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FoodchemistryFoodchemistryFoodchemistryBelitzFoodchemistryBelitz٠٠GroschGroschSpringerSecondEdition1999SpringerSecondEdition1999FoodchemistryOwenFennema3rdFoodchemistryOwenFennema3rded.,MarcelDekker,NewYorked.,MarcelDekker,NewYorkPrinciplesofFoodChemistryJohnPrinciplesofFoodChemistryJohndeMan,3rdedition,AspenPublishersdeMan,3rdedition,AspenPublishersInc.,Gaithersburg,Md.Inc.,Gaithersburg,Md.CourseObjectivesCourseObjectivesItisdesirabletoestablishanItisdesirabletoestablishananalyticalapproachtothechemistryanalyticalapproachtothechemistryoffoodformulation,processingandoffoodformulation,processingandstoragestability,sothatfactsstoragestability,sothatfactsderivedfromthestudyofonefoodorderivedfromthestudyofonefoodormodelsystemcanenhanceourmodelsystemcanenhanceourunderstandingofotherproducts.understandingofotherproducts.OwenFennema,1996OwenFennema,1996GoalofthisclassGoalofthisclass1.1.Understandfoodsasmechanistic,chemicalUnderstandfoodsasmechanistic,chemicalsystems.systems.2.2.AllthephenomenaobservedinpreparingAllthephenomenaobservedinpreparingfoodcan,inprinciple,beunderstoodinfoodcan,inprinciple,beunderstoodinclassicallychemicalterms.Understandingclassicallychemicalterms.Understandinghowchemicalsystemsbehaveenablesustohowchemicalsystemsbehaveenablesustobettercontrolthemtomeetourmanybettercontrolthemtomeetourmanyneeds.needs.3.3.StudentswillentertheclasswithabasicStudentswillentertheclasswithabasicknowledgeofchemistryandleaveknowledgeofchemistryandleaveunderstandinghowitappliestofood.understandinghowitappliestofood.GoalofthisclassGoalofthisclass4.4.UnderstandingtheimportantchemicalUnderstandingtheimportantchemicalbasisoffoodquality.basisoffoodquality.5.5.SuccessfulstudentswilldevelopaSuccessfulstudentswilldevelopaconceptualframeworktounderstandconceptualframeworktounderstandunfamiliarphenomenaintermsoffoodunfamiliarphenomenaintermsoffoodchemistry.chemistry.6.6.AchievementofthesegoalsrequiresbothAchievementofthesegoalsrequiresbothanaccumulationoffactsandtheanaccumulationoffactsandthedevelopmentofananalyticalapproachtodevelopmentofananalyticalapproachtofoodquality.foodquality.IntroductionIntroductionFoods:Foods:arematerialswhich,intheirarematerialswhich,intheirnaturallyoccurring,processedorcookednaturallyoccurring,processedorcookedforms,areconsumedbyhumansasforms,areconsumedbyhumansasnourishmentandforenjoyment.nourishmentandforenjoyment.Nourishment:Nourishment:thenutritionalvaluethenutritionalvalueEnjoyment:Enjoyment:thehedonicvaluethehedonicvalue、、visualvisualappealappeal、、smellsmell、、tasteandtexture,whichtasteandtexture,whichinteractwiththesenses.interactwiththesenses.Conveniencefoods:Conveniencefoods:accordingtoaccordingtopropertieswhichdeterminehandlingpropertieswhichdeterminehandlingAdditionalrequirement:Additionalrequirement:freefromtoxicfreefromtoxicmaterialsfoodinthefirstofnecessitytomaterialsfoodinthefirstofnecessitytothepeoplethepeopleFoodchemistry:Foodchemistry:isinvolvednotnolyinisinvolvednotnolyinelucidatingthecompositionoftherawelucidatingthecompositionoftherawmaterialsandend-products,butalsowithmaterialsandend-products,butalsowiththechangeswhichoccurinfoodduringitsthechangeswhichoccurinfoodduringitsproduction,processing,storageandproduction,processing,storageandcooking.cooking.Desiredandundesiredreactions:Desiredandundesiredreactions:compositionalanalyses(detection,compositionalanalyses(detection,isolationandstructuralisolationandstructuralcharacterizationoffoodconstituents)characterizationoffoodconstituents)ThereactionsofasingleconstituentThereactionsofasingleconstituentorofasimplemixtureorofasimplemixtureAfoodinwhichanindividualreactionAfoodinwhichanindividualreactiondominatescanbemade.dominatescanbemade.AgivencompoundisthusnotAgivencompoundisthusnotrestrictedtoanyonefoodorgroupofrestrictedtoanyonefoodorgroupoffoods.foods.ThenfocusonchemicalprocessesinThenfocusonchemicalprocessesinindividualfoods.individualfoods.Biochemistry:Biochemistry:tostudythetostudythemechanismofsynthesisofprotein,mechanismofsynthesisofprotein,carbohydrates,lipids,etcinlivingcarbohydrates,lipids,etcinlivingtissues.tissues.WaterWaterWater(moisture):Water(moisture):thepredominantthepredominantconstituentinmanyfoods.Asamediumconstituentinmanyfoods.Asamediumwatersupportschemicalreactionsandwatersupportschemicalreactionsanditisadirectreactantinhydrolyticitisadirectreactantinhydrolyticprocesses.Removedofwaterfromfoodprocesses.Removedofwaterfromfoodorbindingitbyincreasingtheorbindingitbyincreasingtheconcentrationofcommonsaltorsugarconcentrationofcommonsaltorsugarretardsmanyreactionsandinhibitstheretardsmanyreactionsandinhibitsthegrowthofmicroorganisms,thusgrowthofmicroorganisms,thusimprovingtheshelflivesofanumberofimprovingtheshelflivesofanumberoffoods.foods.ThroughphysicalinteractionwithThroughphysicalinteractionwithproteins,polysaccharides,lipidsandproteins,polysaccharides,lipidsandsalts,watercontributessignificantlysalts,watercontributessignificantlytothetextureoffood.tothetextureoffood.Table0.1Table0.1..Component:Component:anyofthepartsthatanyofthepartsthatmakeupasubstancemakeupasubstanceConstituent:Constituent:oneoneStructureStructureWaterMoleculeWaterMoleculeO:1SO:1S222S2S222P2P443S3SThesixvalenceelectronsofoxygenThesixvalenceelectronsofoxygenarehybridizedtofoursparehybridizedtofoursp33bondingbondingorbitalswhichformtetrahedron.orbitalswhichformtetrahedron.WaterMolecularStructureWaterMolecularStructureHOH105°~1A(http://www.martin.chaplin.btinternet.co.uk/molecule.html)TheHydrogenBondTheHydrogenBond(http://www.martin.chaplin.btinternet.co.uk/molecule.html)HOHO--OO…………HH++HHHH0.096nm0.096nmO-Hcovalentbond,bondangleO-Hcovalentbond,bondangleelectronegativeoxygen,electronegativeoxygen,electropositivehydrogenelectropositivehydrogenFig0.1Fig0.1..105°EachwatermoleculeistetrahedrallyEachwatermoleculeistetrahedrallycoordinatedwithfourotherwatercoordinatedwithfourotherwatermoleculesthroughhydrogenbonds.moleculesthroughhydrogenbonds.ThetwounsharedelectronpairsofThetwounsharedelectronpairsofoxygen:H-bondacceptorsitesoxygen:H-bondacceptorsitesTheH-Obondingorbitals:hydrogenTheH-Obondingorbitals:hydrogenbonddonorsitesbonddonorsitesThedissociationenergy:25kJmolThedissociationenergy:25kJmol-1-1Fig0.2Fig0.2..ThesimultaneouspresenceoftwoThesimultaneouspresenceoftwoacceptorsitesandtwodonorsitesinacceptorsitesandtwodonorsitesinwaterpermitsassociationinathree-waterpermitsassociationinathree-dimensionalnetworkstabilizedbyH-dimensionalnetworkstabilizedbyH-bridges.bridges.PhysicalpropertiesofwaterandicePhysicalpropertiesofwaterandiceComparisonwithCHComparisonwithCH44,NH,NH33,H,H22SSTable2Table2Meltingandboilingpoints,surfaceMeltingandboilingpoints,surfacetension,tension,Permittivity,heatcapacityexhibitPermittivity,heatcapacityexhibitunusuallylargevaluesunusuallylargevaluesThethermalconductivityoficeat0℃isThethermalconductivityoficeat0℃isapproximatelyfourtimesthatofwaterapproximatelyfourtimesthatofwaterIcehasathermaldiffusivityIcehasathermaldiffusivityapproximatelyninetimesgreaterthanapproximatelyninetimesgreaterthanthatofwaterthatofwaterTissuesfreeze&thawTissuesfreeze&thawThespecialphysicalpropertiesofwaterThespecialphysicalpropertiesofwaterisunusualforothersmallmolecules.isunusualforothersmallmolecules.HForNHHForNH33withisoelectricdipoleswithisoelectricdipolessimilartothoseofwater,formonlysimilartothoseofwater,formonlylinearortwo-dimensionalassociations.linearortwo-dimensionalassociations.PolarizationofH-ObondsistransferredPolarizationofH-Obondsistransferredviahydrogenbondsandextendsoverviahydrogenbondsandextendsoverseveralbonds.severalbonds.ThedipolemomentofacomplexThedipolemomentofacomplex(Multimoleculardipole)iscertainlymuch(Multimoleculardipole)iscertainlymuchhigherthanthatofasinglemolecule.higherthanthatofasinglemolecule.ProtontransporttakesplacealongtheProtontransporttakesplacealongtheH-bridges.H-bridges.AhydratedHAhydratedH33OO⊕⊕ionisformedwithanionisformedwithanexceptionallystronghydrogenexceptionallystronghydrogenbond(dissociationenergyaboutbond(dissociationenergyabout100kJmol100kJmol-1-1))((0.10.20.10.2))AsimilarmechanismisvalidinAsimilarmechanismisvalidintransportofOHtransportofOHΘΘions.ions.H-bridgesiniceextendtoalargerH-bridgesiniceextendtoalargerspherethaninwater.spherethaninwater.ThemobilityofprotonsiniceishigherThemobilityofprotonsiniceishigherthaninwaterbyafactorof100.thaninwaterbyafactorof100.LiquidWaterandIceLiquidWaterandIceThepronouncedtendencyofwaterThepronouncedtendencyofwatermoleculestoassociatethroughH-bridges,moleculestoassociatethroughH-bridges,liquidwaterandicearehighlystructured.liquidwaterandicearehighlystructured.Stableice-Iisformedat0˚Cand1atmStableice-Iisformedat0˚Cand1atmpressure.pressure.Figure3Figure3NineknowncrystallinepolymorphicNineknowncrystallinepolymorphicstructures,eachofwhichisstableinastructures,eachofwhichisstableinacertaintemperatureandpressurerange.certaintemperatureandpressurerange.Thecoordinationnumberinice-Iisfour,Thecoordinationnumberinice-Iisfour,theO-HtheO-H……Odistanceis0.276nm(0˚C),theOdistanceis0.276nm(0˚C),theH-atombetweenneighboringoxygenisH-atombetweenneighboringoxygenis0.101nmfromtheoxygentowhichitis0.101nmfromtheoxygentowhichitisboundcovalentlyand0.175nmfromtheboundcovalentlyand0.175nmfromtheoxygentowhichitisboundbyahydrogenoxygentowhichitisboundbyahydrogenbridge.bridge.Fivewatermolecules,formingaFivewatermolecules,formingatetrahedron,arelooselypackedandkepttetrahedron,arelooselypackedandkepttogethermostlythroughH-bridges.togethermostlythroughH-bridges.WhenicemeltsandtheresultantWhenicemeltsandtheresultantwaterisheated,boththecoordinationwaterisheated,boththecoordinationnumberandthedistancebetweenthenumberandthedistancebetweenthenearestneighborsincrease.nearestneighborsincrease.Table0.2Table0.2..AnincreaseincoordinationnumberAnincreaseincoordinationnumberincreasesthedensity.increasesthedensity.AnincreaseindistancebetweenAnincreaseindistancebetweennearestneighborsdecreasesthenearestneighborsdecreasesthedensity.density.TheeffectofincreasingcoordinationTheeffectofincreasingcoordinationnumberispredominantduringanumberispredominantduringatemperatureincreasefrom0to4˚C.temperatureincreasefrom0to4˚C.Unusualproperty:itsdensityintheUnusualproperty:itsdensityintheliquidstateat0˚C(0.9998g·cmliquidstateat0˚C(0.9998g·cm-3-3)is)ishigherthaninthesolidstate(ice-Ihigherthaninthesolidstate(ice-I0.9168g·cm0.9168g·cm-3-3).).WaterisastructuredliquidwithaWaterisastructuredliquidwithashorttime-rangeorder.shorttime-rangeorder.ThroughH-bridges,formshort-livedThroughH-bridges,formshort-livedpolygonalstructureswhicharerapidlypolygonalstructureswhicharerapidlycleavedandthenreestablishedgivingacleavedandthenreestablishedgivingadynamicequilibrium.dynamicequilibrium.lowerviscosityofwaterlowerviscosityofwaterThehydrogen-boundwaterstructureThehydrogen-boundwaterstructureischangedbysolubilizationofsaltsischangedbysolubilizationofsaltsormoleculeswithpolarand/orormoleculeswithpolarand/orhydrophobicgroups.Insaltsolutionshydrophobicgroups.Insaltsolutionsthen-electronsoccupythefreethen-electronsoccupythefreeorbitalsofthecations,formingorbitalsofthecations,forming““aquaaquacomplexescomplexes””.Otherwatermolecules.OtherwatermoleculesthencoordinatethroughH-bridges,thencoordinatethroughH-bridges,formingahydrationshellaroundtheformingahydrationshellaroundthecationanddisruptingthenaturalcationanddisruptingthenaturalstructureofwater.structureofwater.IonicSolutesIonicSolutesHOHHOHHOHHOHHOHHOHHOHD:\\lecturenotes\\Lectures-UCLABC205-Biol_Chem_andNut_Lecture.htmHydrationshellsareformedbyHydrationshellsareformedbyanionsthroughion-dipoleinteractionanionsthroughion-dipoleinteractionandbypolargroupsthroughdipole-andbypolargroupsthroughdipole-dipoleinteractionorH-bridges,againdipoleinteractionorH-bridges,againcontributingtothedisruptionofthecontributingtothedisruptionofthestructuredstateofwater.structuredstateofwater.HOHHOHHOHHOHHOHHOHHOHHOHHOHIonicSolutesIonicSolutesIon-dipoleinteractionsIon-dipoleinteractionsPolarSolutesPolarSolutesHydrogenbondsHydrogenbondsDipole-dipoleattractionsDipole-dipoleattractionsAliphaticgroupswhichcanfixtheAliphaticgroupswhichcanfixthewatermoleculesbydispersionforceswatermoleculesbydispersionforcesarenolessdisruptive.Aminimumofarenolessdisruptive.Aminimumoffreeenthalpywillbeattainedwhenanfreeenthalpywillbeattainedwhenanice-likewaterstructureisarrangedice-likewaterstructureisarrangedaroundahydrophobicaroundahydrophobicgroup(tetrahedral-four-coordination).group(tetrahedral-four-coordination).Suchice-likehydrationshellsaroundSuchice-likehydrationshellsaroundaliphaticgroupscontribute,foraliphaticgroupscontribute,forexample,tostabilizationofaprotein,example,tostabilizationofaprotein,helpingtheproteintoacquireitshelpingtheproteintoacquireitsmostthermodynamicallyfavorablemostthermodynamicallyfavorableconformationinwater.conformationinwater.ThermodynamicsofBindingThermodynamicsofBindingG=G=H-TH-TSSEntropycosttosolvent(Entropycosttosolvent(S<0,-TS<0,-TS>0)S>0)STRONGEnthalpygaintosolvent(STRONGEnthalpygaintosolvent(H<0)H<0)NetNetG<0;reactionproceedsG<0;reactionproceedsD:\\lecturenotes\\Lectures-UCLABC205-Biol_Chem_andNut_Lecture.htmNonPolarSolutesNonPolarSolutesWeaktransientdipole(VanderWaalsWeaktransientdipole(VanderWaalsattraction)attraction)StrongstructuringofwaterintoahydrogenStrongstructuringofwaterintoahydrogenbondedclathratecagebondedclathratecage––entropycostentropycostD:\\lecturenotes\\Lectures-UCLABC205-Biol_Chem_andNut_Lecture.htmNon-polarSolutesNon-polarSolutesOILHOHHOHHOHHOHHOHHOHEffectonStorageLifeEffectonStorageLifeDryingand/orstorageatlowDryingand/orstorageatlowtemperaturesareamongtheoldesttemperaturesareamongtheoldestmethodsforthepreservationoffoodmethodsforthepreservationoffoodwithhighwatercontents.withhighwatercontents.WaterActivityWaterActivity1952,W.J.Scott1952,W.J.ScottThestoragequalityoffooddoesnotdependonThestoragequalityoffooddoesnotdependonthewatercontent,butonwateractivity(thewatercontent,butonwateractivity(AwAw).).Aw=P/PAw=P/P00=ERH/100=ERH/100P=partialvaporpressureoffoodmoistureP=partialvaporpressureoffoodmoistureattemperatureTattemperatureTPP00=saturationvaporpressureofpurewateratT=saturationvaporpressureofpurewateratTERH=equilibriumrelativehumidityatTERH=equilibriumrelativehumidityatTThetypicalwateractivityofsomefoodstuffsTypeofproductWaterActivity(aw)Freshmeatandfish.99Bread.95Agedcheddar.85Jamsandjellies.8Plumpudding.8Driedfruit.6Cookies.3Milkpowder.2Instantcoffee.2TherelationshipbetweenwatercontentTherelationshipbetweenwatercontentandwateractivityisindicatedbytheandwateractivityisindicatedbythesorptionisothermofafood.sorptionisothermofafood.Atalowwatercontent(<50%),evenAtalowwatercontent(<50%),evenminorchangesinthisparameterleadtominorchangesinthisparameterleadtomajorchangesinwateractivity.majorchangesinwateractivity.ThecourseofadryingprocessliesThecourseofadryingprocessliesslightlyabovetheadsorptionisothermslightlyabovetheadsorptionisothermpertainingtothestorageofmoisture-pertainingtothestorageofmoisture-sensitivefood.sensitivefood.MoistureSorptionIsothermMoistureSorptionIsothermawMoisturecontent(d.w.b.)http://www.pentech.ac.zaMoistureSorptionIsothermawMoisturecontent(d.w.b.)MoistureSorptionIsothermawMoisturecontent(d.w.b.)Zone3Zone2Zone1ZonesinIsothermsZonesinIsothermsZone3:BulkwaterZone3:BulkwaterZone2:LooselyboundwaterZone2:LooselyboundwaterZone1:Tightlyboundwater.Zone1:Tightlyboundwater.TemperatureDependencyawMoisturecontent(d.w.b.)coldhotMoisturecontent(d.w.b.)SorptionandDesorptionSorptionandDesorptiondesorptionsorptionHysteresis:Typically,atanygivenp/pHysteresis:Typically,atanygivenp/p00,the,thewatercontentofthesamplewillbegreaterwatercontentofthesamplewillbegreaterduringdesorptionthanduringresorption.duringdesorptionthanduringresorption.Asarule,thepositionofthehysteresishoopAsarule,thepositionofthehysteresishoopchangeswhenadsorptionanddesorptionarechangeswhenadsorptionanddesorptionarerepeatedwiththesamesample.repeatedwiththesamesample.DecreasedwateractivityretardsthegrowthDecreasedwateractivityretardsthegrowthofmicroorganisms,slowsenzymecatalyzedofmicroorganisms,slowsenzymecatalyzedreactions(particularlyinvolvinghydrolases)reactions(particularlyinvolvinghydrolases)and,lastly,retardsnonenzymaticbrowning.and,lastly,retardsnonenzymaticbrowning.Fig0.4Fig0.4awMoisturecontent(d.w.b.)MoistureSorptionIsothermawMoisturecontent(d.w.b.)MoistureSorptionIsothermWaterActivity(Aw)WaterActivity(Aw)WaterActivity(Aw)MoistureSorptionIsothermsMoistureSorptionIsothermsHighlyproductspecific(physicalandHighlyproductspecific(physicalandchemicalstructure)chemicalstructure)HighlytemperaturedependantHighlytemperaturedependantShowsorption/desorptionhysteresisShowsorption/desorptionhysteresisAffectbothphysical/chemicalAffectbothphysical/chemicalreactivityofthefoodandthereactivityofthefoodandthedynamicsofwatertransportdynamicsofwatertransportReactionRatesandWaterReactionRatesandWaterActivityActivityawMoisturecontent(d.w.b.)log(RATE)LipidoxidationMostreactionsMicrobialgrowthSORPTIONISOTHERMhttp://www.pentech.ac.zaIncontrast,therateoflipidautoxidationIncontrast,therateoflipidautoxidationincreasesindriedfoodsystems.increasesindriedfoodsystems.FoodswithAwvaluesbetween0.6andFoodswithAwvaluesbetween0.6and0.9areknownas0.9areknownas““intermediatemoistureintermediatemoisturefoodsfoods””(IMF).Thesefoodsarelargely(IMF).Thesefoodsarelargelyprotectedagainstmicrobialspoilage.protectedagainstmicrobialspoilage.OneoftheoptionsfordecreasingwaterOneoftheoptionsfordecreasingwateractivity,andthusimprovingtheshelflifeactivity,andthusimprovingtheshelflifeoffood,istouseadditiveswithhighoffood,istouseadditiveswithhighwaterbindingcapacities(humectants).waterbindingcapacities(humectants).Table0.5Table0.5HumectantsHumectantse.g.:sucrose,propyleneglycol,e.g.:sucrose,propyleneglycol,glycerolglycerolBecarefulof:Becarefulof:Solubility,MWSolubility,MWFlavorFlavorCrystallizationonstorageCrystallizationonstorageChemicalreactivityChemicalreactivityToxicityToxicityHowever,theyarealsosweetenersandwouldHowever,theyarealsosweetenersandwouldbeobjectionablefromaconsumerstandpointbeobjectionablefromaconsumerstandpointinmanyfoodsintheconcentrationsrequiredinmanyfoodsintheconcentrationsrequiredtoregulatewateractivity.toregulatewateractivity.WaterActivityasanIndicatorWaterActivityasanIndicatorWateractivityisonlyoflimiteduseWateractivityisonlyoflimiteduseasanindicatorforthestoragelifeofasanindicatorforthestoragelifeoffoodswithalowwatercontent,sincefoodswithalowwatercontent,sincewatersolutionswhichareatawatersolutionswhichareatathermodynamicequilibrium.thermodynamicequilibrium.However,foodswithalowwaterHowever,foodswithalowwatercontentarenon-idealsystems.contentarenon-idealsystems.PhaseTransitionofFoodsPhaseTransitionofFoodsContainingWaterContainingWaterThephysicalstateofmetastableThephysicalstateofmetastablefoodsdependsontheircomposition,foodsdependsontheircomposition,temperatureandstoragetime.temperatureandstoragetime.Temperature:glassy,Temperature:glassy,rubbery(plastic)orhighlyviscous.rubbery(plastic)orhighlyviscous.ThekineticsofphasetransitionscanbeThekineticsofphasetransitionscanbemeasuredbymeansofdifferentialmeasuredbymeansofdifferentialscanningcalorimetry(DSC),producingascanningcalorimetry(DSC),producingathermogramthatshowstemperatureTgthermogramthatshowstemperatureTgasthecharacteristicvaluefortheasthecharacteristicvalueforthetransitionfromglassytorubbery.transitionfromglassytorubbery.Fig0.5Fig0.5FoodsbecomeplasticwhentheirFoodsbecomeplasticwhentheirhydrophiliccomponentsarehydrated.hydrophiliccomponentsarehydrated.ThusthewatercontentaffectstheThusthewatercontentaffectsthetemperatureTg.temperatureTg.DuringthecoolingofanaqueousDuringthecoolingofanaqueoussolutionbelowthefreezingpoint,partofsolutionbelowthefreezingpoint,partofthewatercrystallizes,causingthethewatercrystallizes,causingthedissolveddissolvedsubstancetobecomeenrichedinthesubstancetobecomeenrichedintheremainingfluidphase(unfrozenwater).remainingfluidphase(unfrozenwater).InthecaseofoligosaccharidesInthecaseofoligosaccharidescomposedofthreeglucosemolecules,composedofthreeglucosemolecules,mactotriosehasthelowestTgmactotriosehasthelowestTg’’valueinvalueincomparisonwithpanoseandcomparisonwithpanoseandisomaltotriose.isomaltotriose.ThereasonisprobablythatinaqueousThereasonisprobablythatinaqueoussolution,theeffectivechainlengthofsolution,theeffectivechainlengthofhomologousseriesofoligoandhomologousseriesofoligoandpolysaccharides,TgandTgpolysaccharides,TgandTg’’increaseincreasewiththemolecularweightuptoawiththemolecularweightuptoacertainlimit.certainlimit.Fig0.6Table0.8Fig0.6Table0.8ConclusionConclusionTherateofafoodTherateofafood’’schemicalandschemicalandenzymaticreactionsaswellasthatofitsenzymaticreactionsaswellasthatofitsphysicalprocessesbecomesalmostzerophysicalprocessesbecomesalmostzerowhenthefoodisstoredatthephasewhenthefoodisstoredatthephasetransitiontemperatureofTgorTgtransitiontemperatureofTgorTg’’..MeasurestoimprovestoragelifebyMeasurestoimprovestoragelifebyincreasingTgorTgincreasingTgorTg’’canincludethecanincludetheextractionofwaterthroughdryingand/orextractionofwaterthroughdryingand/oranimmobilizationofwaterbymeansofanimmobilizationofwaterbymeansoffreezing,orbyaddingpolysaccharides.freezing,orbyaddingpolysaccharides.