令人不寒而栗

Chilling(Refrigeration)Dr.YiWangReductionintemperatureretardsallchangesoffood.Thelongerthestorageperiod,thelowertherequiredtemperature.Lowtemperatureinhibitsmicro-organismsandenzymes,thereforestabilizesfoods.Productsarenotsterilizedandcarefulcontrolishencenecessary.IntroductionInchillingtemperatureisreducedto-1~8oC.Chemical,biochemicalandmicrobiologicalchangesarereducedandtheshelflifeoffoodsisextended.PostharvestphysiologicalchangesinplantsPost-slaughtermetabolicchangesinanimaltissuesDeteriorativechemicalreactions(lipidoxidation,browning,colordegradation,autolysisoffish,nutrientloss,enzymaticchanges)IntroductionChillingcausesminimalchangestosensoryandnutritionalqualitiesoffoods.Thus,chilledfoodsareperceivedasconvenienthighquality“healthy”,“natural”and“fresh”

Chillingisoftenusedincombinationwithotherunitoperationstoextenttheshelflifeofmildlyprocessedfoods(fermentation,pasteurizationirradiationorMAP).Thesupplyofchilledfoodsisheavilydependentondistributionsystems(chilledstores,transport,retaildisplay,anddomesticrefrigerators).Low-acidchilledfoods(susceptibletopathogenicbacteria)mustbepreparedandpackagedunderstrictlycontrolledhygienicconditions.Chilledfoodsaregroupedintothreecategoriesaccordingtothechilledstoragetemperatureranges:-1to1oC--freshfish,meats,sausages,groundorsmokedmeatsandbreadedfish;0to5oC--pasteurizedcannedmeat,milk,cream,yogurt,preparedsalads,sandwiches,bakedfoods,pasta,pizzas,unbakedpastry;0to8oC--fullycookedmeatsandfishpies,cookedoruncookedcuredmeats,butter,margarine,cheese,fruitjuicesandmostfruitsandvegetables.Theory1.FreshfoodsRateofbiochemicalchangescausedbymicro-organismsorenzymesincreaseslogarithmicallywithtemperature;Chillingalsoretardstherespirationoffreshfoods.Thefactorscontroltheshelflifeoffreshfoodsinchilledstorage:typeandvarietyorcultivar;partused(stem,leaforroot;fastgrowing=>highmetabolicrate=>shortstorage);conditionatharvest(contamination,damage,maturity);conditionsduringharvest,processingorpackaging(suchastemperature,humidity,materials);conditions(temperatureandhumidity)ofstorage,distributionandretaildisplay.Respirationrateoffreshfruitsisnotconstant(climactericandnon-climactericfruits):climactericfruits(A)showanincreaseinrespirationrateneartooptimumripenessnon-climactericfruits(B)haveafairlyconstantrespirationrateTheclimactericfruitsincludeapple,apricot,avocado,banana,mango,peach,pear,plumandtomato.Non-climactericfruitsincludecherry,cucumber,grape,grapefruit,lemon,pineappleandstrawberry.Vegetablesrespireinasimilarwaytonon-climactericfruits.Theshelflifeoffreshcropfoodsisrelatedtotheirrespirationrateasshownbelow:ArtichokeTurnipBeetrootAsparagusAvocadoWhenthetemperatureistoolow,chillinginjurymayoccurtofruitsandvegetablesChillinginjury--causedbythetemperaturebelowaspecificoptimum,andresultedinvariousphysiologicalchanges(internalorexternalbrowning,failuretoripenandskinblemishes).Incorrectrelativehumiditymayalsocauseundesirablechangesduringlongtermstorage.ThetablebelowshowsthechillinginjurytemperaturesofsomefruitsInanimaltissues,aerobicrespirationrapidlydeclinesafterslaughter.Thenanaerobicrespiration(glycogentolacticacid)takesplace,whichcausespHofthemeattofall,onsetofrigormortis,inwhichmuscletissuebecomesfirmandinextensible.

Coolingduringanaerobicrespirationisnecessarytoproducetherequiredtextureandcolorofmeatandtoreducebacterialcontamination.However,coolingmeatbeforerigormortismaycauseundesirablechanges,whicharetermedcoldshortening.Coldshortening-whichiscausedbycoolingpriortotheonsetofrigormortisandresultsinthetougheningofmeat.ThusanagingprocessisnecessaryforallowingthechangefromrigortoagedmuscleTochillfreshfoodsitisnecessarytoremovebothsensibleheatandrespirationheat.Sensibleheat-heatthatmustbetransferredtoraiseorlowerthetemperatureofasubstance.Respirationheat--heatthatisgeneratedbyrespiratoryactivityoffreshfoods,andC6H12O6+6O2

6CO2+6H2O+

H

where

H=2835kJ/kmolC6H12O6at20oC&1atmHeatproducedbyrespiratoryactivityofselectedfoods2.ProcessedfoodsLowtemperaturepreventsorretardsreproductionofmicro-organismsThereare4broadcategoriesofmicro-organismsbasedontheirgrowthtemperatureranges:thermophilic(min.30-40,opt.55-65oC)Mesophilic(min.5-10,opt.30~40oC)psychrotrophic(min.<0-5,opt.20-30oC)psychrophilic(min.<0-5,opt.12-18oC)Greek:philicmeanslove,mesomeansmoderateormiddle,psychromeanscold,trophicmeanspertainingtofoodorsurviveEx.hydrophilic,mesophase,(hydrophobic)Themainmicrobiologicalconcernswithchilledprocessedfoodsarethepathogensthatcangrowat<5oC,orthatcancausefoodpoisoningwhenthetemperatureincreased(byabuse)ChillingpreventsthegrowthofthermophilicandmanymesophilicmicrobesButsomepathogenscaneithergrow,orbesufficientlyvirulenttocausepoisoningafteringestionofonlyafewcellsAsummaryofthesepathogens(sources,growthtemperatures,typeofinfection/spoilage,andtypicalhigh-riskfoods)aregiveninTable19.4PathogensMin.growtemp.(C)FoodsinriskA.hydrophiliaEntero.E.coliV.parahaemolyticusBacilluscereusY.enterocoliticaCamp.jejuniSalm.enteritidisC.botulinumIC.botulinumIIStaphy.aureusC.perfringensLis.monocytogenes1-54-75-104-10-1-7205.2-6103.36(10fortoxin)12-0.4-3water,milk,meat..meat,vegetables..rawfish,shellfish..cerealorspiceprod.meats,seafoods,tofu..milk/products,watereggs,meats,milk..cannedvegetables,otherlowacidfoods..milk/products,meat..meat,dairy,driedfoodmilk,sandwich,salad..Spoilagemicro-organismsMin.growtemp.(C)FoodsinriskBro.thermosphactaLacticacidbacteriaPseudomonassppYeasts(Candidaspp)Moulds(Mucorspp)-0-5-3-0<0Vacuumpackedmeats..meat,vegetables..milk/products,meats,fruitjuices,beverages..MostchilledfoodsFruitjuices,meat/dairyproducts,vegetables.Bacilluscereus(芽孢桿菌)Campylobacter(彎曲菌)Clostridiumbotulinum(肉毒桿菌)Clostridiumperfringens(梭狀芽孢桿菌)Escherichiacoli(大腸桿菌)Listeriamonocytogenes(李斯特菌)Proteus(變形桿菌)Pseudomonas(單假孢桿菌)Salmonella(沙門氏菌)Shigellas(志賀氏桿菌)Staphlococcusaureus(葡萄球菌)Streptococci(鏈球菌)GMP(goodmanufacturingpractice)mustbeenforcedduringtheproductionofchilledfoods:HygienedesignofthechillingplantCleaningschedulesQualitymanagementprocedure(HACCP)Shelflifeofchilledprocessedfoodsisdeterminedbythefactors(as“hurdles”):TypeoffoodDegreeofmicrobial/enzymedestructionsControlofhygieneinprocessingandpackagingBarrierpropertiesofthepackageTemp.duringprocessing,distributionandstorage3.Cook-chillfoodsDefinition:Individualfoodsorcompletemealsproducedbycook-chillorcook-pasteurise-chillprocesses.Thecook-chillfoods:usedininstitutionalcateringtoreplacethetraditionalwarmholdingfoodshighernutritionalandeatingqualitieslessexpensive,andsalesincreaseowingtotheirconvenience,mildprocessingandhealthyimageTheclassofmicrobialriskofready-to-eatchilledfoodscanbecharacterizedas:Class1:containingraworuncookedingredients(e.g.saladorcheese)asready-to-eatfoodsClass2:madefromamixtureofcookedandlowriskrawingredientsClass3:cookedproductsthatarethenpackagedClass4:cookedafterpackaging,includingready-to-eat-products-(orrefrigerated-pasteurised-foods-)for-extended-durability(REPFEDs)“Cooking”referstoaheatprocessthatresultsinaminimum6Dreductionintargetpathogens.SomeClass1productsmayrequirecooking;whereasothersmayonlyrequireashortre-heatingSafetyofthesefoodscanbecontrolledby:minimizingthelevelsofpathogensontheincomingingredientensuringnotintroducingpathogensinprocessingandstorageproceduresensuringnotallowingthenumberofpathogenstoincreaseAspecial“hygienicarea”isrequiredforproductsinClasses1,2&4(3?)

inadditiontonormalhygienicmanufacturingareasdesignedtobeeasilycleanedtopreventbacteriaestablishinginitAlsoanadditional“high-carearea”isrequiredforproductsinClasses2&3physicallyseparatedfromotherareastoisolatecookedfoodsduringprocesses(preparation,mealassembly,chillingandpackaging)specifiedhygienerequirements:positivepressureventilationwithmicro-filteredairatcorrecttemp&humidityentryandexitonlythroughchangingrooms“no-touch”washingfacilitiesconstructionforeasycleaningonlyfullyprocessedfoodsandpackagingmaterialsadmittedthroughhatchesorair-locksspecialhygienetrainingandfullyprotectiveclothingoperationalprocedurestolimittheriskofcontaminationproductionstoppedforcleaninganddisinfectionatpredeterminedtimeinterval(e.g.every2hours)Processingstepsofcook-chillfoodstheinitialstepsareasnormal(cooking).thenportionedandtobechilledwithin30minchilledto3oCwithin90minstoredat0~3oCwithshelflifeupto5daysCook-pasteurize-chillfoodhasapasteurizationstepbetweenthecookingandchillingsteps.hotfill,sealatpartialvacuumtoremoveO2pasteurizedto80oCfor10minthefoodshaveashelflifeof2~3weeks4.EquipmentsizeandchillingtimeCalculationsofequipmentsizeandchillingtimearesimpleforprocessedfoodsForfreshfoods,anumberofassumptionsshouldbemadetosimplifythecalculationsCalculationofequipmentsize: Heatload=sensible+respiration+…Incalculationofchillingtime,unsteady-stateheattransfershallbeused.Unsteady-stateheattransferWhenasolidpieceoffoodisheatedorcooledbyafluid,theresistancestoheattransferare:thesurfaceheattransfercoefficientthethermalconductivityofthefoodSeveraldimensionlessparameters,e.g.theBiotnumber(Bi),Fouriernumber(Fo)andTemperaturefactor,areused.TheBiotnumber:whereh=heattransfercoefficient(W/m2.K) k=thermalconductivity(W/m.K)

=characteristic“halfdimension” (radiusforcylinderorsphere, half-thicknessforslab)TheTemperaturefactor:where

h=medium’stemperature

f=finaltemperatureoffood

i=initialtemperatureoffoodTheFouriernumber:where c=heatcapacityoffood(kJ/kg.K)

=densityoffood(kg/m3) t=timeofheatingorcooling(s)Thecalculationsarecomplex,andaseriesofchartsareavailableExample1:Freshberriesmeasuring2cmindiameterarechilledfrom18oCto7oCinachillerat-2oC,withasurfaceheattransfercoefficientof16W/m2K.Calculatethetimerequiredtocooltheberriesinthechiller.Datafortheberriesare:thermalconductivity0.127W/mK,specificheat3778J/kgK,density1050kg/m3.t?------Fo=kt/crotheta^2-----Fo?h=16;k=0.127;c=3778;ro=1050;theta=2/2;Example1:Freshberriesmeasuring2cmindiameterarechilledfrom18oCto7oCinachillerat-2oC,withasurfaceheattransfercoefficientof16W/m2K.Calculatethetimerequiredtocooltheberriesinthechiller.Datafortheberriesare:thermalconductivity0.127W/mK,specificheat3778J/kgK,density1050kg/m3.EquipmentforchillingChillingequipmentisclassifiedbasedonthemethodusedtoremoveheatas:mechanicalrefrigeratorscryogenicsystemsBothtypesofequipmentmayoperatebatchorcontinuously.Tolowerthetemperatureasquicklyas(<4hr)possiblethroughthecriticalwarmzone(65~5

oC)wheremax.growthofmicro-organismsoccurs.Mechanicalrefrigerators:

Thissystemhasfourbasicelements,namelyevaporator,compressor,condenserandexpansionvalve.Componentsofrefrigeratorsarefrequentlyconstructedfromcopper.Arefrigerantcirculatesbetweenthefourelements,changingstatefromliquidtogas,andbacktoliquid.Fig.21.1SinglestagemechanicalrefrigerationcomponentsMechanical(compression-expansion)refrigerationsystemABCDEThecycleareasfollows:Inevaporator-theliquidrefrigerantevaporatesunderreducedpressure,absorbslatentheatandcoolsthechilling(freezing)medium;thisisthemostimportantpart;theremainingpartsareusedtorecyclerefrigerantIncompressor-therefrigerantvaporfromtheevaporatorpassestothecompressorwhereitspressureaswellastemperatureareincreased.Incondenser-thevaporthenpassestothecondenserwherethepressureismaintainedandthevaporiscondensed(temperatureisreduced)byreleaseitsheattoacoolingmediumInexpansionvalve-thecondensedliquidrefrigerantpassesthroughthevalvewherethepressureisreducedandtherefrigerationcyclere-startsFig.21.2Pressure-enthalpychart(a)courtesyofIneosFluorFellows,2009Fig.21.2Pressure-enthalpychart(b)adaptedfromSinghandHeldman(2001)Fig.21.3Pressure-enthalpychartshowingvapor-compressioncycleSinghandHeldman,2001PerformanceofrefrigerationsystemCoefficientofperformance(COP):Definition:Ratiooftheheatabsorbedbytherefrigerantintheevaporatorandtheenergysuppliedtothecompressorwhere Hi–enthalpyofrefrigerantatpositioni,

position1–leavingthecondenser,

2–enteringthecompressor, 3–leavingthecompressor,workdoneontherefrigerantbythecompressoris: whereqw–rateofworkdoneontherefrigerant

m–massflowrateoftherefrigerantheatremovedinthecondenseris:Heatabsorbedintheevaporatorisknownas“refrigerationeffect”,andcanbefoundby:Ifthetotalrateofheatremoved(knownascoolingorrefrigerationload)isknown,therefrigerantflowratecanbefoundby: wheremf–refrigerantflowrate

q–actualcoolingloadExample2AcoldstoreiscooledusingR-134arefrigerantinavapor-compressorrefrigerationsystemthathasacoolingloadof35kW.Theevaporatortemperatureis-5oCandthecondensertemperatureis43oC.Assumingthatthecompressorefficiencyis80%,calculatethecompressorpowerrequiredandtheCOPofthesystem.q=35k=m(H2-H1);inevaporatorqw(compressor)=m(H3-H2);W=qw/80%;incompressorCOP=(H2-H1)/(H3-H2);COP=qw/q=80%*W/q;H3-H2=c(T3-T2);incompressoralowboilingpointandhighlatentheatofvaporizationadensevaportoreducethesizeofthecompressorlowtoxicitynonflammablelowmiscibilitywithoilincompressorlowcostImportantpropertiesofrefrigerantsThecommonlyusedrefrigerantsare/were:halogens(CFCs,e.g.R-11andR-12),partiallyhalogenatedCFCs(HCFCs,e.g.R-21,R-22),ammonia(NH3orR-717)andcarbondioxide(CO2orR-744).

CFCswerethemostwidelyusedrefrigerants,buttheyhaverelativehighmiscibilitywithoilandmaydamagetotheozonelayer.Theyhavebeenphase-out.

HCFCsarelessharmfultotheozonelayer,andarebeingtemporarilysubstitutedforCFCs;butwillbephase-outby2015.Ammoniahasexcellentheattransferpropertiesandisnotmisciblewithoil,butitistoxicandflammable,andcausescorrosionofcopperpipes.CO2isnon-flammableandnon-toxic,butrequiresconsiderablyhigheroperatingpressures.Newozone-friendlyliquids,suchasHFCs(e.g.HFC23(CHF3),HFC32(CH2F2),HFC125(C2HF5))andnewerHCFCsaredevelopedandlikelytobecomeimportantrefrigerants.Thechillingmediumcanbeair,waterormetalsurfaces.Thecommontypesofequipmentare:Airchillers(e.g.blastchillers):convectionbyforcedcirculationofair,thusincreaseheattransferrate;commoninrefrigerationplantsandrefrigeratedvehicles.Chillcabinets:convectionbynaturalcirculationofchilledair;commonlyforholdingpre-chilledfoods,andheatloadissmall.Waterchillers:waterisusedaschillingmedium,directimmersioninchilledwaterisusedtocoolfruitsandvegetables.Heatexchangers:theyareusedtocoolliquidfoods.cryogenicchillingAcryogen

isarefrigerantthatchangesphasebyabsorbinglatentheattocoolthefood.The3statediagramofCO2isshownbelow:CryogenicchillersusesolidCO2,liquidCO2orliquidN2.SolidCO2removesheatbysublimation.Liquidcryogensremoveheatbyvaporization.Thevaporfromsublimationorvaporizationalsoabsorbsheatasitwarmsfromaverylowtemperature(-78oCforCO2or-196oCforN2).Incryogenicchilling,CO2ismorepreferredthanN2dueto:CO2hasahigherboilingpointandsublimationpointthanN2,andthereforealesssevereeffectonthefood;mostheatremovedbyCO2isduetotheconversionofitsstate,thustheequipmentissimplerbecauseadditionalgashandlingequipmentisnotrequired.ThemainlimitationofCO2isitstoxicity(asphyxiaif>0.5%).Anadditionalcostsmaybeincurredforairextractionsystems.

LiquidN2hasadvantagesovercarbondioxideinfreezingapplications.BothLiquidandSolidCO2couldbeusedforrapidcoolingwithoutdehydration.LiquidCO2isinjectedintoairtoproducefineparticlesofsolidCO2(“snow”)

,whichrapidlysublimetovapor.SolidCO2(“dry-ice”)haslargerparticlesandisdepositedintotraysorcartons.AsmallexcessofsolidCO2maybenecessarytoproducecoolingduringtransportation.ChillstorageTostorechilledfoodsCooledbycirculationofcoldairfrommechanicalsystemsControlofstorageconditionsbyadequatecirculationofairusingfans:<5oCforhigh-riskfoodstomeetsafety,qualityandlegalrequirementsrequiredhumidityforfreshfoodsatmosphericcompositionCalculationofheatloadsforachillstoreTotalheat=loadHeatof+respirationSensibleheatof+containerHeatevolvedby+man&lightsHeatlossesPre-chilledfreshberriesat7oCareloadedin250kgbatchesintocontainers(at18oC)andheldfor12hinacoldstoreoperatingat-2oC.Thecoldstoreholdsanaverageof2.5toffoodandmeasures3x10x10m(HxWxL).[Thewallsandroofareinsulatedwith300mmofpolyurethanefoam,andthefloorisconstructedfrom450mmofconcrete.Theambientairtemperatureaverages12oCandthesoiltemperature9oC].Anoperatorspendsanaverageof45min/daymovingthecontainersinthestoreandswitchesonfour100Wlightswheninthestore.Eachcontainerweighs50kg.Determinewhethera5kWrefrigerationplantwouldbesuitableforthecoldstore.(Additionaldata:thethermalconductivitiesare0.026W/m.Kfortheinsulationand0.87W/m.Kfortheconcrete;thespecificheatofthecontaineris480J/kg.K,theheatproducedbytheoperatoris240W,andtheaverageheatofrespirationofberriesis0.275J/kgs)Example3.Heatload