《DEHYDRATION》PPT课件教案

AddtheauthorandtheaccompanyingtitleDEHYDRATIONVocabularyDrying,dehydrate,rehydrate,equilibriumrelativehumidity,wateractivity,isothermssorptiondesorptionhysteresisbehaviorhypothesiscapillarysemiempiricalempirical,criticalmoisturecontentDEHYDRATIONDryingoffoodsisanimportantfoodprocessingoperationusedtopreservefoods.Thedistinguishingfeaturesbetweendryingandconcentrationarethefinallevelofwaterandnatureoftheproduct.Concentrationleavesaliquidfood,whereasdryingtypicallyproducesproductwithwatercontentsufficientlylowtogivesolidfood.ReasonsfordryingfoodsHistorically,therewasaneedtopreservefoodsforlongertimessothatfoodwasavailableduringtimesoflimitedfoodproductionoravailability.HuntersneededatechniquetopreservemeatformorethanafewdaystoensureacontinuousfoodsupplyInthesamemanner,wehavetechniquesthatallowustopreservefoodsastheyareharvested,sothatwecanenjoythematlatertimes.ReasonsfordryingfoodsOneoftheeasiestwaystopreservefoodsistoremovewater,sincemicroorganismsneedwatertosurviveandgrow,andmanychemicalreactionsrequirewatertoproceed.Earlyhuntersdriedtheirmeattohelpmaintainamorecontinuousfoodsupply.Nowadays,wedryfoodsforthesamereason:toprovideacontinuoussupplyoffoodsthatwecanenjoyatanytime.OtherreasonsfordryingfoodsRemovalofwaterleavesaproductreducedinweightandofteninbulk.Thisreducesshippingcostsandmakesthefoodsupplymoreeconomical.Driedfoodsalsoprovideconvenience.Driedconveniencefoodsmaybeusedforspecialexpedition--type(military)foodswhereweightisamajorconcern.Therearemanymethodsandtechnologiesbywhichwecandehydratefoods.Wemustfirstunderstandthenatureofwaterinfoodproductstoappreciatethedifficultiesinproducinghigh-qualitydriedproducts.Removalofwaterfromfoodsisnotadifficulttask.However,removingthewaterinsuchawaythattheproductregainsitsinitialformwhenrehydratedisnotsoeasy.STATEOFWAERINFOODSIndehydration,itisimportanttounderstandthebehaviorofwatersothatitcanberemovedmosteffectivelyandstillleaveahigh-qualityproduct.Foodtechnologistsoftenusethethermodynamicmeasureofwateractivitytodescribehowwaterinteractsinfoodproducts.WateractivityWateractivity(aw)isdefinedastheratioofthevaporpressureofwatermeasuredatthefoodsurface(Pw)tothesaturationvaporpressureofpurewateratthesametemperature(Pwº)WateractivityForacupofwater,thevaporpressureoverthesurfaceismeasuredasthesaturationvaporpressure,andawis1.Whentherearesolutesinthewatersuchassugars,salts,etc.thevaporpressureoverthewatersurfaceislowerthanthesaturationvaporpressure,andawisreducedtosomevaluelessthan1.Thereductioninwateractivitydependsonthetypeofsolutespresentandtheirlevels.WaterActivityForfoodproducts,thewateractivityisgenerallylessthan1.awisrelatedtothemoisturecontentofthefood,thetypesandconcentrationsofdifferentsolutes,andthestructureorphysicalcharacteristicsofthefood.RelationshipbetweenRH&awThewateractivityofafoodcanberelatedtoanequilibriumrelativehumidityintheairaroundtheproduct.Thatis,atonlyonerelativehumiditywilltheairbeinmoistureequilibriumwiththefoodproductwherethefoodneithergivesuporadsorbswater.Thisrelativehumidityisthe"equilibriumrelativehumidity"orERH.Free"water&"bound"waterInthepastpeoplesimplifiedthestateofwaterinfoodsbydenotingtwotypes:"free"wateror"bound"water."free"wateror"bound"water.Theworkingdefinitionforthesetermsis:Freewateristhatwhichgiveswateractivityof1,boundwatergiveswateractivitylessthan1.Freewaterisrelativelyeasytoremovefromafoodproductwhileboundwatertakesmoreenergytoreleasefromthefood.Thus,thelatentheatrequiredtoremoveamoleculeofwaterfromafoodincreasesasthewateractivitydecreases.Thisisimportanttothosewhodesigndryingoperations,sincetheenergyrequirementtoprovidesufficientdrivingforcefordryingisrelatedtothelatentenergyofvaporization.physicalchangesAsafoodproductdriesoutandthewatermoleculesbecomelessmobile,physicalchangesalsooccurinthefood.Aswaterisremoved,theremainingproductgenerallybecomesincreasinglyviscous.Theproductmaygothroughseveralregionsofproperties,whereviscositiesareintermediatebetweenapumpableliquidandastationarysolid.ThestatediagramForasimplesystemofsoluteandsolventTheglasstransitioncurverepresentsametastabletransitionwhereviscosityissohighthattheproductdoesnot"flow".Belowthiscurve,thefoodisstabletodiffusion-limitedprocessesforextremelylongtimesForexample,powderedmilkproductsremaindryandstablewhenmaintainedbelowtheglasstransitiontemperature.However,ifthepowderpicksupmoisturefromtheairorexperienceselevatedstoragetemperature,itmayexceedtheglasstransitioncurveandbelessstable.Inthiscase,powderedmilkwouldbelikelytogetsticky,andthepowderwouldcaketogether.EFFECTSOFDRYINGONPRODUCTQUALITYAfterrehydratingthefoodcannotreachtheoriginalquality.Thereisalwayssomechangethatgivesalossofqualityintheproduct.Thegoalistominimizethesechanges,whileoptimizingprocessefficiencyandminimizingcosts.Severaltypesofchangescanoccurduringdrying.Twomainproblemsarelossandchangeofflavors,andchangeinphysicalqualitiesofdriedproducts.EffectonflavorOneproblemwithdriedfoodsisthattheflavoroftherehydratedproductisnotthesameasthatoftheoriginal.Duringdrying,flavorcompoundsthataretypicallymorevolatilethanwaterareremovedinthedryingprocess.Thephysicalforcesthatcausewatermoleculestoberemovedfromthefoodduringdryingalsocausevolatilecompounds(alcohols,aldehydes,ketones,etc.)toberemoved.burntflavorDriedproductshavelessofthesevolatileflavoringcompoundsthantheoriginalstartingmaterial.Inaddition,theratesofchemicalreactionsareenhancedattheelevatedtemp.,andmanyofthesereactionsgenerateundesiredflavorcompounds.Forexample,thebrowningreaction(betweenreducingsugarsandproteins)isenhancedandgeneratesaburntflavor.(reconstitutedmilkfromadriedpowder)BrowningOtherchemicalreactionsmayalsotakeplaceduringdrying.Browningoccursinmanyfoodswhichresultsincolorchanges.Proteindenaturationcanoccurduringdrying,whichcausesincreasdviscosity,Thermaldegradationofvitaminsandproteinsmayalsoinfluencethenutritionalstatusofdriedproducts.Theextentofthesechangesdependsonthenatureofthedryingprocess.Sometypesofdryersproduceproductshavingsuperiorpropertiesonreconstitution.Theinstantcoffeespray-driedandfreeze-driedisdifferent.Sincefreeze-dryingdoesnotinvolveavapor-liquidinterface,thevolatileflavorandaromacompoundsarenotlostduringdrying,andfreeze-driedproductshavehigherqualityMOISTURESORPTIONANDDESORPTIONDuringdrying,bothmoisturecontentandwateractivitychange.Atanygivenrelativehumidityofairusedfordrying,thereisanequilibriumwatercontentwiththeproduct,Atthispointtheactivityofwaterintheairisthesameasthatintheproduct.Thisrelationshipspecifiesthewatercontentinafoodproductthatcanbereachedforanyconditionofdryingair.IsothermsByholdingafoodproductinairatdifferentrelativehumiditiesandmeasuringtheequilibriumwatercontent,thecurveofwatercontentandwateractivitycanbeobtained.It’snaturedependsonwhetherthefoodproductisbeingdriedorallowedtopickupmoisturefromtheair.Thedirectionoftheexperimentalmeasurementaffectstherelationshipbetweenwatercontentandwateractivity.IsothermsMOISTURESORPTIONANDDESORPTIONMoisturesorption(pickingupwater)curvestypicallyareslightlylowerinwatercontentsthanmoisturedesorption(drying)curves.Severalmechanismshavebeenproposedforthishysteresisbehavior.capillaryforcesvolumeexpansionRATEOFDEHYDRATIONIndrying,watermoleculesmustmaketheirwaythroughthefoodtothesurface(internalresistancetodrying)incontactwithdryingair.Onceatthesurface,watermoleculesaretransferedintotheair(externalresistancetodrying)basedonthedifferenceinvaporpressurebetweentheairandthesurface.Whenthevaporpressureintheairreachesthesamevalueasthevaporpressureofwateratthesurfaceofthefood,dryingceases.Therateofdryingmaybelimitedbyeithertherateofinternalmigrationofwatermoleculestothesurfaceortherateofevaporationofwatermoleculesfromthesurfaceintotheair,dependingontheconditionsofdrying.Infactmostfoodsswitchfromanexternaldryingprocessduringinitialstagestoaninternaldryingprocessastheproductdriesout.RATEOFDEHYDRATIONDryingCurvesConstantRatePeriodFallingRatePeriodDryingCurvesAcurveoflossofmoistureduringdryingofafoodproductaretypicallygeneratedbyweighingasampleoffoodundergoingdryingandrelatingweightlosstomoisturecontent.Moisturecontentismostoftenexpressedaskgofwaterperkgofdryproduct(ormatter).Thekgofdrymatter(initialproductweightminusweightfromwater)arealwaysconstantduringdrying,soaconstantreferencepointisusedwhenreferringtodryinginkgwater/kgdrymatter.Theshapeofthedryingcurveissimilarformanyfoodproducts.Aftershortinitialequilibrationperiod(forthermalequilibration),themoisturecontentdecreasesrapidly,andalmostlinearly,withtime.Thisinitialdryingperiodisfollowedbyamuchslowerrateofdryingasthemoisturecontentoftheproductdecreases.Therateofdryingistheslopeofthemoisturecontentchangewithtime,expressedinkgwater/kgdrymatter-minute.constantrateperioddryingrateisplottedagainstthemoisturecontent(insteadoftime).Sincemoisturecontentgoesfromhightolowduringdrying,theinitialdryingconditionisgivenbythepointattherightofthegraph.Initially,therateofdryingmaybenearlyconstantuntilsomecriticalmoisturecontentXc,isreached.Xcrepresentsthemoisturecontentwheredryingchangesfromconstantratetofallingrate.Thisinitialperiodofconstantratedryingiscalledthe"constantrateperiod,"orCRP.fallingrateperiodAftertheproductisdriedbelowXc,therateofdryingdecreases.Thisiscalledthe"fallingrateperiod,"orFRP.Here,dryingratedependsonthemoisturecontentremainingintheproduct.Iftheproductisdriedextensively,theproducteventuallyequilibrateswiththedryingair.Theequilibrationpointdependsontemperatureandrelativehumidityoftheairusedinthedryer.ConstantRatePeriodTheinitialrateofdrying--Therateatwhichwatermoleculesarriveatthesurfacebymigrationfromtheinteriorisgreaterthan(orequalto)therateatwhichwatermoleculesarelostfromthesurfacetothedryingair.Sothereissufficientwatertobeevaporated,thethermalenergytothefoodisusedaslatentheat,thetemp.offoodisnotelevated.EnergyequationForthesimplestcase,whereonlyconvectiveheattransferoccurs,alloftheheatenergygoesintovaporizingmoistureduringtheconstantrateperiod.Thatis,therateofheattransferintotheproductisbalancedbytherateofenergyremovalduetothevaporizingmoisture.Therateofenergyremovalwithvaporizedwatercanbefoundastheproductoftherateofdryingandthelatentheatofvaporization.Thatis,foreachmoleculeofwatervaporizedatthesurface(liquidtovapor),anamountofenergyequivalenttothelatentheatofvaporizationisrequired.Theconstantratedryingperiodlastsaslongastherateofmoisturemigrationfromtheinterioroftheproducttothesurfaceissufficientlyrapidthatthemoisturecontentatthesurfaceisconstant.Atthepointwheremoisturemigrationfromtheinteriorisslowerthanthesurfacevaporization,theconstantrateperiodendsandthetimeforconstantratedrying,tCRPcanbefoundas:FallingRatePeriodAfterthecriticalmoisturepoint,therateatwhichmoisturemigratestothesurfacelimitsdrying.Thatis,therateofmoisturelossfromthesurfacetothedryingairisfasterthantherateatwhichthatmoistureisreplenishedatthesurface.Mechanismsofinternalmasstransfer1.Liquiddiffusion.2.Vapordiffusion.3.Capillaryflow.4.Pressureflow.5.Thermalflow.FACTORSTHATINFLUENCEDRYING1.ProcessConditionsTemperature/AirVelocity/RelativeHumidity/Pressure2.FoodPropertiesSurfaceArea/Constituent/Orientation/CellularStructure/TypeandConcentrationofSolutes.DRYINGMETHODS1.DirectContactDryersSunDryer/BinDryer/KilnDryer/TrayorCabinetDryer/TunnelDryer/BeltorConveyorDryer/FluidizedBedDryer/RotaryAirDryer/SprayDryer.2.IndirectContactDryersDrumDryer3.InfraredorDielectricDryersInfraredDryers/MicrowaveDryers.TrayorCabinetDryerThefoodproductplacedinapanisplacedinsideadryingchamberwithhotairblowingacrosstheproductuntildryingiscomplete.SomeofthehotairusedfordryingmayberecirculatedthroughthedryertoconserveenergyHowever,increasedrelativehumidityoftherecirculatedairdecreasesdryerefficiency.TunnelDryerThefoodproductisloadedontotraysthatareplacedintocarts.Thecartsareinputatoneendofthetunneldryerandmovethroughtotheoutlet.Airblowingwithinthetunnelcausesdryingataspecifiedrate,sothatthefoodproductreachestheexitoncompletionofdrying.(1)cocurrent;(2)countercurrent;or(3)mixedflow,BeltorConveyorDryerProductmayalsobemovedthroughadryerbyplacingitonabeltorconveyor.Inordertoextendthetimewithinaconveyordryer,aseriesofconveyorsmaybearrangedoneabovetheother.Inthiscase,productdropsfromanupperconveyortoalowerconveyor.airflowcanbethroughtheconveyorandthroughthebedoffoodproductlayingontheconveyor.FluidizedBedDryerAirflowthroughabedofproductissufficienttolifttheproduct.Sincethereisintimatecontactbetweenairandproductdryingratesinthistypeofdryerarequitegood.Thistypeofdryerislimitedtogranulatedpowders,orsmallpiecesofproduct.Theairvelocitydependsonparticlesize,density.RotaryAirDryerThesedryersaretypicallyarrangedashorizontalcylindersthatrotatealongtheirmainaxis.Wetproductentersoneendofthedryerandmovestowardstheotherendbyacombinationofgravityandthebafflearrangementwithinthecylinder.Asthecylinderrotates,airforcedintothecylinderblowsacrosstheproductasittumbles,toprovideeffectivecontactbetweenairandproduct.VocabularySpraydrying,highpressurenozzle,dropletatomization,centrifugalatomizer,spinningdisk,dryerchamber,ambientair,accelerate,cycloneseparator,tangentially,segregate,conical,stickinessandagglomeration,reconstitution,dispersabilitySPRAYDRYINGAtomizationAirHandlingDryerChamberPowderSeparationAtomizationAtomizationproducesacloudofdropletswithverylargesurfaceareafordrying.Highpressurenozzle:dropletsizeiscontrolledbypressureofthefluidfoodagainstthenozzle.Centrifugalatomizer:Liquidfoodispumpedintoaspinningdisk,whereitisacceleratedbycentrifugalforceandexpelledfromtheendsofthedisk-shapedatomizer,becomeacloudofdroplets.AirHandlingAmbientairistakeninthroughaventandheatedpriortocirculationintothedryingchamber.Heatingcanbeaccomplishedinseveralways.Aircanbepassedeitherthroughsteamcoilsoranelectricheatertoattainelevatedtemperatures,typicallybetweenl50and500℃.DryerChamberIntheresidencetimeofdropletsinthespraydryingchamber,thedropletsgofromamoisturecontentintherangeofabout40%toonlyabout5-10%.Thefooddropletsaresprayedatthetopofthechamberandfalldowntothebottombygravity.Bothairandfooddropletsenterthechamberatthetopandfalltothebottomofit,whereairisseparatedfromdrypowderandtheproductisremovedfromthedryer.PowderSeparationPrimaryseparationofpowderisaccomplishedbygravitationalsettingoftheheavierpowderparticles.Separationofairandfinerpowderparticlesisusuallyaccomplishedinacyclonedevice.Thestreamiscirculatedtangentiallyintothecycloneseparator.Centrifugalforcecausestheparticlestosegregatefromtheairandsettletothebottomoftheconicalseparator.Atextileorbagfilterissometimesused.DryingintheSprayDryerConstantRateDrying.FallingRateDrying.StickinessandAgglomerationProductQualityStickinessandAgglomerationDuringsprayatomization,theparticlesaresprayedoutwardstowardsthewallofthedryingchamber.Ifthesedropletshavenotsufficientlydriedwhentheycomeincontactwiththewall,theystickandformascaleontheinsideofthedryingchamberorsticktooneanothertoformagglomeratedparticles.Thechambermustbedesignedtoensurethatthedropletshavedriedsufficientlysothattheyarenolongerstickyastheyapproachthechamberwall.ProductQualityTheabilitytobewettedbywaterduringreconstitution,dispersabilityofthepowderintowaterandsolubilityinwater.Changesinproductattributes,particularlyatthecase-hardenedsurfaceofthedroplets,decreasestheabilityofapowdertobewettedanddispersedintowater.Agglomerationofparticlesmayalsoinfluencetheamountofsurfaceavailableforwetting.AgglomerationProcess,orInstantizatingInsomecases,wettabilityanddispersabilityofapowderareenhancedbyanagglomerationprocess,orinstantizing,immediatelyfollowingthespraydryer.Inaninstantizer,thesurfaceofpowderparticlesisslightlywettedbyafinesprayofsteam.Theseparticlesarefluidizedinairtocausecontactsbetweenindividualparticles,formingagglomeratedpowders,withenhancedwettabilityanddispersability.VocabularyFreezedrying,sublimation,sublimationfront,primaryandsecondarydryingstages,radiationcollapse,icecrystals,pore,porouschannels,diffuse,stresscracking,rehydrateFREEZEDRYINGMoistureisremovedfromthesolidstate(ice)directlytothevaporstatebysublimation.Dryingactuallyoccursintwosteps,primaryandsecondarydryingstages.Itisintheprimarystagethatwaterisremovedbysublimation,whereasvaporizationofunfrozenliquidwatermoleculesoccursinthesecondarystageofdrying.ProductqualityofFreezeDryingTheoriginalstructureofthefoodismaintainedandflavorretentioninisexcellent.Thecostoffreezedryingisveryhigh.StepsinFreezeDryingFreezing.PrimaryDrying.SecondaryDrying.HeatandMassTransferinFreezeDryingPrimaryDryingSublimationoficeisaccomplishedbycontrollingthevacuumlevelinthefreezedryerandthroughcarefulheatinput.Ahighvacuumisdesiredtoenhancesublimationrate.Introductionofheatistosupplyenergytoaplateonwhichthefoodissitting(conductionheating),whilealsoprovidingaradiationsourceabovetheproduct.SublimationFrontTheicerecedesintothefoodproductasdryingoccurs.Thisboundarybetweenfrozenanddriedproductiscalledthesublimationfront.Heatmustbetransferedintotheproducttothisfronttopromotesublimation,andwatervapormustthenberemovedbymasstransferthroughthedriedproductSecondaryDryingOncealltheiceissublimedoutofthefrozenfood,thesecondarydryingprocessbegins.Heatiscontinuallyadded,butataslowerratesincemoisturelossoccursonlybydiffusionofwatermoleculesoutofthefreeze--driedmatrix.CollapseBehaviorRapidheatadditioncausesthetemperatureoftheproducttoexceeditscollapsetemperature.productbecomessuffiientlyflowablethatit"collapses!'Duringcollapse,thepocketswhereicecrystalshavesublimeddisappearasthefoodslowlyflowsintotheseregions.Thiscausesproducttohavehigherdensityandreducesitsabilitytoberehydrate.VaporCondensorAcondensorcollectsthevaporsastheyexitthefreezedryertoenhanceefficiencyandpreventfoulingofthevacuumpump.FrontViewofFreezeDryerHeatingPlateintheDryerTraysMovingShelvesforTraysCondenser&VacuumSystemVacuumSystemBirdViewofWorkshopProblemsDescribethedefinitionofwateractivity.Describetherelationshipbetweenwateractivityoffoodsandtherelativehumidityofenvironment.WhataretheCRP&FRP?Describethecauseofthemandthedifferencesbetweenthem.Describeprinciplesoftunneldryerandspraydryer.Addtheauthorandtheaccompanyingtitle生活图标元素商务图标元素商务图标元素商务图标元素商务图标元素