experiment was used to optimize the operating parameters such as extraction solvent, extraction time and solvent to feed ratio. The clean up procedure for the chromatography analysis of the Soxhlet extract sample was the same as for MAE. HPLC analysis The antioxidant compounds was quantified using Agilent 1200 Series HPLC device with Agilent ZORBAX Eclipse Plus C18 column, 5 µm (4.6 mm 150 mm) according to the analytical method proposed by Bonaccorsi et al. (2008). The linear gradient of acetonitrile in water was used as the mobile phase: 5 20% (0 15 min), 20 30% (15 20 min), 30 50% (20 30 min), % (30 35 min), 100% (35 40 min), and 100 5% (40 50 min) at flow rate of 1.0 ml/min. The sample injection volume was 10 µl. The separation of flavonoids was analysed at 350 nm (isoquercitrin and rutin) and 280 nm ((-)-epicatechin) using UV-DAD detector. The extraction yield is expressed as the mass of extracted active compounds (mg) per mass of sample used (g). The total extraction yields of isoquercitrin, (-)-epicatechin and rutin were the responses of the optimization study. Determination of APD and AED In this optimization study, the two intensive parameters namely absorbed power density (APD) and absorbed energy density (AED) are investigated. APD of the extraction system indicates the microwave power absorbed per unit volume of solvent (W/ml). This can be determined by measuring the amount of energy required to heat up a blank extraction solvent under a corresponding solvent loading ( ml) and nominal microwave power ( W) for certain heating time using Eq. (1). Q APD Eq. (1) V t H where Q is the total heat absorbed by the solvent during microwave heating (J), V is the solvent loading (ml) and th is the microwave heating time (min). Q can be determined from the temperature profile of the solvent during microwave heating using calorimetric method (Incropera, 2007). In this calculation, two heating cases have to be considered: Case 1: When the final heating temperature is less than the boiling point of the solvent Q m C T Eq. (2) L p

5 Case 2: When the final heating temperature equals to the boiling point of the solvent Q mlc p T mv Hvap Eq. (3) where ml is the initial mass of the extraction solvent, Cp is the heat capacity of the extraction solvent, ΔT is the temperature difference before and after the microwave treatment, mv is the mass of the vaporized solvent, Hvap is the heat of vaporization of the extraction solvent. The first heating case considers the absorbed energy required to increase the solvent temperature from room temperature to the boiling point ( 70 o C) while the second heating case combines both the energy required to heat and to vaporize the extraction solvent during boiling. The original volume of blank solvent and the volume of solvent immediately after microwave heating were recorded to determine the amount of evaporated solvent. To get accurate APD value, microwave heating was conducted at different heating times (th) that includes both the heating cases and then the average APD of these conditions were determined. The sample calculation and the APD values at solvent loading of ml and nominal microwave power of W are given in Table 1. The other intensive parameter, AED, which indicates the microwave energy absorbed per solvent volume (J/ml), is related to APD and the extraction time via Eq. (4). AEDt APD t Eq. (4) where AEDt is the total amount of microwave energy absorbed per solvent volume during the extraction (J/ml) and t is the extraction time (min). Optimization of MAE based on APD and AED In this optimization strategy, the optimum values of S/F ratio, AED and APD were investigated sequentially using single factor experiments. These experiments were performed using fresh sample in duplicate except for the determination of AED. In the determination of the optimum AED, the optimum value can be determined from the overall trend of MAE extraction curve i.e. Y/Ysat, where Y is the extraction yield at certain extraction time and Ysat is the equilibrium extraction yield. Therefore, replication of the experiment in this case is not necessarily as it only indicates the reproducibility of extraction yield at each extraction points. In this study, the effect of S/F ratio at ml/g was first investigated at arbitrary extraction conditions e.g. 100 W, 50 ml, 10 min before the optimum S/F ratio was determined. In the subsequent experiment, the effect of AED at the extraction conditions of 100 W, 50 ml and optimum S/F ratio were studied by conducting the extraction at different extraction time (2-20 min) using a fresh sample. The optimum extraction time obtained facilitates the determination of optimum

6 AED using Eq. (4). The same strategy was selected for the subsequent experiments. Ultimately, the effect of APD at the optimum S/F ratio and AED was investigated at the specific solvent loading with varying microwave powers of W. To execute MAE at one specific AED implies that the extraction time for each microwave power level must be determined based on their APD values using Eq. (4). The optimum S/F ratio, AED and APD are referred as intensive optimum conditions of MAE in this study. The intensive optimum conditions were then validated and verified at larger scale MAE of ml. With the establishment of the intensive optimum conditions, the optimum operating parameters of MAE (S/F ratio, power, time) at larger scale MAE can be determined such that the optimum microwave power was selected based on the optimum APD and the optimum extraction time was determined based on the optimum AED via Eq. (4). Table 1: APD values at different microwave irradiation power and solvent loadings Solvent loading, V (ml) Microwave irradiation power, P (W) ± ± ± ± ± ± ± 0.06 Absorbed power density, APD (W/ml) ± ± ± ± 0.03 Example of calculation of APD by calorimetric method MAE operating Heating time, Total heat Absorbed power conditions th absorbed, Q density, APD Average APD (min) (J) (W/ml) (W/ml) ml, 100 W

7 Optimization of MAE based on RSM The optimized result generated by the AED-APD method was evaluated in a comparative study with the conventional RSM method. RSM with Box-Behnken design (BBD) was employed to optimize microwave nominal power (X1) of W, S/F ratio (X2) of ml/g and extraction time (X3) of min in 17 set of duplicated experiments as shown in Table 2. The result from RSM optimization study was fitted into the second-order polynomial model as shown in Eq. (5) o i i ii i ij i j i 1 i 1 i j Y B B X B X B X X Eq. (5) where Y represent the predicted response; Xi was the actual value of an independent variable, B0 denotes the model intercept; Bi, Bii, and Bij are the coefficients of the linear, quadratic, and interactive effects, respectively. Analysis of variance (ANOVA) was used to determine the regression coefficients and the predicted model was verified by conducting the experiments in triplicate under optimum extraction conditions. Table 2: Optimization of MAE based on RSM-BBD Standard order Microwave Power, X 1 (W) Solvent to feed ratio, X 2 (ml/g) Extraction time, X 3 (min) Total extraction yields, Y (mg/g)

8 3. Results and discussion APD-AED optimization method This method optimizes MAE operating parameters based on the extraction mechanisms as demonstrated in Fig. 1. Basically, MAE consists of three extraction mechanisms and each of them is affected by a group of operating parameters. The first mechanism is associated with the penetration of solvent into the plant matrices. In the second, the polar solvent in the plant cells is heated up by microwave and gradually the built up internal pressure ruptures the cells. The final mechanism involves the elution of active compounds from the ruptured cells and dissolution of the compounds into the solvent. Hypothetically, the rupturing of plant cells in mechanism 2 is rate limiting as it requires heating energy to proceed. The mechanism 1 and 3 are relatively fast and the operating parameters associated are extraction solvent, solvent to feed (S/F) ratio (constant volume) and particle size of sample. These parameters do not exhibit interactive effects with each other thus they can be investigated individually and can be specified prior to the optimization of the mechanism 2. The rate limiting mechanism of MAE (rupturing of plant cells) is crucial as it determines both the rate of extraction and the yields of the extraction significantly. The operating parameters that affect the mechanism are microwave power and extraction time. They exhibit interactive effect and are usually optimized together with other interactive parameters such as S/F ratio at constant sample mass (Chen et al., 2010; Yang and Zhai, 2010) and also extraction temperature when involved with thermal sensitive compounds (Liazid et al., 2011; Wang et al., 2009). It was reported that APD and AED could serve as an appropriate alternative in place of the microwave power and extraction time in the study of MAE (Chan et al., 2013). As independent variables, they can be optimized separately and sequentially using single factor experiment as demonstrated in this study.

9 Specify prior to the optimization of mechanism 2 Parameter Extraction solvent (Good dielectric properties) Quantity of solvent Penetration of solvent Surface contact area Mechanism 1 Penetration of solvent into plant matrix Rate of microwave heating Optimization using RSM Parameter Microwave power Optimization using single factor experiments Parameter APD Parameter Solvent to feed ratio (Constant volume) Solubility of compound in solvent Mechanism 2 Rupture of plant cell by internal pressure Rate limiting Duration of microwave heating Interaction Parameter Extraction time No Interaction Parameter AED Parameter Particle size of sample Solvent concentration gradient Surface contact area Mechanism 3 Diffusion and dissolution of compounds into solvent Fig. 1: Strategy of optimizing MAE based on its extraction mechanisms

10 Optimization of S/F ratio The effect of S/F ratio on the MAE yields at the arbitrary conditions of 50 ml, 100 W and 10 min is plotted as shown in Fig. 2. The result shows that increasing S/F ratio at constant volume improves the extraction yields of MAE. Once the ratio is increased beyond its optimum value, i.e. 50 ml/g, the increment in the extraction yield is insignificant (Spigno and De Faveri, 2009). On the other hand, if the S/F ratio is evaluated at constant mass of sample, the change in solvent volume due to different ratio would affect the absorption of microwave energy (Kingston and Jassie, 1988). This may give rise to poor extraction yield due to insufficient heating power (Mandal and Mandal, 2010). Considering both the solvent consumption and the extraction performance of MAE based on the extraction trend in Fig. 2, S/F ratio at constant volume of 50 ml/g was selected for subsequent optimization of AED and APD. 7 Total extraction yield (mg/g) Solvent to feed ratio (ml/g) Fig. 2: Single factor optimization on solvent to feed ratio at constant volume. MAE conditions: 50 ml, 100 W, 10 min and APD of 0.43 W/ml. Optimization of AED The effect of AED was investigated by profiling MAE extraction using AED as a basis as shown in Fig. 3. Both the extraction profiles of MAE at varying time and AED exhibit similar trends. The extraction achieves equilibrium yield at 12 min extraction time and 300 J/ml AED. In spite of the similarity in the extraction profiles using two different bases, they have different implications on the optimization of MAE. Unlike extraction time which only addresses the

11 heating time, AED denotes the amount of microwave energy required to heat up the solvent in order to rupture the plant cells in achieving certain degree of completion for MAE. It also indicates the progress of MAE to reach equilibrium regardless of the microwave irradiation power (or APD) of the extraction system and solvent loading (Chan et al., 2013). In other words, the optimum AED is constant regardless of the microwave heating power, provided that the extraction time is regulated to reach the optimum AED value using Eq. (4). The optimum AED value obtained at 300 J/ml, will be used to investigate the optimum APD of the MAE Extraction yield Y/Y sat Temperature ( o C) AED (J/ml) Extraction time (min) Fig. 3: Single factor optimization on AED. MAE conditions: 50 ml/g, 50 ml, 100 W and APD of 0.43 W/ml; Equilibrium extraction yield (Y sat) is 6.95 mg/g; regressed temperature profile Optimization of APD The effect of APD on the performance of MAE was investigated at AED of 300 J/ml with varying microwave power from 100 to 300 W. This is to ensure all extractions could reach the same degree of completion or reaching equilibrium. It is impractical and meaningless to evaluate the effect of heating power at the same extraction time as the extraction time required

12 for high microwave power differs from that needed for low microwave power to achieve equilibrium yield. Furthermore, the nominal microwave power is merely an indication of the power setting of the microwave system, the crucial evaluation of the heating power in this studies depends on the APD to reflect the real heating power in the system. Once APD of a specific nominal microwave power is determined, the corresponding extraction time at AED of 300 J/ml can be calculated using Eq. (6). 300 (J/ml) t (s) Eq. (6) APD ( W/ml) The relationship between APD and the extraction time at AED of 300 J/ml is plotted as shown in Fig. 4. The figure shows that the equilibrium extraction time decreases exponentially with the increase of microwave heating power. The equilibrium extraction time varies from 30 min to 5 min when the APD of the extraction changes from 0.15 to 0.95 W/ml ( W at 100 ml). MAE conducted at these extraction conditions and the results plotted in Fig. 4 suggests that the MAE favors extraction conditions at low APD (< 0.35 W/ml). MAE conducted at APD greater than 0.35 W/ml gives slightly lower yields (< 5% difference) with shorter extraction time. The decrease in yields at high APD could be caused by the thermal degradation of active compounds. By considering both the performance of MAE and the thermal stability of the active compounds, APD of 0.3 W/ml is taken to be the optimum value for the MAE in this study. Overall, the optimum extraction conditions of MAE were determined to be at S/F ratio (constant volume) of 50 ml/g, AED of 300 J/ml and APD of 0.3 W/ml which corresponds to the optimum microwave power of 150 W and the optimum extraction time of 16.7 min for solvent loading of 100 ml. Verification and comparison of optimum extraction conditions of MAE Verification of the feasibility of the APD-AED optimization method was performed by comparing its optimized results obtained with that from RSM. From the optimized results of RSM tabulated in Table 3, the empirical relationship for the total extraction yield with extraction parameters was generated as follows: Y X X X X X X X X X X X X Eq. (7) where X1 is microwave power (W), X2 is S/F ratio (ml/g) and X3 is extraction time (min),

13 40 Extraction time (min) W 130 W t(min) 150 W 300 J/ml APD 60 s/min 170 W 200 W 300 W 7.4 Total extraction yield (mg/g) W 130 W 150 W 170 W 200 W 300 W APD (W/ml) Fig. 4: Single factor optimization on APD. MAE conditions: 50 ml/g, 100 ml and AED of 300 J/ml) respectively. The models are significant (P < 0.05) with insignificant lack of fit and its response surface curves are plotted in Fig. 5. As observed, the response surface curves have no obvious optimum point since the S/F ratio was investigated at constant volume similar to that in the APD-AED optimization study. Nevertheless, the interaction between microwave power and S/F ratio in Fig. 5 (a) shows that for every S/F ratio, there is a specific microwave power for optimum extraction yield. The specific optimum microwave powers ranges from 161 W to 155 W corresponded with S/F ratios of 30 ml/g to 70 ml/g regardless of its extraction time. The corresponding optimum extraction time can be determined at specific S/F ratio and the specific optimum power. As to compare the optimum points obtained from the APD-AED method,

14 solvent to feed ratio of 50 ml/g was selected for this RSM optimization study. By iterating the above procedure, the optimum microwave power and extraction time were respectively determined to be 156 W and 18 min respectively. The optimum extraction conditions obtained from the two optimization methods were verified with 3 sets of experiments and their extraction results were compared with the optimized Soxhlet extraction as shown in Table 3. Though the optimum operating conditions obtained from the two optimization strategies differ slightly in terms of microwave power (150 W vs. 156 W) and extraction time (16.7 min vs. 18 min), 98% total recoveries of active compounds were achieved in comparison to the Soxhlet extraction. Table 3: Verification of optimum extraction conditions of MAE and comparison with Soxhlet extraction Extraction technique MAE Optimization strategy APD-AED RSM with BBD Soxhlet Optimum condition 85% EtOH, 50 ml/g, 2 g sample, 150 W and 16.7 min 85% EtOH, 50 ml/g, 2 g sample, 156 W and 18 min 100% EtOH, 100 ml/g, 2 g sample and 6 hr Total yields (mg/g) 6.97 ± ± ± 0.20 IQ yield (mg/g) 1.05 ± ± ± 0.09 EC yield (mg/g) 1.38 ± ± ± 0.13 RT yield (mg/g) 4.49 ± ± ± 0.01 Total recovery (%) a / a calculated based on total extraction yield of Soxhlet extraction.

15 (a) (b) Fig.5: Response surface plots of the (a) effect of solvent to feed ratio and microwave power, and (b) effect of extraction time and solvent to feed ratio on total extraction yield

16 Application of intensive optimum MAE conditions The optimum operating conditions obtained from the APD-AED optimization method are the intensive optimum MAE conditions which can be used to determine the specific optimum operating conditions of MAE (S/F, Power, time) for various solvent loading, e.g ml as shown in Table 4. The procedure involves adjusting the microwave power of MAE system at fixed APD value of 0.3 W/ml for each solvent loading. Generally, when the desired APD is difficult to be achieved due to power setting for certain microwave system, the best tuning gives the nearest APD value in the range of 0.30 ± 0.04 W/ml for each solvent loading (Table 4) has to be considered. The APD values obtained for specific solvent loading was then used to calculate their respective extraction time based on AED of 300 J/ml using Eq. (6). Table 4 shows that MAE conducted at different solvent loadings and at their respective optimum conditions produce less than 3% discrepancy in their total extraction yields. This signifies the reliability of the intensive optimum MAE conditions in determining the optimum operating parameters for MAE at varying extraction scales and probable scaling up of the process. Table 4: Validation of intensive optimum MAE condition (50 ml/g, 300 J/ml, 0.3 W/ml) at larger scales extraction Solvent loading (ml) Optimum microwave power (W) a APD (W/ml) Optimum extraction time (min) b Total extraction yield (mg/g) 6.97 ± ± ± ± ± 0.08 IQ yield (mg/g) 1.05 ± ± ± ± ± 0.04 EC yield (mg/g) 1.38 ± ± ± ± ± 0.03 RT yield (mg/g) 4.49 ± ± ± ± ± 0.06 Percentage difference of total extraction yield (%) c / a determined based on APD of 0.3 W/ml. b determined based on AED of 300 J/ml. c determined based on the total extraction yields at solvent loading of 100 ml.

17 The intensive optimum extraction conditions offer great operational flexibility as they describe the intrinsic criteria for optimum extraction regardless of the size of extraction. The intensive parameter such as S/F ratio is closely related to the concentration gradient effect and which indirectly affects the diffusion and dissolution of the active compounds. Meanwhile, APD and AED are the parameters indicating the amount of heating power and energy required to heat up the extraction solvent in order to rupture the plant cells to achieve equilibrium or optimum extraction yield. In view of the above, the intensive optimum conditions of MAE can be easily modified to fit certain optimization objective without having the need to investigate the interactions between the parameters concerned. In this study, MAE was optimized at optimum solvent consumption of 50 ml/g. By fixing the intensive parameters AED and APD respectively at the same value of 0.3 W/ml and 300 J/ml, the extraction yields can be maximized with higher S/F ratio e.g. 80 ml/g at constant volume. Apart from varying the S/F ratio, different combinations of APD and AED result in different MAE performances. There are nine performance regimes of MAE which can be classified under the effects of APD and AED as illustrated in Fig. 6. Based on the extraction results, the optimum region of the MAE is confined within APD of W/ml and AED of J/ml. MAE which conducted outside this optimum region gives different characteristic and performance. MAE conducted below optimum AED values give incomplete extraction due to inadequate heating time; above the optimum AED, the extraction is risked of thermal degradation due to prolonged extraction and hence may give poor equilibrium extraction yields. On the other hand, MAE which is conducted below optimum APD would give poor extraction yield as the heating power is insufficient to rupture all the plant cells, whereas high APD beyond the optimum value would expose the extraction to high temperature causing undesirable effect on thermal sensitive compounds. 4. Conclusion The APD-AED optimization method simplifies and standardizes the optimization of MAE based on its extraction mechanisms. This method is easy when compared to the conventional optimization method using RSM. It can be performed using a series of single factor experiments unlike in the optimization using RSM whereby screening of suitable range of parameters is prerequisite to achieve reliable results. The intensive parameters namely APD and AED can replace microwave irradiation power and extraction time in the optimization of MAE. The intensive optimum conditions of MAE for the extraction of active compounds from

18 APD (W/ml) Diffusive extraction Equlibrium region Excessive thermal exposure Incomplete extraction Overheating Overheating Overpower microwave heating 0.35 Incomplete extraction Optimum extraction Overheating Optimum microwave heating 0.25 Incomplete extraction Incomplete extraction Incomplete extraction Underpower microwave heating AED (J/ml) Fig. 6: Performance regimes of MAE based on APD and AED cocoa leaves are S/F ratio of 50 ml/g, APD of 0.3 W/ml and AED of 300 J/ml. Due to their intrinsic properties, they can be used to determine the optimum operating parameters (S/F ratio, Power, Time) for various scale of the extraction. Acknowledgements This work was carried out under the Centre for Separation Science and Technology (CSST), University of Malaya and financially supported through University of Malaya Research Grant (UMRG: RP002A-13AET). References Alfaro, M.J., Belanger, J.M.R., Padilla, F.C., Pare, J.R.J., Influence of solvent, matrix dielectric properties, and applied power on the liquid-phase microwave-assisted processes (MAP (TM)) extraction of ginger (Zingiber officinale). Food Research International 36, Ballard, T.S., Mallikarjunan, P., Zhou, K., O'Keefe, S., Microwave-assisted extraction of phenolic antioxidant compounds from peanut skins. Food Chemistry 120, Bonaccorsi, P., Caristi, C., Gargiulli, C., Leuzzi, U., Flavonol glucosides in Allium species: A comparative study by means of HPLC-DAD-ESI-MS-MS. Food Chemistry 107,

19 Chan, C.-H., Yusoff, R., Ngoh, G.-C., Modeling and prediction of extraction profile for microwave-assisted extraction based on absorbed microwave energy. Food Chemistry 140, Chen, Y., Zhao, L., Liu, B., Zuo, S., Application of Response Surface Methodology to Optimize Microwave-assisted Extraction of Polysaccharide from Tremella. Physics Procedia 24, Part A, Chen, Y.Y., Gu, X.H., Huang, S.Q., Li, J.W., Wang, X., Tang, J., Optimization of ultrasonic/microwave assisted extraction (UMAE) of polysaccharides from Inonotus obliquus and evaluation of its anti-tumor activities. International Journal of Biological Macromolecules 46, Incropera, F.P., Fundamentals of heat and mass transfer, 6 ed. Wiley, Hoboken, NJ. Kingston, H.M., Jassie, L.B., Introduction to microwave sample preparation: theory and practice. American Chemical Society. Li, Y., Han, L., Ma, R., Xu, X., Zhao, C., Wang, Z., Chen, F., Hu, X., Effect of energy density and citric acid concentration on anthocyanins yield and solution temperature of grape peel in microwave-assisted extraction process. Journal of Food Engineering 109, Liazid, A., Guerrero, R.F., Cantos, E., Palma, M., Barroso, C.G., Microwave assisted extraction of anthocyanins from grape skins. Food Chemistry 124, Mandal, V., Mandal, S.C., Design and performance evaluation of a microwave based low carbon yielding extraction technique for naturally occurring bioactive triterpenoid: Oleanolic acid. Biochemical Engineering Journal 50, Prakash Maran, J., Sivakumar, V., Thirugnanasambandham, K., Sridhar, R., Optimization of microwave assisted extraction of pectin from orange peel. Carbohydrate Polymers 97, Sparr Eskilsson, C., Björklund, E., Analytical-scale microwave-assisted extraction. Journal of Chromatography A 902, Spigno, G., De Faveri, D.M., Microwave-assisted extraction of tea phenols: A phenomenological study. Journal of Food Engineering 93, Wang, J.L., Zhang, J., Wang, X.F., Zhao, B.T., Wu, Y.Q., Yao, J., A comparison study on microwaveassisted extraction of Artemisia sphaerocephala polysaccharides with conventional method: Molecule structure and antioxidant activities evaluation. International Journal of Biological Macromolecules 45, Yang, Z., Zhai, W., Optimization of microwave-assisted extraction of anthocyanins from purple corn (Zea mays L.) cob and identification with HPLC-MS. Innovative Food Science & Emerging Technologies 11,