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ReviewAcceleration of microwave-assisted extraction processes of foodcomponents by integrating technologies and applying emergingsolvents: A review of latest developmentsFlora-Glad Chizoba Ekezie a, b, c, Da-Wen Sun a, b, c, d, *, Jun-Hu Cheng a, b, c, **a School of Food Science and Engineering, South China University of Technology, Guangzhou 510641, Chinab Academy of Contemporary Food Engineering, South China University of Technology, Guangzhou Higher Education Mega Center, Guangzhou 510006, Chinac Engineering and Technological Research Centre of Guangdong Province on Intelligent Sensing and Process Control of Cold Chain Foods, Guangzhou HigherEducation Mega Center, Guangzhou 510006, Chinad Food Refrigeration and Computerized Food Technology (FRCFT), Agriculture and Food Science Centre, University College Dublin, National University ofIreland, Belfield, Dublin 4, Irelanda r t i c l e i n f oArticle history:Received 7 May 2017Received in revised form2 June 2017Accepted 6 June 2017Available online 15 June 2017Keywords:MicrowavesFood materialsTarget ingredientsExtraction efficiencySolvent mediumsa b s t r a c tBackground: Microwave-assisted extraction (MAE) have gained enormous popularity as a preferredmethod for the recovery of various active compounds from food materials. They proffer benefits such asthe reduction of extraction time, environmental friendliness, low cost and enable automation or on-linecoupling to other analytical procedures. Recently, newer add-ons or technological modifications havebeen incorporated into MAE systems, in an effort to continuously improve extraction efficiency andensure a greener implementation.Scope and approach: The core pathways of such meliorations include integrating microwave extractionwith other technologies, e.g., ultrasound assisted extraction (UAE), negative pressure cavitation (NPC),enzyme assisted extraction (EAE), hydrodiffusion extraction (HDE), and supercritical fluid extraction(SFE) or replacement of extraction medium with suitable alternatives including ionic liquids, deepeutectic solvents, non-ionic surfactants, etc. This review confers their underlying principles and mechanisms, equipment and apparatuses, practicalities, as well as resultant benefits such as increased yield,intensification of mass transfer and reduction of energy consumption, in a manner not achievable byMAE alone or previous intermediary modifications.Key findings and conclusions: It is hoped that this paper reinforces the need to initiate more studiesfocused on process validation and optimization of such emerging MAE systems, in furtherance of theirscale-up, sustainability and robust adoption by the food industry.© 2017 Elsevier Ltd. All rights reserved.1. IntroductionLike drying (Cui, Sun, Chen, & Sun, 2008; Pu & Sun, 2016; Yang,Sun, & Cheng, 2017), cooling (McDonald, Sun, & Kenny, 2000; Sun,1997; Sun & Brosnan, 1999; Sun & Hu, 2003; Sun & Wang, 2000;Wang & Sun, 2002a, 2002b; Wang & Sun, 2004; Zheng & Sun,2004) and freezing (Cheng, Sun, & Pu, 2016; Cheng, Sun, Zhu, &Zhang, 2017; Kiani, Zhang, Delgado, & Sun, 2011; Ma et al., 2015;Pu, Sun, Ma, & Cheng, 2015; Xie, Sun, Xu, & Zhu, 2015; Xie, Sun, Zhu,& Pu, 2016), extraction is a common processing method used in thefood industry. Driven by technical, scientific and economical impediments associated with traditional extraction techniques, suchas high energy cost, residual solvent impurities and thermaldegradation, in the past decade, the food industry has experienceda revolution in the development of greener technologies for therecovery of active ingredients (Alexandre, Castro, Moreira, Pintado,& Saraiva, 2017; Barba, Zhu, Koubaa, Sant’Ana, & Orlien, 2016;Chemat, Rombaut, Fabiano-Tixier, Pierson, & Bily, 2015; Zhu et al.,2016). Such novel methods for the screening, separation andextraction of various food components includes microwaveassisted extraction (MAE), enzyme-assisted extraction (EAE),* Corresponding author. School of Food Science and Engineering, South ChinaUniversity of Technology, Guangzhou 510641, China.** Corresponding author. School of Food Science and Engineering, South ChinaUniversity of Technology, Guangzhou 510641, China.E-mail address: dawen.sun@ucd.ie (D.-W. Sun).URL: http://www.ucd.ie/refrig, http://www.ucd.ie/sunContents lists available at ScienceDirectTrends in Food Science & Technologyjournal homepage: http://www.journals.elsevier.com/trends-in-food-scienceand-technologyhttp://dx.doi.org/10.1016/j.tifs.2017.06.0060924-2244/© 2017 Elsevier Ltd. All rights reserved.Trends in Food Science & Technology 67 (2017) 160e172high-voltage electrical discharges (HVED), pulsed electric field(PEF), ultrasound-assisted extraction (UAE), high pressure processing (HPP), pulsed ohmic heating (POH), supercritical fluidextraction (SFE) and their combinations, coupled with various advances in extraction solvents/mediums. Among them, microwaveassisted extraction (MAE) is the most diffused method for the recovery of food constituents due to its high extraction efficiency,eco-friendliness, low-cost, reduced recovery time and ease ofassembling in large or small scale (Felkai-Haddache et al., 2016; Papet al., 2013).The recovery of organic constituents by microwave irradiationwas first documented in a study by Ganzler, Salgo, and Valko(1986). Since then, several enhancement attempts have beenmade and implemented, in furtherance of MAE sustainability andoperational effectuality. In a typical MAE procedure, microwaveirradiation penetrates a material, interact with polar compoundsand produce voluminous heating via ionic conduction or dipolerotation thereby inducing high yield (Deo et al., 2015). However, inorder to assure continuality, certain constraints such as the need toremove the solvent from the material upon process terminationand its limitation to polar extractants should be vanquished (Chan,Yusoff, Ngoh, & Kung, 2011). In addition, studies propose that tofurther promote extraction efficacy, the elementary closed systemand open MAE systems, which were previously in existence, can bemodified with various add-ons or synergistically used with othertechnologies. For instance, MAE can be combined simultaneouslyor sequentially with other techniques such as UAE, SFE, EAE,hydrodiffusion etc. Correspondingly, the conventionality was toemploy organic solvent or water as extractant in MAE systems, butnowadays, various hydrotropic liquids, two-phase solutions, miscellars, etc. are employed (Chen, Liu et al., 2016; Chen, Zhang et al.,2016; Li et al., 2016; Liu, Chen et al., 2016; Liu, Yu et al., 2016; Tanget al., 2017; Xie et al., 2017; Zhang, Liu et al., 2016; Zhang, Yao et al.,2016). These approaches provide a greener implementation strategy through minimizing solvent usage and less toxicity, reducingwaste production or the emission of CO2 released into the atmosphere and minimal energy consumption. In addition, some ofthese hybrid modes such as MHG ensure recyclability of solvents,re-use of extraction residues for subsequent operations and promote overall extraction efficiency (Boukroufa, Boutekedjiret,Petigny, Rakotomanomana, & Chemat, 2015).Considering the sundry number of reviews (Chan et al., 2011;Mandal & Tandey, 2016; Tatke & Jaiswal, 2011; Wang, Ding, &Ren, 2016; Zhang, Yang, & Wang, 2011) published in the past onMAE systems, which clearly demonstrates the intensity of attentionbestowed on the extraction setup. However, an up-close analysis ofthese reviews and findings from recent experimental investigationsindicates that the engineering of MAE systems have long evolvedfrom elementary systems or their intermediary modifications intoemployment of greener solvent-based MAE systems and integration with other advanced or newer techniques, which remarkablyimproves the overall process efficiency in an unprecedentedmanner. Although, Wang et al. (2016) pointed out some of theseadvancements are in the area of using greener analytical solvents, itbecomes imperative to thoroughly update information available onthese newly developed hybrid modes of MAE systems and ecofriendly solvents in order to substantiate their potentiality andbuttress the need for process validation and optimization in a stepwise approach. Therefore, this review presents a collation ofrecently published studies focused on fresh modifications/ameliorations to MAE systems/procedures. Specifically, from the viewpoint of alternative extractants, combination modes andsubsequent applications during extraction of target compoundsfrom food matrix are discussed. Additionally, the principles andmechanisms as well as equipment setup/configuration arehighlighted, and MAE technological modification trajectory experienced over the years and future directions are elucidated. It ishoped that the current review will promote their potency, popularity and transition from lab-bench to industrial utilization.2. Fundamentals of microwave-assisted extractions2.1. Principles and mechanism of MAEThe rationale of microwaves during extraction lies in thetransfer of energy by electrical field via two coinciding mechanisms. Firstly, dipole rotation occurs by the interaction of dipoleswith polar components, and engendering the dipoles to realignwith the applied field, which causes coerced molecular movementsthat produces heat (Adetunji, Adekunle, Orsat, & Raghavan, 2017;Chan et al., 2011). Secondly, ionic conduction induces the movement of charged ions inside the solvent when electromagnetic radiation is applied and generates a resistance within the solution,resulting in friction and consequently heat is released. Notwithstanding, the aptitude of the food matrix to absorb microwaveirradiation and produce heat is dependent on its dielectric loss.Hence, the absorbed energy is derived from the dissipation factor(d) equation, expressed as follows:tan d ¼ ε00.ε0 (1)where ε00 represents the efficiency of transforming microwaveirradiation into heat and called dielectric loss factor, while ε0 conveys the capacity of an irradiated molecule to become polarized bythe electric field. Meanwhile, the transformation of electrical energy into thermal energy is expressed as:P ¼ K: f ε0E2 tan d (2)where tan d is termed as dielectric loss tangent, E is called theelectric field strength, ε0 is the dielectric constant, f denotes thefrequency employed, K is a known constant and P represents microwave power dissipation per unit volume. When microwave heatcomes in contact with tiny traces of moisture inside the cell matrix,evaporation occurs and builds intense pressure on the cell wall,which ruptures and causes the release of the active constituents.Higher throughputs can be achieved by careful selection of operating conditions such as temperature, since increase in temperaturepromotes faster penetration of solvent into the cell matrix. Likewise, considering the existence of a permanent dipole moment thatinteracts with microwaves, polar molecules and ionic mixturesfacilely absorbs microwave energy. However for non-polar solvents, e.g., hexane, the solvents do not heat up spontaneously whenthey are in contact with microwaves. Other parameters influencingthe performance of MAE systems include solid-to-liquid ratio,extraction duration, microwave power, nature of samples, andstirring. The effects of these parameters have previously beenreviewed and the details can be found elsewhere (Chan et al., 2011).2.2. Configuration and instrumentation in MAE systemsTypically, a basic MAE system comprises of a magnetron, anisolator, a wave guide, a cavity and a mode stirrer (Routray & Orsat,2012). The magnetron generates microwave energy, propagatingthrough the wave guide into the cavity and the mode stirrer disperses the energy via several routes. The cavity is used to confineenergy pending complete absorption by the material matrix. Theisolator is used to prevent reflected energy from reaching themagnetron, otherwise power output would be diminished, i.e.,F.-G.C. Ekezie et al. / Trends in Food Science & Technology 67 (2017) 160e172 161energy is allowed to move from the magnetron to the cavity but notvice versa. The entire set-up can be ameliorated to contain aturntable that ensures uniform energy distribution when a sampleis loaded into the system, regardless of its position (Tatke & Jaiswal,2011). In general, MAE devices are classified as those that operateabove or atmospheric pressure, namely a ‘closed system’ (pressurized MAE) or an ‘open system’ (focused MAE).In the closed system, the vessel is hermetically sealed with amulti-mode supply of microwave radiation and extraction is heldunder homogenous microwave heating. The properly regulatedhigh pressure contained in the vessel permits quick and effectiveextraction. A simultaneous rise in pressure and temperature accelerates MAE extraction process, because solvent absorption ofmicrowave energy is augmented (Chan, Yusoff, & Ngoh, 2013). Inaddition, although closed systems provide high extraction efficiency, a major disadvantage is its vulnerability to thermal degradation coupled with limited analyte output. On the other hand,open MAE systems are associated with advantages such as lesssafety issues, minimal degradation of heat-sensitive compounds,increased productivity and solvent can be injected at any timeduring extraction. Besides, it works at atmospheric pressure,operates under mono-mode or multi-mode in conjunction with areflux unit to condense vaporized solvent. Open MAE systems aresatisfactorily efficient but certain drawbacks as mentioned earlierhave spurred several research groups to develop more efficientMAE systems (Lu, Pan, Chen, Qiu, & Xie, 2014; Yao et al., 2015a,b;Zhang, Liu et al., 2016; Zhang, Yao et al., 2016).2.3. Intermediary modifications to MAE systemsSince its inception, several modifications have been made tobasic MAE system in an effort to further improve overall extractionefficacy. These enhancements were achieved either by changing thebasic design or by integrating an auxiliary operating condition/system. For example, according to (Aguilera-Herrador, Lucena,C ardenas, & Valcarcel, 2007 ), a closed MAE was modified into anon-line MAE performed in a microwave oven equipped with avessel, alongside high pressure supplied via a pump and used todeliver solvent into the system. The sample is soaked in the solventand microwave treatment is exerted. After extraction, an aliquot ofthe extract is aspirated by another pump, through a membrane,which retains analytes while the rest of the materials are passedout as waste. An acetonized steam is subsequently used to rinse theanalytes that is followed by quantitative measurement using alight-scattering detection system.In order to promote process efficiency and minimize losses, Yuet al. (2009) transformed a closed MAE system to operate undernitrogen-protection since reaction of active ingredients with oxygen can be inhibited when inert gases like nitrogen is exercisedalongside. The microwave-assisted extraction under nitrogen gas(NPMAE) was executed to enhance the yield of ascorbic acid (AA)from selected fruit and vegetables, followed by a quantitativeestimation using high performance liquid chromatography (HPLC).Better yields of AA were obtained with NPMAE in comparison withthe traditional MAE and soxhlet technique.In addition, unlike the previously discussed methods, MAE device can be integrated with an analytical step to allow automationand simultaneous operation, which is called dynamic microwaveassisted extraction. In contrast to the batch MAE, this continuousprocess eliminates extraction cycle, trims the risk of contaminationor analyte loss and reduces thermal degradation of compounds. Asrevised by Chan et al. (2011), dynamic microwave-assisted extraction has been successfully employed for an expedited extraction ofsafflower yellow from Flos carthami and flavonoids fromH. epimedii.3. Extraction enhancement by technology integration3.1. Microwaves and negative pressure cavitation (MW-NPC)Negative pressure cavitation (NPC) extraction method is a newlyformulated form of hydrodynamic cavitation established to besignificantly efficient for the separation and recovery of variousfood components such as sugars, flavonoids, phenols and alkaloids,etc. (Liu et al., 2009; Zhang et al., 2010). Customarily, in an NPCextraction device, the mechanism of action is primarily cavitationarising from the production, multiplication and collapse of an indefinite number of minute vapor bubbles, formed at either liquid orliquid-solid interfaces (Roohinejad et al., 2016). This causes anunusual high energy output around a substantial number of reaction sites, leading to increased temperature and pressure inside thesystem (Arrojo, Nerin, & Benito, 2007). Apparently, the increasedenergy supplied magnifies reaction rate in the cavitation system.Meanwhile, it is important to note that the entire phenomenon ofcavitation is primarily induced by negative pressure, constantlysupplied by an attached vacuum pump.As widely acclaimed, application of microwaves during extraction offers several advantages but the process is limited due to itsinability to achieve a thoroughly mixed solution of liquids andsolids for enhancing mass transfer (Roohinejad et al., 2016). Recentstudies suggest that NPC extraction method can be successfullycombined with other extraction methods such as MAE, EAE, mechanical pulverization, etc. Therefore as shown in Fig. 1, a system(MW-NPC) simultaneously employing MW and NPC was proposedto ameliorate the extraction of natural ingredients. The assemblagecomprised of a three-neck reaction flask to accommodate bothsolvent and food materials. An airflow meter was used to allow aninflux of nitrogen into the inlet pipe and unto the center neckportion of the flask. The inherent temperature of the supplied microwave irradiation is regulated using a temperature receptor,placed in the microwave resonance cavity. Accordingly, Yao et al.(2015a,b) employed MW-NPC to intensify the output of threebioactive components namely, chimaphilin, 20-O-galloyl andhyperin from pyrola, and found that MW-NPC significantly shortened extraction time by 40% and promoted overall process efficiency. They also found the extracts obtained using MW-NPCdepicting higher DPPH radical scavenging activity with an IC50value of 0.121 mg/mL, when compared to 0.144 mg/mL and0.167 mg/mL obtained by MW and NPC alone, respectively.Likewise, Zhang, Hu et al. (2013); Zhang, Yao et al. (2013) successfully utilized MW-NPC to extract phenolic compounds fromFig. 1. Schematic diagram of combination of negative pressure cavitation method andmicrowave (Yao et al., 2015a,b).162 F.-G.C. Ekezie et al. / Trends in Food Science & Technology 67 (2017) 160e172pyrola, using 1-butyl-3-methylimidazolium tetrafluoroborate asthe extraction solvent. The most suitable conditions for efficientextraction were found to be 700 W, 0.07 MPa, 40 C, 20:1, 0.5 M and15 min for microwave power, negative pressure, extraction temperature, liquid-to-solid ratio, ionic liquid (IL) concentration andextraction time, respectively. It is noteworthy to mention that theseparameters must be carefully selected, due to their influence on theperformance of MW-NPC extraction system. Nonetheless, MW-NPCextraction systems are highly efficient, environmentally friendlyand economic. Moreover, they can be broadly employed in variousextraction applications, especially for recovering heat sensitivecompounds.3.2. Ultrasonic microwave-assisted extraction (UMAE)Lately, the synergistic application of microwaves and ultrasoundirradiation for the extraction of food constituents has receivedenormous attention. UMAE amplifies mass transfer mechanismduring extraction by supplying lofty momentum and energy torupture cell integrity and consequently liberate constituents intothe extraction solvent (Chen et al., 2010; Ochoa-Rivas, Nava-Valdez,Serna-Saldívar, & Chuck-Hernandez, 2017 ). Microwaves act byenhancing moisture permeability to capillaries and increasingwater absorption capacity of the biological material (Kratchanova,Pavlova, & Panchev, 2004). It also heats the material homogeneously and instantaneously in a short time. Conversely, ultrasonication is advantageous due to cavitation, which destabilizescell wall and heightens mass transfer kinetics and equilibrium(Kaufmann & Christen, 2002). The utilization of ultrasound duringMAE contributes mechanical and thermal effect, which cumulatively boost extraction efficiency.Table 1 summarizes recent studies conducted on UMAE. Amongthem, a new direction applying bronsted ionic liquid([HO3S(CH2)4mim]HSO4) during UMAE was projected as a befittingalternative to conventional solvents, which was shown to deepenpectin throughput from the albedo of pomelo peels (Liu, Qiao, Guet al., 2017; Liu, Qiao, Yang et al., 2017). Liew, Ngoh, Yusoff, andTeoh (2016) also conducted a similar investigation using UMAE toextract pectin from pomelo peel using citric acid as the extractingsolvent, and showed that at optimum conditions, the yield of pectin(36.33%) from UMAE was higher compared with ultrasoundassisted extraction (UAE) and MAE. Additionally, Xu, Yang, and Fu(2016) opined that combining ultrasonication and MAE may be anovel waste valorization strategy for walnut green husk, especiallywhen recovering juglone from its green husk. In general, UMAEpromotes overall extraction efficiency by energy savings, extractiontime reduction, less solvent consumption, as well as improvedextraction yield as compared to traditional extraction techniques ortheir individual techniques when used separately.3.3. Microwave-assisted subcritical and supercritical fluidextractionSupercritical (SC) state is the characteristic attribute of a substance acquired when subjected to temperature and pressure levelsabove its critical point (Dejoye et al., 2011; Pereira & Meireles,2010). Certain enhanced properties such as viscosity, surface tension and solvation capacity, makes SC fluids particularly suitable forTable 1Effect of ultrasonication on microwave-assisted extraction procedures.Matrix Target Component Solvent Optimal Conditions Major Findings ReferencesPomelo peels Pectin 10 mM bronsted IL[HO3S(CH2)4mim]HSO4SLR ¼ 27 mL/g,MP ¼ 360 W, ET ¼ 15 min,UP ¼ 50 WIncreased pectin yield (i.e.,328.64 ± 4.19 mg/g)Liu, Qiao, Gu et al. (2017);Liu, Qiao, Yang et al. (2017)Clinacanthus nutans Polyphenols, flavonoids,triterpenoids,vitamin CDistilled water SLR ¼ 1:55 g/ml,MP ¼ 90 W, ET ¼ 75 sExtraction yields ofpolyphenols ¼ 8.893,flavonoids ¼ 25.936,triterpenoids ¼ 16.789 andvitamin C ¼ 0.166 mg/gQun et al. (2017)Walnut green husk Juglone 70% trichloromethaneSLR ¼ 70:1, UP ¼ 585.42 W,UT ¼ 25.57 min,MT ¼ 103.27 sSignificantly increasedextraction efficiency ofUMAE (836.5l g/g), incontrast to UAE and MAE,with values of 179.8 and187.2 l g/g, respectively.Xu et al. (2016)Pomelo peel Pectin Distilled water UT ¼ 27.52 min,MT ¼ 6.40 min,MP ¼ 643.44 W, pH ¼ 1.80UMAE achieving thehighest pectin yield of36.33%Liew et al. (2016)Polygonum cuspidatum Polydatin, resveratrol,emodin-8-O-b-D glucoside,emodin10% hydroxypropyl-bcyclodextrinSLR ¼ 1:50 g/mL,ET ¼ 1 min,extractiontemperature ¼ 80 CEnhancement of the releaseof the hydrophilic andhydrophobic components,as compared to HRE, UE andMAE methodsGao et al. (2016)Cabbage outer leaves Glucosinolates,sulforaphane, vitamin C andphenolicsEthanol UT ¼ 30 min, UP ¼ 320 W,MP ¼ 100 W, MT ¼ 2 minHigh contents ofextractable bioactivecompounds, as compared toUAE or MAE alonePongmalai, Devahastin,Chiewchan, andSoponronnarit (2015)Rhubarb Anthraquinones 2.0 mol L1 of1-butyl-3-methylimidazoliumbromide [bmim]Br solutionSLR ¼ 1:15 g mL1,Time ¼ 2 min, MP ¼ 500 WUp to 18.9e24.4% increasein extraction efficiency andreduced extraction timefrom 6 h to 2 minLu et al. (2011)Dioscorea Zingiberensis Diosgenin 0.5 molL-1 1-ethyl-3-methylimidazoliumtetrafluoroborate ([EMIm]BF4) solutionSLR ¼ 1:5, MP ¼ 500 W,UP ¼ 50 W and ET ¼ 8 minMaximum yield ofdiosgenin(10.24 ± 0.31 mg g1)Wang et al. (2014)Hemp Seed Cake Polyphenols 10% ethanol SLR ¼ 1:6, MP ¼ 700 W,UP ¼ 200 W, ET ¼ 20 minEnhanced extractionthroughputTeh, Niven, Bekhit, Carne,and Birch (2014)Note: SLR ¼ Solid-liquid ratio, UP ¼ Ultrasound power, UT¼ Ultrasound time, MP ¼ Microwave power, MT ¼ Microwave time, ET ¼ Extraction time.F.-G.C. Ekezie et al. / Trends in Food Science & Technology 67 (2017) 160e172 163extracting food components, especially oil and fats. In addition, SFEleaves no solvent impurities and operates at low temperatures. Theonly disadvantage associated with SFE is the low polarity of carbondioxide (i.e., the most diffused SC fluid) but this can be tackled usingvarious modifiers. On the other hand, subcritical water extractionemploys water as an extractant, which is superheated to temperatures between 100 and 374 C at elevated pressure levels thatensure that liquids remain in their liquid form (Matusiewicz &Slachci nski, 2014 ). It is also referred to as pressurized or low polarity water and over the years has received enormous attention asa suitable solvent for the extraction of both polar and non-polarcompounds.A contemporary add-on to MAE is subcritical or supercriticalfluid extraction for increased extraction effectuality. For example,Dejoye et al. (2011)) conducted a preliminary treatment of Chlorellavulgaris with microwaves before supercritical CO2 extraction (MWSCCO2) and showed that the yield of fatty acids from the samplematrix was increased. As shown in Fig. 2, the oil throughput fromMW-SCCO2 extraction was higher and contained significant quantities of oleic, linoleic, a-linolenic and palmitic acids, whencompared to supercritical fluid extraction without microwavepretreatment (Dejoye et al., 2011). Similarly, Lu et al. (2014) exercised an experiment to recover alkaloids from Gynura Segetum andshowed higher yields of alkaloid with MW-SCCO2 under theoptimal conditions of soaking time of 10 h and microwave powerlevel of 90% than supercritical CO2 extraction alone.With regards MAE extraction using subcritical water, an attemptwas made by Matusiewicz and Slachci nski (2014) to implement asingle-stage microwave-assisted subcritical water extraction (MWSWE) for concurrent determination of inorganic-metallic constituents in selected milieu samples. The process conditions weremicrowave treatment time of 10 min, temperature of 280 C,pressure level of 90 bar, and ethanol-water modified with 1% HNO3as the extracting solvent. Results showed that the method couldincrease efficiency, save running time and reduce associatedoperational costs. Aside metallic compounds, MW-SWE can also beexploited for the recovery of polyphenols, a broad variety of polarto non-polar compounds, which depicts biological activities bypreventing diseases related with excessive oxygen radical formation. To cite an example, a microwave pretreatment (200 C) priorto SWE maximized the release of polyphenols by over 55% fromdefatted rice bran under 10 min only with higher antioxidant activity than untreated samples (Wataniyakul, Pavasant, Goto, &Shotipruk, 2012). More recently, Yusoff and Leo (2017) conducteda similar investigation on recovering polyphenols from roselleseeds subjected to MAE procedures with subcritical water as theextractant. In the absence of temperature regulation, the extractantreached subcritical conditions at 158 C and 16.4 bar, leading toexcellent liberation of phenolic compounds (18.2 mgGAE/g) aftertreatment time of 10 min at a microwave power of 300 W. Despitethe above studies, research on microwave-assisted subcritical orsupercritical fluid extraction is still at its embryonic stage althoughthe process appears to possess futuristic significance in separationscience.3.4. Microwave-assisted enzymatic extraction (MAEE)Enzyme-assisted extraction (EAE) is a potential panacea fortraditional extraction, owing to its peculiar advantages includingenvironmental compatibility, reduced solvent usage, increasedextraction efficiency and ease of operation (Cheng et al., 2015; Puri,Sharma, & Barrow, 2012). The intrinsic properties of enzymes i.e.high selectivity, keen specificity and ability to catalyze certain reactions are the mechanical pathways for effectual extraction (Puriet al., 2012), EAE has been utilized to recover constituents fromcanola, sunflower, olive, sesame, etc. They are also employed injuice processing to disrupt cell wall integrity and enhance liberation of phenolic compounds into the juice, thus extending productshelf life and overall quality (Puri et al., 2012). However, majordrawbacks of EAE are high cost, problems in commercial scale upand inability of some enzymes to completely hydrolyze cells thatleads to low extraction yield.A formidable approach for mitigating these issues involvesapplying microwave irradiation during EAE. The procedure knownas microwave-assisted enzymatic extraction (MAEE) is acknowledged to have great prospects because the energy supplied by thenonionizing radiation of microwave permits rotation of dipoles andinduces fragmentation of the cell matrix, as heat is continuouslysupplied (Adetunji et al., 2017; Wang et al., 2007). For achievingdesirable performance, affecting factors such as surface area of foodmaterial, enzyme structure, process time and solid-liquid ratio areimportant and should be considered.Fig. 2. Extraction of lipids from Chlorella vulgaris using (C) microwaves/supercritical CO2 extraction and (B) supercritical CO2 extraction, both operating at 28 MPa/40 C (Dejoyeet al., 2011).164 F.-G.C. Ekezie et al. / Trends in Food Science & Technology 67 (2017) 160e172Several studies have been published addressing MAEE. The mostrecent probe conducted by Nguyen, Jones, Kim, San MartinGonzalez, and Liceaga (2017) was focused on the production offish protein hydrolysates (FPH) from rainbow trout using alcalase asthe hydrolytic enzyme, at different enzyme substrate ratio (E:S) in amicrowave system operating at 1200 W power level. The techniquewas found to promote hydrolysis (p