食品保鮮學課件Chapter 6 Drying and aw

Chapter6

Drying&WaterActivityinFoodPreservationOutline6.1Introduction6.2HowDoesDryingPreserveFoods6.3ApplicationofDryinginFoods6.4DryingTechniques6.5ChangesinDrying6.1IntroductionTime-honoredmethodofpreservingfood15,000and10,000BCFish,meatandfruitDryinginthewind&sunAround8,000BCNaturaldrying,smokingand&salting1795artificialdrying20thcenturySpraydrying&freezedryingCostofDryingProcessingPackagingTransportationStorageCannedandfrozenfoodWateractivity(aw)aw

aw=pw/pw0=ERH/100Pw:VaporpressureofwaterPw0:VaporpressureofpurewaterInterestinawin1950sawCorrelatewithmicrobialgrowthandmetabolicactivityGoodindicatorofwateravailabilityformicrobialactivity6.2HowDoesDryingPreserveFoods6.2.1aw&MicroGrowth,Death&SurvivalLimitingawEachmicrohasalimitingaw(minimumaw)awbelowlimitingawNotgrowNotformsporeNotproducetoxicmetabolitesImportanceofminimumaw0.95-0.91Wateractivityvaluesofselectedfoods&associatedmicrobialspoilageRangeofAwMicroorganisminhibitedExamplesoffoodsSomeyeasts,G-rods,bacteriasporesFreshfoods;foodscontaining40%sucroseor7%salt(cannedfoods,severalsausages,bread)Mostcocci,lactobacilli,vegetativecellsofbacilli,somemouldsFoodscontaining59%sucroseor12%salt(mayonnaise,bacon,rawham)

0.94GrowthandtoxinproductionbyalltypesofClostridiumbotulinum0.91-0.86MostyeastsFoodscontaining65%sucroseor15%salt(fruitjam,fruitjuiceconcentrate)0.86AerobicgrowthofStaphylococcusaureus1.00-0.95Instantcoffee,spices,crackers,flour,cereals.NomicrobialgrowthBelow0.60Confetioneryproducts,driedfruitscontaining15-20%water,honeyOsmophilicyeast0.65-0.60Practicallimitforfungi0.68Foodscontaininglessthan10%water(dates,figs,nuts)Xerophilicmoulds0.75-0.65Productionoftoxins0.80Foodscontaining15-20%water(highmoistureprunes,sweetenedcondensedmilk)Mostmoulds0.86-0.80ExamplesoffoodsMicroinhibitedRangeofAwWateractivityvaluesofselectedfoods&associatedmicrobialspoilage(cont.)InhibitingawforMicrosCommonspoilagebacteria0.97Exception:Clostridium:0.94Staphylococcus

aureus–mostAwtolerant0.86~0.91Yeasts&moulds0.86Osmophilicyeasts&xerophilicmoulds:0.6MinimumAw:0.66.2.2.GeneralMechanismsofDrying

EffectofdryingReducewateractivity,preservingfoodsbyavoidingmicrobialgrowth&deteriorativechemicalreactionsEffectofheat(thermaldrying)Inhibitbothmicros&enzymesEffectofheatanddrying6.2.3EndpointofDryinginFoods

Meat,fish,anddairyproducts3%orlessVegetableproducts5%Cerealproducts12%6.3ApplicationofDryinginFoodsFruits&VegetablesApples,cherries,peaches,peas,corn,peppers,zucchini,okra,onions,greenbeans…LeanmeatsBeef,lamb,&venisonFishFoodsnotsuitablefordryingHighmoisturecontentLettuce,melons,andcucumbers6.4DryingTechniquesDryingprocesses

Thermaldrying

Mechanicaldewatering

Osmoticdehydration

6.4.1ThermalDryingSolaroropenair-DryingConvectionair-DryingSpray-dryingFluidizedbeddryingSmokingVacuum-DryingFreeze-DryingSolarorOpenAir-Drying

RadiationenergyfromsunisusedNonpolluting,renewable,abundantLimitationsLargeareasofspace&highlaborinputsDifficultcontrolrateofdryingInsectinfestationµbialcontaminationRaisinsNotsuitableforlargescaleproduction（自然干燥）

SundriedtomatoesConvectionAir-DryingSimplestdryingtechniqueTakeplaceinenclosed&heatedchamberDryingmedium:hotairUsedforproductsthatarerelativelylowinvalueGrains（熱風干燥）

Spray-DryingRemovewaterfromfree-flowingliquidmixtureTransformliquidintopowderproductProducts:smalldropletswithlargesurfaceareasComeintocontactimmediatelywithhotgasLiquidisevaporatedrapidlyMinimizeheatdamageDisadvantagesSizeofequipmentisverylargeOilmaterialsrequirespecialpreparationFluidizedBedDryingMovementofparticulatematterinupward-flowinggasstream(hotair)FluidizationmobilizethesolidparticulatesIntimatecontactbetweengasandsolidsDryingoccurbyconvectionHotgasintroducedintothebottomandexitsatthetopMainadvantageUniformtemperature&highdryingrates,lessthermaldamage(流化床干燥)

RotaryDrumDryingDrumdryers1-5mindiameter10-40minlengthRotateat1-8rpmwithacircumferentialspeedof~0.2-0.4m/secDesignedtooperateatanearlyhorizontalposition（滾筒干燥）

SmokingSmokeisnearlyasoldasopenair-dryingHeataccompaniedwithsmokehasdryingeffectMainlyusedwithmeatandfishImpartsdesirableflavor,colorSomecompoundsformduringsmokingAntimicrobialAntioxidantVacuum-DryingLowpressure,heatingsourceWatervaporizeatalowertemperatureFoodsdriedwithoutexposuretohightemperatureAbsenceofairdiminishesoxidationColor,texture,&flavorimprovedFreeze-DryingFrozenmaterial,<0°CWaterremovedfromsolid-vaporNoheatdamageoccursLittleornolossinqualitiesRemovaloficecrystalsleavesporoushoneycomb-typestructureProductrehydraterapidlySlowandexpensiveMainlyusedforhigh-valueproductsSpacefoods6.4.2OsmoticDehydration(OD)DefinitionTheprocessofwaterremovalbyimmersionofawater-containingcellularsolidinconcentratedaqueoussolutionsyrup(Impregnation)brine(Salting)OsmoticProcessTwomajorsimultaneouscountercurrentflowsFlowofwaterfromfoodintoosmoticsolutionFlowofsolutesfromsolutionintofood

CharacterizedbyequilibriumUsualprocesstodecreaseproductawinfruits&vegetablesCandiedfruitsJamSolutionSolutionSolutesWaterFoodComponentsWater&soluteTransferOsmoticProcessAdvantagesofODQualityimprovementintermsofcolor,flavorandtextureEnergyefficiencyPackaginganddistributioncostreductionChemicaltreatmentnotrequiredProductstabilityandretentionofnutrientsduringstorage6.4.3MechanicalDewatering

KeepliquidandsolidphaseseparatedPhysicalforceisusedVacuum(filter)PressurefiltersCentrifuges6.4.4FactorsConsideredinSelectingaDryingProcessTypeofproducttobedriedPropertiesoffinishedproductdesiredProduct'ssusceptibilitytoheatPretreatmentsrequiredCapital&processingcostEnvironmentalfactorsNOONEBESTTECHNIQUEFORALLPRODUCTS

6.5ChangesinDrying6.5.1PhysicalChanges

1.ShrinkageorcollapseChangeinvolumeduringprocessingMoisturelossduringdryingIceformationduringfreezingPoreformationduringpuffingImpactdriedfoodproductqualityReduceproductwettabilityChangeproducttextureDecreaseproductabsorbencyResultofcapillarycollapseFluidevaporatedorremovedfromcapillaryTypesofShrinkageIsotropicUniformshrinkageofallgeometricdimensionsFruits&vegetablesAnisotropicNonuniformshrinkageofdifferentdimensionsFish&seafood2.CrustFormation

OuterlayerlosemoisturebeforeinteriorpartMoistureconcentrationislessinsurface–shrinkagechecking,crackingCrustformationMoisturegradient&resistantnearthesurfaceImpervioustomoistureEnclosesthebulkInteriormoisturenotremovedMinimizebyreducingdryingr