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Kumar, Chandan & Karim, Azharul(2019)Microwave-convective drying of food materials: A critical review.Critical Reviews in Food Science and Nutrition, 59(3), pp. 379-394.
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https://doi.org/10.1080/10408398.2017.1373269
1
110867Microwave‐ConvectiveDryingofFoodMaterials:ACriticalReview1
C.Kumara1,M.AKarima2
aChemistry, Physics and Mechanical Engineering, Queensland University of 3Technology, Brisbane, Australia 4
5
1. Abstract:6
Microwave convective drying (MCD) is gaining increasing interest due to its7
uniquevolumetricheatingcapabilityandabilitytosignificantlyreducedryingtimeand8
improvefoodquality.Themainobjectiveofthispaperistodiscuss,criticallyanalyze9
andevaluatetherecentadvancesinMCDandsuggestthefuturedirectionsinthisfield.10
The main focus of this paper is the mathematical modelling and experimental11
investigationsinmicrowaveconvectivedryingoffoodmaterials.Recentdevelopments12
inmthamaticalmodellingofMCDisdiscussedandexistingexperimentalsetupandtheir13
advantagesanddisadvantagesarediscussedandanalysed.Longdryingtimeisaconcern14
infoodindustries.ReductionsindryingtimebyapplyingMCDcomparedtoconvection15
drying are calculated and discussed. It was apparent that the proper integration of16
mathematicalmodellingandexperimentaltechniqueisthebestwaytomaximizethe17
advantages of this dryingmethod. Although there is a plethora of research is being18
carried out in this topic, there is still need for research to develop fundamental19
modellingtooptimizetheprocessparametersandscaleupthistechnologyforindustrial20
application.Overall,thereviewprovidesanin‐depthinsightintothelatestdevelopment21
ofMCDanditsmathematicalmodellingapproachesandwillhopefullyservetoinspire22
futureworkinthefield.23
24
Keywords: Food Drying, Microwave, Drying time, Modelling, Food quality,25
Experimentalsetup,Energyefficiency26
1Correspondingauthor:Tel+61415489441,Email:[email protected]
2
2. Introduction:27
Energyefficiency,productqualityanddryingtimearethreemainconcernsinfood28
drying. Drying research evolved to optimize these three parameters. Based on the29
availabledryingtechnologies,dryingmethodscanbedividedintofourgenerations.First30
andsecondgenerationincludedifferentconvectivedryingmethods.Cabinet,kiln,belt,31
conveyordryerareconsideredas the firstgeneration,andsprayanddrumdrierare32
considered as second generation drying. Then the freeze and osmotic drying are33
consideredasthethirdgeneration.MicrowaveandRFrelateddryingarereferredtoas34
innovative and fourth generation drying technology (Malafronte et al., 2012; Vega‐35
Mercado,MarcelaGóngora‐Nieto,&Barbosa‐Cánovas,2001).Microwaverelateddrying36
is gaining increasing interest due to the following reasons: (a)volumetric heating:37
microwaveenergyheatsupvolumetricallyandpumpsmoisturetosurfacetheproduct38
thuscasehardeningoftheproductcouldbeeliminated(I.W.Turner,Puiggali,&Jomaa,39
1998); (b)increase drying rate:microwave increase diffusion of heat andmass thus40
significantly reduce thedrying time (Mujumdar,2004;M.Zhang,Tang,Mujumdar,&41
Wang,2006);(c)qualityimprovement:qualityofthedriedproductcanbeimprovedby42
combining microwave with other drying method(Dev, Geetha, Orsat, Gariépy, &43
Raghavan, 2011); (d) applying MW energy in a pulsedmanner to maximize drying44
efficiencywhen the process ismass transfer controlled; (e) Boundwatermolecules45
whicharedifficulttoremovecanalsobeexcitedbymicrowaves(Gunasekaran,1999).46
Inordertoimprovetheperformance,microwaveisgenerallycombinedwithhotair47
drying, freeze drying, vacuumdrying, spouted bed drying and osmotic drying. Since48
freezeandvacuumdryinginvolvehighercapitalandoperatingcost,convectiondrying49
ismostwidelycombinedwithmicrowave(Andrés,Bilbao,&Fito,2004).Moreover,the50
maindrawbacksofconvectivedryingarelongerdryingtimesandformationofacrust51
on the surface due to the elevated temperature. Microwaves can mitigate these52
problemsbyincreasingthediffusionrateandsupplyingenoughmoisturetothesurface.53
Thus,combiningmicrowavewithconvectiondryingcansignificantlyshortenthedrying54
timeandimproveproductqualityandenergyefficiency(M.Zhangetal.,2006).Internal55
heat generatedby themicrowave increasepressurewithin thepores anddrives the56
moisture to the surface. The convective air takes themoisture by evaporation, and57
sometimesevaporativecoolingoccursatthesurface(Kumar,Millar,&Karim,2016)..58
3
There have been tremendous progress in experimental investigation and59
mathematical modelling of MCD in the last few decades. However, there is no60
comprehensive review of the recent advances in this field to analyze the progress61
systematically and prospect future direction. This paper presents a comprehensive62
reviewof the currentdevelopments inMCDof foodmaterials.Thiswork focuseson63
mathematicalmodellingofMCD,experimentalmethodsanddryingtimesavingsinMCD,64
First,areviewonMCDmethodwascriticallydiscussedbycategorizingtheliterature65
intotwobroaddivisions:(1)continuousmicrowave‐convectivedrying(CMCD)and,(2)66
intermittent microwave‐convective drying (IMCD). In this section, the drying time67
savingsandfoodqualityinMCDarediscussed.Thenthecurrentexperimentalprocess68
andtheiradvantagesandlimitationwerepresentedanddiscussed.Finally,anextensive69
review of the mathematical models available for MCD are critically analysed with70
relevantanalysisandreasoning.71
3. Principleofmicrowaveheating72
Microwavereferselectromagneticradiationinthefrequencyrangeof300MHz‐73
300GHz with a wavelength 1mm‐1m. Electromagnetic energy propagates through74
spacesbymeansoftime‐varyingelectricandmagnetic fields(HaoFeng,Yin,&Tang,75
2012).Microwavepenetrates thematerialuntilmoisture is locatedandheatsup the76
materialvolumetrically thus facilitateshigherdiffusionrateandpressuregradient to77
driveoffthemoisturefrominsideofthematerial(I.W.Turner&Jolly,1991).Thereare78
two main mechanisms of MW heating; dipolar reorientation and ionic conduction.79
Water molecules are dipolar in nature and try to follow the electric field which80
alternatesatveryhighfrequency.Foracommonlyusedmicrowavefrequencyof245081
MHz,theelectricfieldchangesdirections2.45billiontimesasecond,makingthedipoles82
movewithit(Kumar,2015).Suchrotationofmoleculesproducesfrictionandgenerate83
heatinsidethefoodmaterial(M.Zhang,Jiang,&Lim,2010).Foodmaterialsespecially84
fruits and vegetables contain about 80%of dipolarwater, therefore,microwave can85
generatesignificantvolumetricheat.Thesecondmajormechanismisionicconduction.86
Theions,particularlyinsaltyfoods,migrateundertheinfluenceofelectricfieldmigrate87
andgenerateheat.Figure1showsthetwoheatingmechanisminthefrequencyregion88
usedinindustryforheatinganddrying.89
4
90
Figure1:Schematicdiagramdepictingthedipolarandioniclossmechanismsand91
their contributions to the dielectric properties as a function of frequency (Metaxas,92
1996b).93
4. Continuousmicrowave‐convectivedrying(CMCD)94
Anextensivecompilationof literatureregardingmicrowaveassistedconvective95
dryingof food ispresented in Table1. ThesecondcolumnofTable1represents the96
drying time savings for different studies when compared to convective drying. The97
percentageofdryingtimesavingswerecalculatedbydividingthereductionindrying98
timecomparedtoconvectivedryingwiththetotalconvectiondryingtime.Asubstantial99
dryingtimesavingwasreportedby(Sadeghi,MirzabeigiKesbi,&Mireei,2013),where100
theyreportedthattheconvectiondryingoflemonsliceat500Ctook1850min,whereas101
microwaveconvectivedryingwithspecificmicrowavepower2.04W/g tookonly44102
min.Thisisanenormoussavingindryingtime,accounting97%.Anotherconsiderable103
dryingtimesavingwasreportedbyMcMinnetal.(2003)whichisapproximately96%104
forlemonslicedryingusingMCD.OthersignificantdryingtimesavingsusingMCDwas105
reportedbyPrabhanjanetal.(1995)forcarrot(82%),Bantleetal.(2013)forclipfish106
(90%),Gowenetal.(2008)forsoybean(62.5%).Nevertheless,thedryingtimesavings107
maydifferwiththemicrowavepowerlevel.Thehigherthemicrowavepower,themore108
thedryingtimesavings.However,thehigherpowerlevelcandamageoroverheatthe109
product and eventually degrade the food quality. Therefore, the microwave power110
5
shouldbetakenintoconsiderationwhenapplyingCMCDdrying.Insummary,itcanbe111
concludedthatsubstantialreductioninthedryingtime(25–90%)havebeenachieved112
inMCDdryingwhencomparedwithconvectiondrying(Cinquanta,Albanese,Fratianni,113
LaFianza,&DiMatteo,2013;Izli&Isik,2014;Prabhanjanetal.,1995). However,as114
mentioned earlier, heat sensitive food materials like banana may char due to115
overheating. In thatcase, themicrowaveshouldbeappliedcarefully toavoidquality116
degradationinCMCD.117
Table1:Summeryofmicrowaveassistedconvectiveheatinganddryingoffoodmaterial118
Material Drying time saving
(Maximum)
Modelling of power
distribution
Reference
Apple N/A N/A (Marzec, Kowalska, & ZadroŻNa, 2010)
Apple cylinders N/A N/A (Andrés et al., 2004)
Apple cube 50% N/A (Chong, Figiel, Law, & Wojdyło, 2014)
Apple and
Mushroom
N/A N/A (Funebo & Ohlsson, 1998)
Beetroot 75% N/A (Figiel, 2010)
Carrot N/A Lamberts law (Sanga, Mujumdar, & Raghavan, 2002)
Carrot cubes 82% N/A (Prabhanjan et al., 1995)
Chinese jujube 61% N/A (Fang et al., 2011)
Clipfish 90% N/A (Bantle et al., 2013)
Cooked soybeans 62.5% N/A (Gowen et al., 2008)
Corn N/A N/A (Nair, Li, Gariepy, & Raghavan, 2011)
Cranberries N/A N/A (Sunjka, Rennie, Beaudry, & Raghavan,
2004)
Garlic N/A Lambert’s law (Abbasi Souraki & Mowla, 2008)
Garlic cloves 65% N/A (Sharma, Prasad, & Chahar, 2009)
Gel N/A Maxwell* (Pitchai, Birla, Subbiah, Jones, &
Thippareddi, 2012)
Green peeper N/A Empirical (H. Darvishi, M. H. Khoshtaghaza, G.
Najafi, & F. Nargesi, 2013)
6
Material Drying time saving
(Maximum)
Modelling of power
distribution
Reference
Green pepper N/A Empirical (H. Darvishi, M. H. Khoshtaghaza, G.
Najafi, & F. Nargesi, 2013)
Lemon slice 97% Empirical (Sadeghi et al., 2013)
Mashed potato N/A Maxwell* (Chen et al., 2014)
Minced beef N/A Lambert’s Law* (Campañone & Zaritzky, 2005)
Moringa oleifera
pods (Drumsticks)
79% N/A (Dev et al., 2011)
Mushrooms N/A N/A (Argyropoulos, Heindl, & Müller, 2011)
Oyster mushroom 80% Empirical (Bhattacharya, Srivastav, & Mishra, 2013)
Pineapple N/A N/A (Botha, Oliveira, & Ahrné, 2011)
Pistachios N/A Empirical (Kouchakzadeh & Shafeei, 2010)
Potato N/A Maxwell’s Equation (Malafronte et al., 2012)
Potato 96% Lamberts law (McMinn et al., 2003)
Potato spheres N/A Maxwells (Gulati, Zhu, Datta, & Huang, 2015)
Pumpkin slices 76% N/A (Ilknur, 2007)
Swede, potato,
bread, and
concrete
N/A N/A (Holtz, Ahrné, Rittenauer, & Rasmuson,
2010)
Sour Cherries N/A N/A (Wojdyło, Figiel, Lech, Nowicka, &
Oszmiański, 2014)
Tomato N/A N/A (Swain, Samuel, Bal, & Kar, 2013)
Tomato slice 85% Empirical (Workneh & Oke, 2013)
Two-percent agar
gel
N/A Lambert’s and
Maxwell’s*
equations
(Yang & Gunasekaran, 2004)
Wheat seeds N/A Lambert’s Law (Hemis, Choudhary, & Watson, 2012)
*‐heatingonly(nomasstransfer);N/A‐Notavailable119
Microwaveassisteddryingalsoapplied tonon‐foodmaterial likewood(Lehne,120
Barton,&Langrish,1999),kaolin(Kowalski,Musielak,&Banaszak,2010),brick(I.W.121
Turner&Jolly,1991),agglomeratedsand(Hassini,Peczalski,&Gelet,2013)andfound122
7
tobehelpfulintermsofenergyefficiencyandproductquality.Regardingqualityofnon‐123
foodmaterials,MCDdriedproductsresultedinsuperiorqualitywhencomparedtohot124
airdrying(Argyropoulosetal.,2011;Cinquantaetal.,2013).Jindaratetal.(2011)also125
foundthatusingmicrowaveenergyindryingofanon‐hygroscopicporouspackedbed126
reduceddryingtimebyfivetimeswhencomparedtoconvectivedryingmethod.From127
theabovediscussion,itcanbesaidthatMCDisapotentialoptiontoachieveabetter128
qualityofdriedfoodandareductionindryingtime.129
The third columnofTable1 indicates the availability ofmodellingworkof the130
relevantstudyandtypesofmodel,i.e.whethertheyappliedLambert’slaworMaxwell’s131
equationformicrowavepowerabsorption.Moredetailsofthemodellingworkincluding132
Lambert’sLawandMaxwell’sequationarediscussed in themodellingsectionof this133
paperpresentedinsection7.134
However,supplyingcontinuousmicrowaveenergytoheatsensitivemateriallike135
foodmaycauseunevenheatingoroverheatingorevencreatehotspotsandcoldspots136
(Kumar, Joardder, Karim, Millar, & Amin, 2014). Heat and mass transfer should be137
carefullybalancedtoavoidsuchoverheating(Gunasekaran,1999).Thisproblemcould138
beovercomebyapplyingmicrowaveenergyintermittently.Intermittencyalsoallows139
limiting the temperature rise and moisture redistribution which improves product140
qualityandenergyefficiency(Soysal,Ayhan,Eştürk,&Arıkan,2009a).141
5. IntermittentMicrowave‐ConvectiveDrying(IMCD)142
Table 2 summarizes the studies relevant to Intermittent Microwave‐Convective143
Drying (IMCD). Intermittentapplicationofmicrowaveenergy in convectivedrying is144
more advantageous than continuous application and can overcome the problems of145
overheatingandunevenheating.Thishasbeendemonstratedindifferentstudiesshown146
on Table 2. For instance, Soysal et al. (2009a) reported that IMCD of red pepper147
producedbettersensoryattributes,appearance,color,textureandoverallliking,than148
MCD and commercial drying. Soysal et al. (2009b) compared IMCD and convective149
dryingfororeganoandfoundthattheIMCDwas4.7–11.2timesmoreenergyefficient150
comparedtoconvectivedryingandwasabletoprovidebetterqualitydriedfood.151
152
8
Table2:Summary of intermittent microwave assisted convective heating and drying of food material 153
Material Modelling of power
distribution
Single phase or
multiphase models Reference
Bananas N/A N/A (Ahrné, Pereira, Staack, &
Floberg, 2007)
Carrots, mushrooms N/A N/A (Orsat, Yang, Changrue, &
Raghavan, 2007)
Carrot Slices N/A N/A (D. Zhao et al., 2014)
2% agar gel Lambert’s Law* Single Phase (Yang & Gunasekaran,
2001)
Dill leaves Empirical N/A (Esturk & Soysal, 2010)
2% agar gel N/A* N/A* (Gunasekaran & Yang,
2007a)
Mashed potato Maxwell’s* Single Phase (Gunasekaran & Yang,
2007b)
Oregano N/A N/A (Soysal et al., 2009b)
Pineapple N/A N/A (Botha, Oliveira, & Ahrné,
2012)
Red pepper N/A N/A (Soysal et al., 2009a)
Sage(Salvia
officinalis) Leaves N/A N/A
(Esturk, 2012; Esturk,
Arslan, Soysal, Uremis, &
Ayhan, 2011)
Apple Labmert’s Law Single Phase (Kumar, Joardder, Farrell,
Millar, & Karim, 2015)
Apple Labmert’s Law Multipahse (Kumar, Joardder, Farrell,
& Karim, 2016)
AdvantagesofIMCDintermsofimprovingenergyefficiencyandproductquality,154
andsignificantlyreducingdryingtimehavebeenfoundinmanyotherproductssuchas155
Oregano(Soysaletal.,2009b),Pineapple(Bothaetal.,2012),RedPepper(Soysaletal.,156
2009a),SageLeaves(Esturk,2012;Esturketal.,2011),Bananas(Ahrnéetal.,2007),157
andCarrotsandMushrooms(Orsatetal.,2007).158
9
ThesecondandthirdcolumnofTable2presentsanoverviewofmodellingstudies159
regardingIMCD.Itcanbeseenfromthetablethatthereareverylimitedstudyregarding160
modellingofIMCD,whichhasbeendiscussedmoredetailsinsection7.161
6. ExperimentsinMCDdrying162
ThissectiondiscussestheexperimentalproceduresusedinliteratureforMCD.163
The limitation and strength of the experimental facilities are analysed and164
recommendations for future experimental setup and challenges in scaling up MCD165
experimentalfacilityaresuggested.166
The main components of an MCD drying test facility are: 1 variable power167
microwavegenerator(magnetron),2.waveguide,3.microwavecavityand4.Aheating168
source forhotair supply (Tulasidas,1995).Microwavegeneratorcanbeavarietyof169
devices such asmagnetrons, klystrons, power grid tubes, travelingwave tubes, and170
gyrotrons,butthemostcommonlyusedsourceisthemagnetron,whichismoreefficient,171
reliable, and available at lower cost than other sources (National Research Council,172
1994).Microwavegeneratedinmagnetronisguidedthroughwaveguidestothecavity173
wherethematerialsareplaced.At thesametimehotairneedstobesuppliedtothe174
cavitytocombinewiththeconvectivedrying.TheMCDdryingsetupscanbeclassified175
intotwomaincategoriesbasedonthecavitytype:singlemodecavityandmultimode176
cavity(Chandrasekaranetal.,2011).177
6.1 Experimentalprocesswithsinglemodecavity178
Singlemodecavitiesareusedwhenprecisefocusatagivenlocationisneeded.179
Forthesecavities,thetransverseelectric(TEorH)wavesandtransversemagnetic(TM180
orE)wavesarecommonlyusedwhicharedesignedeitherrectangularorcircularcross181
section.Theelectric intensity inthedirectionofpropagation iszero fortheTEwave182
whereasfortheTMwave,themagneticintensityinthedirectionofpropagationiszero183
(Chandrasekaran, Ramanathan, & Basak, 2012). Single mode applicators are used184
extensivelywheretheloadconditionscanbekeptconstant(andthusrepeatedtuningis185
not required). These conditions are present in heating a fluid stream, for example,186
whereby the fluid flowsthroughtheapplicatorsuchthat itspropertiesarerelatively187
constant.188
10
AbasicsinglemodeapplicatorsetuphasbeenshowninFigure2.Thewaveguide189
circulator(as inFigure2) isa three‐portdevice ideallyhavingnopowerattenuation190
capability.Becausecirculatorsarenotdesignedtoabsorbpower,aseparate“dummy”191
loadisconnectedtothecirculatorandusedtoabsorbreversepowerthatmayotherwise192
damagethemagnetron(Gerling,2000).193
194
Figure2:Basicsinglemodeapplicatorsetup(Gerling,2000)195
Impedancematchingisusuallyachievedusingthreemanualstubtuners,which196
are used to alter the capacitance of the waveguide system (Gerling, 2000). As an197
example,twostrawberriesofslightlydifferentshapewouldrequiredifferentstubtuner198
settings (position and penetration depth) for optimum impedance matching.199
Furthermore, the same strawberry at different drying stages (and thus different200
moisture contents)may require a drastic change in stub tuner settings to keep the201
strawberrymatchedwith the load. Failure tomatch two impedances results inpoor202
microwavepowercouplingwiththeload.203
Therearemanyotherstudiesthatusedspeciallydesignedsinglemodeapplicator204
intheirstudy(Holtz,Ahrné,Karlsson,Rittenauer,&Rasmuson,2009;Holtzetal.,2010;205
Malafronteetal.,2012).Thedryingequipment(asshowninFigure3)comprisedofa206
straightaluminiumwaveguide (8643mm) fittedwithanair‐cooledmagnetron (2.45207
GHz,2M244‐M16,Panasonic,Cwmbran,U.K.).Thedryingcavitywassituatedinsidethe208
waveguide.Betweenthemagnetronandwaveguideathree‐stubtuner(stainlesssteel)209
11
wereincorporated,andthefarendofthewaveguidewasadjustable,enablingthelength210
ofthewaveguidetobeadjusted.211
212
Figure3.Microwaveconvectivedryingequipment.Schematicimageand213dimensionsofthedryer(applicator)(Malafronteetal.,2012)214
Jindaratetal.(2011)analysedenergyefficiencyofmicrowaveconvectivedrying215
usingaTE10modemicrowaveconvectivedryerforporouspackedbed.Figure4shows216
the experimental setup using a rectangular waveguide with dimension of 110mm217
⤬55mm. The isolator was used to trap microwave reflection from sample to save218
magnetron.The temperatureatvariouspointofsamplewasmeasuredby fiberoptic219
probes.220
12
221
222
Figure4.Schematicofexperimentalfacility:(a)equipmentsetup;(b)223microwavemeasuringsystem.(Jindarat,Rattanadecho,&Vongpradubchai,2011)224
Inconclusionofsingleofcavitydiscussion,itcanbesaidthattheexperimental225
setupwithasinglemodelcavityaremainlydesignedforstudyingthephysicsinvolved226
ratherthanbeingoptimizedforadryingprocess.Inthesesetups,couplingmicrowave227
powertoaloadrequirestherespectivecompleximpedancesbetweentheloadandthe228
microwavepowersourcetobematched.Thisisthegreatestdisadvantageofasingle229
mode applicator, since the complex impedance of the load changes with geometry,230
chemical composition and temperature (Gerling, 2000). Precision control or target231
heatingcanbeachievedwithsinglemodeapplicator.However,multimodeapplicatoris232
beingwidelyusedintheindustrialsystem.233
6.2 Experimentalprocesswithmultimodemodecavity234
Multimode microwave applicators are closed volume, totally surrounded by235
conductingwallsandhavealargecavitytopermitmorethanonemode(pattern)ofthe236
electric field (Funebo & Ohlsson, 1998). Unlike single mode applicators, multimode237
applicators are less sensitive to product position or geometry (Chandrasekaran,238
13
Ramanathan,&Basak,2013).Figure5showsamultimodecavitywithhotairinletand239
outlet fordryingofpumpkin (Wang,Wang,&Yu,2007).Manydrying researchused240
modifiedhouseholdmicrowaveovenasmultimodeapplicator.Wangetal.(2007)used241
alaboratorymicrowaveovenwithanoutletattheupperleftsideoftheoventoallow242
exhaustofmoistairasshowninFigure5.Onedish,containingthesample,wasplaced243
atthecenterofaturntablefittedinside(bottom)themicrowavecavity.Moistureloss244
wasrecordedat5minintervalsduringdrying,measuredbytakingoutandweighingthe245
dishonthedigitalbalance(JY1000,1000±0.01g).However,takingthesampleoutmay246
causesomeerrorinmoisturemeasurement.Themajorlimitationofthisstudywasthat247
therewasno temperaturemeasurement of the sample. Therefore, itwas difficult to248
monitortheoverheatingofthesamples.249
250
251
Figure5:Multimode applicatorwithmode stirrer and turntable(Wang et al.,2522007).253
Figure 6 shows a modified microwave (900 W, 2450MHz), having an inside254
chamberwithdimensionsof(345x335x220)whichwasusedtodryOregano(Soysalet255
al.,2009b;Soysaletal.,2009a).Tworectangularopening(10x8cm2)wasmadeonthe256
14
backandtopwalltosupplyandexithotairrespectively.Theheatedairwasforcedinto257
thedryingchamberbya radial fan (100W,180m3/h).Apotentiometerwasused to258
controlthespeedofthefan.Continuousmonitoringoftheweightwasdonewithadigital259
balance(SartoriusTE3102S,Germany,3100±0.01g)thatwasplacedundertherotating260
glasstray(diameter:314mm,mass:1150g).Aprogrammable logiccontroller(PLC)261
systemwasusedtocontroltheintermittencetime,thefanandtheglassturntable.The262
temperature of the air inside the air duct was measured using a PT100 platinum263
resistancetemperaturesensor.Theglassturntableonwhichthematerialwasplaced264
continuously turned (5 rpm) during the drying procedure to obtain a uniform265
distributionofthemicrowaveenergyinthematerial,andtoassurehomogenizeddrying.266
Thisdryingsystemisimpressivebecauseitisequippedwithtemperature,moisture,and267
airvelocitymeasurementsystem.However,onelimitationcouldbetheunevenairflow268
distributioninthedryingchamber.Astheairentersatthebottomandexitatthetop,269
this may result in uneven airflow or poor airflow distribution over the product.270
Moreover,thetemperaturedistributioncanbeuneveninmicrowavesoitisbetterto271
usethermalimagingcamerainmultimodecavityratherthanpointmeasurementusing272
thermocouple.273
274
Figure 6: Experimental microwave–convective drying system: 1. microwave275drying chamber; 2, rotating glass tray; 3, tray rotating rod; 4, digital balance; 5, PID276controlunitandsolid‐staterelay;6,airentrance;7,wiremesh;8,moistairexitopening;2779,temperaturesensor;10,airduct;11,electricheaters;12,fan;13,fanspeedadjuster;27814,digitalwattmeter;15,PLCcontrolunit(Soysaletal.,2009b;Soysaletal.,2009a).279
15
UpritandMishra (2003)usedamodifiedprogrammabledomesticmicrowave280
oven(IBM,ModelElectron),withamaximumoutputof625Wat2450MHzandcavity281
(dryingchamber)dimensionsof300x240x210mmasshowninFigure7forsoy‐fortified282
paneerdrying.Moisturelossinthesamplewasmeasuredbyon‐lineweightrecording283
at 3 min interval. The temperature and relative humidity of the exhaust air were284
measuredperiodicallyusingadigitalhygrometer(ALMEMO,Germany).Thematerialis285
assumedtoreachequilibriummoisturecontentwhenthematerialacquiredaconstant286
weight.Theypaidspecialattentiontoavoidcharringoftheproductwhichisdifficult287
withoutmeasuringthetemperatureoftheproduct.Becausemicrowaveheatingisfast288
and volumetric, their visual inspection may not be sufficient to predict the289
commencementofcharring.290
291
Figure7.Schematicofmicrowaveconvectivedryer(Uprit&Mishra,2003)292
293
Andrés et al. (2004) performed experiments in a specially designed hot air–294
microwaveovenequippedwithcontinuousmicrowaveenergy(Figure8).Theyvaried295
microwavepowerandairtemperaturewithconstantairvelocityat1m/s.296
16
297
Figure8.Schematicillustrationofthecombinedhotair–microwavedrying298equipment(Andrésetal.,2004).299
Ahrnéetal.(2007)Thedryingexperimentswereperformedinadryer(Figure300
9)speciallydesignedintheframeworkoftheEuropeanUnionprojectICA4‐CT‐2002‐301
10034byP.O.RismanincollaborationwithSIKandconstructedbyTIVOXmachineAB302
(TIVOXMaskinAB,Sweden).Itcanbeoperatedwithatemperatureof40‐800Candair303
velocity of 0.3‐0.8m/s. The product temperature was measured by fiber‐optic304
temperaturesensors (ReFLexTM,Neoptix,Canada)placed in four samples locatedat305
differentpositionsofthedryer.Thefiberoptictemperatureprobesgivetemperatureat306
a point which may sometimes deceive in microwave heating experiments since307
microwavegeneratesunevenheatingwithhotspotandcoldspotinthesample.Instead308
of point measurement, Kowalski, Musielak et al. (2010) used a thermal imager to309
measure2Dsurfacetemperature(Figure10)whichcanprovidemoredetailinformation310
ofsurfacetemperatureofthesample.311
312
Figure9.Schematicrepresentationofthemicrowaveconvectivedryer.(Ahrné313etal.,2007)314
17
Kowalski,Musielaketal.(2010)usedamicrowaveovenwithcavitydimensions315
300x400x450 which can supply maximum power of 600W. They found that316
temperaturewashigherinthemiddleofthesample.Intheirsetup,airwith220Cwas317
suppliedwithinstalledmechanicalventilationsystem.However,fromFigure10,itisnot318
clearwheretheventilationsystemwasplacedandhowefficientlyitworked.Airflowis319
importantbecausereducedairflowmayresultsmoistureaccumulationatthesurfaceof320
thesamplewhichreducesdryingrateanddegradessampletextureforfoodapplication.321
322
Figure10.Theexperimentalset‐up:(a)microwave‐convectivedryerand(b)323determinationofthetemperaturedistribution324
Incontrast,Fuetal.(2005)developeddryertooperateatanairvelocityof1‐325
3m/stoprovidesufficientairflowtothedryingchamber(Figure11).Theairwasdrawn326
withanaxialfanthroughcuttingventonthemicrowavewall.However,oneshouldmake327
surethat themicrowave leakageis lessthantheallowable limit(5mW/cm2)through328
thatvent.329
18
330
Figure 11.Apparatus for convective and microwave finish drying (Fu et al.,3312005)332
Temperatureandmoisturearethetwomainparametersneedtobemeasured333
accuratelyduringthedryingprocess.Theefficacyofanexperimentalsetupdependson334
howaccurately these twoparameterscanbemeasured.Additionally, theairvelocity335
distributionplaysanimportantroleindryingratebecauseitaffectstheheatandmass336
transfercoefficientwhichisessentialformodelling.337
The major problem with multimode applicators is non‐uniformity of the338
microwavefieldintheloadwhichcausesnon‐uniformheatdistribution.Thisproblem339
canbeminimisedbyprovidingamodestirrer(rotatingmetalbladedfan)orturntable340
(Schiffmann, 1995). The effect of mode stirrers in a multimode microwave heating341
applicatorareinvestigatedbyKurniawanetal.(2015)using3DCOMSOLMultiphysics342
Simulation.Theyhavefoundthatthetemperaturedistributionismoreuniformforthe343
casewithmodestirrerscomparedtothatobtainedwithoutmodestirrers.Implementing344
intermittency inmicrowave application is another technique to reduce non‐uniform345
heatingofsample.346
In summary, the experimental facilities for a MCD should be equipped with347
continuous weight monitoring, temperature monitoring such as thermal imaging348
camera,andhaveuniformairflowdistributioninthechamber.Inaddition,thesafetyof349
thesetupisanothercriticalissue.Becausethemicrowaveleakagecancausedamageto350
human cell and tissues and cause cataracts. Therefore, the design of vent or any351
19
perforationshouldbecarefullyhandledwhiledesigningandmanufacturingmicrowave352
assisteddryer.353
7. MathematicalModellingofMCD354
Mathematicalmodels can be used to predict the internal temperature that is355
difficult to measure and control the microwave power to avoid overheating. As356
discussedinsection3thatmicrowavegeneratevolumetricheatinsidethesampleand357
thisheatgenerationneedstobeaddedinenergyequationwhenmodellingMCDprocess.358
Thus the theoretical investigationofMCD consists of twoparts. The first part is the359
calculationofmicrowavepowerabsorptionthatconvertedintoheatandthesecondpart360
issolvingtheconjugateheatandmasstransferfordrying.Thissectiondiscussesboth361
microwavepowerabsorptionandheatandmasstransferduringMCDdrying.362
7.1 ModellingofMWpowerabsorption363
InMCDmodel,heatgenerationanalysisiscrucialforaccurateestimationofthe364
temperature of the sample. Volumetric heat generation depends on the dielectric365
propertiesofthematerial,andthefrequencyandintensityoftheappliedmicrowave.366
There is plethora of studies regarding calculation of power distribution or heat367
generationduetomicrowave.Inthepreviousstudieselectromagneticfieldinsidethe368
product was assumed to be (i)uniform; (ii) exponentially decayed; (iii)decayed369
followingempiricalrelations;(iv)decayedfollowingLambert’srelationand(v)decayed370
calculated by solving electric field equations (Maxwell’s equations). Among them,371
Lambert’sLawandMaxwell’sequationsaremostcommonlyusedinmicrowaveassisted372
drying.373
374
7.1.1 Lambert’sLaw:375
Microwaveenergydistribution in foodproductswithbasic geometries (slabs,376
spheres, and cylinders) is calculated by using Lambert’s Law. Especially in drying,377
applicationofthislawismorecommon(AbbasiSouraki&Mowla,2008;J.R.Arballo,378
Campanone,&Mascheroni,2012;Hemisetal.,2012;Khraisheh,Cooper,&Magee,1997;379
Mihoubi & Bellagi, 2009; Salagnac, Glouannec, & Lecharpentier, 2004; Zhou, Puri,380
Anantheswaran,&Yeh,1995).381
20
Lambert’s Law considers exponential attenuation of microwave absorption382
withintheproduct,viathefollowingrelationship.383
x)(-20expP=xP (1)
Here, the incident power at the surface (W), α is he attenuation constant384
(1/m), and x represents the distance from surface (m). The measurement of 0P is385
generallyobtainedviaexperimentation.386
Theattenuationconstant, isgivenbythefollowingequations387
2
1'
''1
'2
2
1
(2)
where isthewavelengthofmicrowaveinfreespace( cm24.12 at2450MHz388
andairtemperature200C)andε'andε"arethedielectricconstantanddielectricloss,389
respectively.390
Thevolumetricheatgeneration, micQ (W/m3)inEquation(3)isthencalculated391
by:392
V
PQ mic
mic , (3)
where,Visthevolumeofapplesample(m3).393
However,Lambert’sLawdoesnotaccuratelypredicttheheatingsituationand394
electricfielddistribution(Chandrasekaranetal.,2012;V.Rakesh,Datta,Amin,&Hall,395
2009).Recently,Kumaretal.(2015)developedmathematicalmodelforIMCDdrying396
usingLambert’slawandhighlightedthelimitationsofLambert’slaw.Theymentioned397
thatthemicrowaveabsorptionshouldreducewithdecreasingmoisturecontent,butthe398
Lambert’s Law fails to take this into account, giving highest power at the surface399
irrespectiveofmoisturecontent.400
Duringthe1970sand1980smostofthepowerabsorptioncalculationwasbased401
onLambert’slaw.However,duringtheearly1990s,theoreticalfoundationofcombined402
electromagneticandthermaltransportusingMaxwell’sequationshasbeenestablished.403
Chandrasekaran, Ramanathan et al. (2012) reviewed the comparison between404
21
Lambert’slawandMaxwell’sequation;theyreportedthatMaxwell’sequationgiveexact405
andmoreaccuratesolutionformicrowavepropagationinsamples.406
7.1.2 Maxwell’sEquations407
Maxwell’sequationsareasetofpartialdifferentialequationsofelectrodynamics408
whichrepresentafundamentalunificationofelectricandmagneticfieldsforpredicting409
electromagneticwavepropagation.Physicistandengineersusecomputersrangingfrom410
simpledesktopmachinestomassivelyparallelsupercomputingarraysforsolvingthese411
equationstoinvestigatewaveguiding,radiation,scattering,andheating(H.Zhao,1997).412
Maxwell’s equations that govern the propagation and microwave heating of413
materialaregivenbelow:414
t
BE
(4)
cJt
DH
(5)
0. B (6)
D. (7)
WhereEiselectricfielddistribution(V/m),Diselectricfluxdensitydistribution415
(C/m2),Hismagneticfielddistribution(A/m),Bismagneticfluxdensitydistribution416
(W/m2), currentdensity(A/m2),ρisvolumechargedensity(C/m3).417
InfrequencydomaintimeharmonicMaxwell’sequationscanbewrittenas418
0''''
1 2
Ei
cE
(8)
where Eistheelectricfieldstrength(V/m), f isthemicrowavefrequency(Hz),419
c isthespeedoflight(m/s), ' , '' , arethedielectricconstant,dielectriclossfactor,420
andelectromagneticpermeabilityofthematerial,respectively.421
422
Theheatgenerationduetomicrowave, mQ (W/m3),givenby(COMSOL,2012),423
mlrhm QQQ . (9)
22
Here, rhQ istheresistiveloss(W/m3)and mlQ isthemagneticloss(W/m3).For424
foodproductsthemagneticlossesarenegligible,i.e. 0mlQ (Chenetal.,2014).425
Theresistivelosscanbecalculatedas(Chenetal.,2014;Wentworth,2004)426
*5.0 EJQrh
(10)
where *EistheconjugateofE
andtheelectriccurrentdensity J
(A/m2)isgiven427
by,428
EfEJ
02 , (11)
where, istheelectricalconductivity(S/m), isthedielectriclossfactorand429
0 ispermittivityinfreespace.430
Substitutingtheaboveintoequation(12),themicrowaveheatgenerationdueto431
canbewrittenas,432
2
0 EfQm (12)
whichconformswiththeheatgenerationsequationderivedbyMetaxas(1996a).433
7.2 Dielectricproperties:434
Dielectricpropertiessuchasdielectricconstantanddielectriclossfactordefine435
how materials interact with electromagnetic energy. These properties of materials436
definehowmuchmicrowaveenergywillbeconvertedtoheat(Chandrasekaranetal.,437
2013). As can be seen from the heat generation formulation above, the dielectric438
properties of the material are one of themost important parameters in calculating439
microwaveheatgenerationduringdrying(Sosa‐Morales,Valerio‐Junco,López‐Malo,&440
García,2010).Therefore,theevaluationofdielectricpropertiesiscriticalinmodelling441
andproductandprocessdevelopment(Ikediala,Tang,Drake,&Neven,2000).442
7.3 HeatandmasstransfermodellinginMCD443
Heatandmasstransfermodel isnecessaryforevaluatingtheeffectofprocess444
parametersonenergyefficiency anddrying time, andoptimizing thedryingprocess445
23
(Kumar,Karim,Joardder,&Miller,2012a;Kumar,Karim,&Joardder,2014).Developing446
aphysicsbaseddryingmodelforagriculturalproductsisachallengingtaskduetothe447
complexstructuralnatureofagriculturalproductsandchangesinthethermo‐physical448
properties during drying. Moreover, the heat generation due to microwave in MCD449
makesthemodellingmorechallenging.Therefore,someassumptionsareindispensable450
if mathematical models are to be developed, but these should be carefullymade to451
represent thephysical phenomenaduring theprocess. In this section, themodelling452
approachesofheatandmass transfer inMCDarediscussed, and their limitationare453
highlighted.TheMCDdryingmodelsavailable in literaturecanbeclassified into two454
categories:empiricalmodels,fundamentalmodels.Thefundamentalsmodelsagainthen455
categorizedintothreedivisionsbasedontheirwatertransportmechanisms:1)single456
phase,2)doublephaseand3)multiphasemodelsasshowninFigure12.Inthefollowing457
sections,themodelbasedontheseclassificationarediscussedandanalysed.458
459
Figure12.Generalclassificationofmicrowavedryingmodels460
461
7.3.1 Empiricalmodel462
TheempiricalmodelsaregenerallyderivedfromNewton’slawofcoolingandFick’s463
lawof diffusions (Erbay& Icier, 2010). These are simpler to apply and oftenused to464
describethedryingcurve.Thedryingconstantandcoefficientsintheempiricalmodels465
are developed with regression analysis. Page model and exponential model are466
frequently used in microwave convective drying. Table 1 and Table 2 present the467
24
empiricalmodelofCMCDandIMCDavailableintheliterature.Therearemanydifferent468
empirical model often referred as thin layer drying models such as Newton, Page,469
Henderson Pabis, Logarithmic, two term, two‐term exponential, and Wang and Sing470
models.Theacceptabilityofthesemodelsareusuallydeterminedbythecoefficientof471
determinationR2, and the reducedvalueofmean squareofdeviationχ2.Theydonot472
providephysical insightintotheprocess.Also,thesemodelsareonlyvalidforspecific473
process condition and material. In contrast to empirical models, the physics‐based474
models can capture the inherent drying mechanism better (Chou, Chua, Mujumdar,475
Hawlader,&Ho,2000;Chua,Mujumdar,&Chou,2003;Ho,Chou,Chua,Mujumdar,&476
Hawlader,2002).The theoreticalmodelsoften canbeused forwide rangeofprocess477
parameterandcanbeusedforparametricanalysisandoptimization.478
7.3.2 Singlephasemodel479
Thesinglephasemodelsconsideronlyonephaseinfooddomaini.e.liquidwater480
diffusionthroughthesolidmedia.Theseareoftencalleddiffusion‐basedmodelsandare481
verypopularbecauseoftheirsimplicityandgoodpredictivecapability(JavierR.Arballo,482
Campañone,&Mascheroni,2010;Kumaretal.,2012b;Perussello,Kumar,deCastilhos,483
&Karim, 2014).Maybe due to this reason,most of themodel related tomicrowave484
assisted drying are single phase. Thesemodels assume conductive heat transfer for485
energyanddiffusivetransportformoisture.486
Thesinglephasemodelconsidersonlydiffusivewatertransportasshowninthe487
equation(13).488
489
0u)(-Deff
www cct
c (13)
490
where,cwisthemoistureconcentration(mol/m3), effD istheeffectivediffusion491
coefficient(m2/s)anduistheconvectiveflow.Theeffectivediffusioncoefficient, effD ,in492
thosemodelshastobedeterminedexperimentally.Thusthismodelcanbesaidassemi‐493
empiricalmodel.494
TheenergybalancewascharacterizedbyaFourierfluxwithaheatgenerationterm495
duetomicrowaveheating, micQ (W/m3).496
25
micp QTkt
Tc
).( (14)
where,Tisthetemperature(0K), isthedensityofsample(kg/m3), pc isthespecificheat497
(J/kgK), and k is the thermal conductivity (W/mK). Theheat generation, micQ (W/m3),498
wascalculatedusingLambert’sLaw.499
Incaseofmicrowaveheatingordrying,theeffectivediffusivityincreasesdueto500
volumetric heating which cause pressure driven flow. This pressure driven flow is501
lumpedtogetherwithdiffusionincaseofsinglephasemodels.Thereforeitcanbesaid502
thatalthoughthismodelcanprovideagoodmatchwithexperimentalresults,itcannot503
provideanunderstandingofothertransportmechanismssuchaspressuredrivenflow504
and evaporation. This is because all water are lumped in one parameter, diffusion505
coefficient,cannotprovideindividualcontributionofthewaterflux.Thedrawbacksof506
thesekindsofmodelsarediscussedindetail intheworkofZhangandDatta(2004).507
Lumping all thewater transport asdiffusion cannotbe justifiedunder all situations.508
Therefore,multiphasemodelsconsideringtransportofliquidwater,vapour,airinside509
thefoodmaterialsareconsideredtobemorerealistic.510
7.3.3 Doublephasemodel511
Double phase models consider two species, namely water vapour and liquid512
waterformasstransferduringthemodellingprocess.Thesemodelsarecomparatively513
more realistic and comprehensive than single‐phase models because of the514
incorporationofevaporationorphasechangebetweenliquidwaterandwatervapour.515
Thegeneralgoverningequation fordoublephasemodel ispresented inEq (15)and516
(16).517
Liquidphaseofwater: Rcuc-Dt
cllll
l
(15)
Vapourphaseofwater:
Rcuc-Dt
cvvvv
v
(16)
Herethe lc and vc aretheconcentrationofliquidwaterandwatervapour, lD and518
vD arethediffusioncoefficientforliquidwaterandwatervapourand lu and vu arethe519
convectiveflowofliquidwaterandvapour.TheRistheproductionorconsumptionof520
26
species (i.e. evaporation or condensation). Malafronte et al. (2012) estimated the521
evaporationrateas,522
0
)( vsatc cTckR when,
vsat
vsat
cc
cc
(17)
WhereRT
TMPTc v
sat
)()( and vapour pressure was calculated from Antoine’s523
equation.524
There are some two phase models in MCD drying process. For example,525
Malafronte et al. (2012) developed a two phase models for combined microwave526
convective drying. The strong point of the model is that they took moisture and527
temperaturedependentdielectricproperties.Moreover,theysolvedbothheatandmass528
transportequationsandMaxwell’sequationsintransientregime.Marraetal.(2010)529
alsodevelopedatwophasemodelforcombinedmicrowaveconvectivedrying.Themain530
strongpointoftheirmodelisthattheyconsideredconjugateapproachofheatandmass531
transfer,whichenabledtocalculaterealisticheatandmasstransfercoefficientwithout532
requiringempiricalaveragebasedcalculations. Jindaratetal. (2011)alsoconsidered533
twophasetransport.However,thedeformationorshrinkagehavenotbeenconsidered534
inallthesemodels.Moreover,theavailablemodelconsideringtwophaseneglectedthe535
Darcyorpressuredrivenflowi.e.bulkflow.Multiphasemodelsdiscussedbelowhave536
consideredbulkflow.537
7.3.4 Multiphaseporousmediamodel:538
The multiphase models consider all phases inside the food materials namely water, air, 539
water vapour and solid matrix. Multiphase models can be categorised into two groups: 540
equilibrium and non-equilibrium approach of vapour pressure. In equilibrium formulations, the 541
vapour pressure, vp , is assumed to be equal with equilibrium vapour pressure, eqvp , , and vice 542
versa. There are some multiphase models considering equilibrium approach applied in vacuum 543
drying of wood (Ian W. Turner & Perré, 2004) and convection drying of wood and clay 544
(Stanish, Schajer, & Kayihan, 1986) (Chemkhi, Jomaa, & Zagrouba, 2009), microwave 545
spouted bed drying of apple (H. Feng, Tang, Cavalieri, & Plumb, 2001) and large bagasse 546
stockpiles (Farrell et al., 2012). Some multiphase porous media models combine the liquid 547
water and vapour equations together to eliminate the evaporation rate term (Farrell et al., 2012; 548
27
Ratanadecho, Aoki, & Akahori, 2001; Suwannapum & Rattanadecho, 2011). By doing this, the 549
concentrations of liquid water and vapour cannot be determined separately (Kumar, Joardder, 550
et al., 2016). 551
However, equilibrium condition may be valid at surface with lower moisture content 552
because equilibrium condition may not be achieved due to lower moisture content at the surface 553
during drying. Therefore, the non-equilibrium approach is a more realistic representation of 554
the physical situation during drying (J. Zhang & Datta, 2004). Moreover, the non-equilibrium 555
formulation for evaporation can be used to express explicit formulation of evaporation, thus 556
allowing calculation of each phase separately. Furthermore, non-equilibrium multiphase 557
models are computationally effective and applied to wide range of food processing such as 558
frying (Bansal, Takhar, & Maneerote, 2014; Ni & Datta, 1999), microwave heating (Chen et 559
al., 2014; V. Rakesh, Seo, Datta, McCarthy, & McCarthy, 2010), puffing (Vineet Rakesh & 560
Datta, 2013) baking (J. Zhang, Datta, & Mukherjee, 2005), meat cooking (Dhall & Datta, 2011) 561
etc. However, application of these non-equilibrium models in drying of food materials is very 562
limited. 563
TherearesomemultiphasemodelsinMCDprocess.Mostrecently,Kumaretal.564
(2016) developed multiphase model for IMCD of apple. They considered non‐565
equilibrium approach and applied Lambert’s law to calculate microwave heat566
generation.Dinčovetal. (2004)developedaFinitevolumemultiphaseporousmedia567
model where the coupling was achieved by considering dielectric properties as a568
functionofmoisture and temperature. Zhuet al. (2015)developeda comprehensive569
multiphase model of microwave drying of sphere. The governing equation used to570
developthemultiphasemodelaresummarisedinTable3(Kumar,Joardder,etal.,2016).571
Table3.Summaryofgoverningequationsusedinmultiphasemodelling572
Physics and Phases Governing Equations Eq. No.
Hea
t and
Mas
s T
rans
port
Mas
s T
rans
port
Wat
e Liquid water evapc
w
wrww
w
wrwwww Rp
kkP
kkS
t
,,
(18)
Gas
Vapor evapvgeffgg
v
grgvgvgg RDSP
kkS
t
,
,(19)
Air va 1 (20)
Gas pressure evap
g
grggg RP
kkS
t
,
g (21)
28
Ene
rgy
Energy mevapfgeffthwwggeffpeff QRhTkhnhn
t
Tc
).( ,
(22)
Nom
encl
atur
e
Sym
bols
=porosity, S=saturation, =density, k= permeability, P=gas pressure, = viscosity, P=gas
pressure, c= concentration, D=Diffusion coefficient, =mass fraction, T=Temperature, n
= flux, kth= thermal conductivity, h= latent heat, E=Electric field, f= microwave frequency, c= speed of light, Qm= microwave heat generation, Revap=evaporation of liquid water, p= pressure,
m = magnetic permeability, ' = dielectric constant , '' =dielectric loss factor, 0=permittivity in free space,
Subscript
w=water=vapor, g=gas, eff=effective, m=microwave, r=relative=capillary, evap=evaporation, m=microwave
573
8. Challengesandfuturedirectionforresearch574
Themajorchallengeinmicrowavesystemisthenon‐uniformityofelectricfield.575
Future research should involve addressing this problem focusing on obtainingmore576
uniform electric filed distribution thus to obtain uniform temperature andmoisture577
distribution.Onepotentialsolutioncouldbeusingmultiplemagnetronatappropriate578
locationofthechamber.Mathematicalmodellingandsimulationwouldbeessentialto579
do such investigations. The plant based food materials are hygroscopic in nature,580
containingfreewaterandboundwater.Thetransportmechanismsoffreewaterand581
boundwateraredifferent.Therefore,thefuturemodellingstudyofMCDshouldinclude582
aseparatemechanismforthetransportoffreeandboundwater.Moreover,themodels583
shouldbeabletopredicttheeffectofprocessparametersondryingkinetics,thenthe584
models can be used to optimize the process and suggest an appropriate strategy of585
applyingMCDtoavoidoverheatingandgainingmaximumfoodqualitywithminimum586
energyusage.587
9. Conclusion588
The present study provides an overview of MCD process focusing on drying589
kinetics,experimentalprocessandmathematicalmodelling.Moreover,thecomparative590
study of drying time betweenmicrowave assisted drying and convective drying for591
differentmaterialsarecalculatedanddiscussed.Itisclearthatdryingtimesavingcan592
beupto90%byapplyingmicrowave.ConsideringthishugedryingtimesavingofMCD,593
there is enourmous potential of this process in industrial applications. The critical594
29
review of experimental facilities showed that the experimental facilities should be595
equippedwith real timemoistureand temperaturemeasurementof the sample, and596
shouldhaveuniformairflowdistribution.Theriskandsafetyissuesofcreatingventfor597
airflowinmicrowavecavitywasinvestigated.Althoughthesetupwithasinglemode598
cavity can provide an understanding of physics, their performance is sensitive to599
product position and geometry. In contrast,multimode cavities are less sensitive to600
productpositionorgeometrybutprovidesunevenheating.However,theseunevenheat601
canbeminimizedbyapplyingmicrowaveintermittently,providingturntableormodel602
stirrersandbysuitablecavitydesign.Continuousmicrowave‐convectivedrying may603
overheat and damage quality which can be overcome by intermittent microwave‐604
convective drying by applying appropriate power level and pulse ratio. The605
mathematical modeling can help to optimize intermittency and power level of606
microwaveforbetterproductqualityandenergyefficiency.Onemajordisadvantageof607
microwave application is the non‐uniform heating due to standing wave pattern. A608
comprehensivemultiphasemathematicalmodelcanalsohelp inselectingmagnetron609
size, position and cavity shape in order to obtain more uniform heating pattern.610
Therefore,thefutureresearchshouldfocusonscalingupandoptimizingthistechnology611
forindustrialapplications.612
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