Doughthermo-mechanicalproperties:influenceofsodiumchloride,mixingtimeandequipmentA.Angioloni*,M.DallaRosaAbstractThermo-mechanicalpropertiesofdoughspreparedfromcommonwheatflourwereinvestigatedunderdifferentkneadingconditionsandwithdifferentamountsofsodiumchloride.Dynamicmechanicalthermalanalysisshowedthathigh-speedmixingandtheadditionofsalttodoughslowedheat-inducedreactionssuchasstarchgelatinisationandproteincoagulation.Theeffectofdoughmixingtechnologywasmoresignificantthantheamountofsodiumchlorideinmodifyingdoughrheologicalcharacteristics.q2004ElsevierLtd.Allrightsreserved.Keywords:Dough；Mixing；Sodiumchloride；Thermo-mechanicalproperties；Starchgelatinization1.IntroductionThefirststepinabakingprocessismixingthedough；howthemixingisperformedandtheingredientsareincorporatedanddispersedlargelydeterminethefinalqualityofthebakedproduct(Aamodtetal.,2003；BasaranandGocmen,2003).Theproductionofwheatdoughisaprocessinwhichrawmaterials(mainlyflour,water,saltandyeast)aremixedandsubjectedtoalargerangeofstrainsituations.Doughisacomplexmixtureofstarch,protein,fatandsalt.Mixinghasthreeimportantfunctions:(i)itblendstheingredientsintoamacroscopicallyhomogeneousmass,(ii)itdevelopsthedoughintoathree-dimensionalviscoelasticstructurewithgas-retainingpropertiesand(iii)itincorporatesairwhichwillformnucleiforgasbubblesthatgrowduringdoughfermentation(Bloksma,1990；ColladoandLeyn,2000；DobraszczykandMorgenstern,2003；HoseneyandRogers,1990；Naeemetal.,2002).Bothmixingintensityandmixingenergymustbeaboveaminimumcriticalleveltodevelopthedoughproperly,thelevelvaryingwithflourandmixertype(KilbornandTipples,1972；MacRitchie,1986；Skeggs,1985；Zhengetal.,2000).Thetimerequiredforoptimumdoughdevelopmentispositivelycorrelatedwiththepolymericproteincompositionandthebalancebetweenproteinpolymersandmonomers(DobraszczykandMorgenstern,2003；MacRitchie,1992；Millar,2004).Rheologicalpropertieschangeduringeverystageofthedoughmakingprocess；stressconditionsarehighwhenthedoughismixedinhigh-speedmixers,tobecomeanelasticandcoherentmass.Mixingspeedandenergy(workinput)mustbehigherthanacertainvaluetodeveloptheglutennetworkandtoproduceasuitablebreadmakingdough.Ontheotherhand,anoptimalmixingtimehasbeenrelatedtooptimumbreadmakingperformancewhichvariesdependingonmixertypeandingredients(DobraszczykandMorgenstern,2003；Manietal.,1992).Forexample,kneadingdoughstoreachoptimumdevelopmentusingelongationalflowinsheeting,requiredonly1015%oftheenergygenerallyimpartedbyconventionalhighspeedshearmixers,suggestingthatmuchhigherratesofworkinputcanbeachievedduetotheimprovedstrainhardeningofdoughunderextension(DobraszczykandMorgenstern,2003；KilbornandTipples,1974；Millar,2004)Starch,themajorcomponentofwheatflour,makingupabout80%ofitsdryweight,influencesdoughrheologicalproperties,especiallyuponheatinginthepresenceofwaterwhenstarchgelatinises(LiandYeh,2001).Thegelatinizationprocessincludesanumberofchanges:absorptionofwaterandswellingofthegranules,changeinsizeandshapeofthegranules,lossofbirefringenceandX-raydiffractionpattern,leachingofamylosefromthegranulesintothesolventandtheformationofapaste(Atwelletal.,1988).Atreducedwatercontents,suchasindough,thechangesresultingfromgelatinisationarestronglydependentontheamountofwateravailable(Eliasson,1983；Seetharamanetal.,2004).Theincreaseinviscosityduetostarchgelatinisationhasbeensuggestedtomodifystructuralpropertiesofdough.Inaddition,thepresenceofsodiumchlorideisknowntoaffectdoughproperties；salttoughenstheproteinandhelpsinconditioningthedoughbyimprovingitstolerancetomixing；theadditionofsaltproducedamorestableandstiffdough(Galaletal.,1978；ShiuandYeh,2001).Moreoveritisknownthatwhensaltisaddedtothedough,heat-inducedreactionssuchasstarchgelatinisationandproteincoagulation,areslowed.Theaimofthepresentworkwastoanalysetheeffectsofincreasingsodiumchlorideconcentrationanddifferentkneadingconditionsonseveraldoughthermo-mechanicalproperties,usingadynamicalstress-straincontrolledrheometer.2.MaterialsandmethodsCommercialwheatflourwasfromMulinoPivetti(Italy),sodiumchloride,fromCarloErba(Italy).AACC(2000)methodswereusedtodeterminemoisture(44-19),ash(08-01),protein(46-10)andgluten(38-12)intheflouranditsAlveographcharacteristics.Doughsampleswith50%moisturewerepreparedinaccordancewithAlveographmethodAACC54-30A(2000),usingtwodifferentmixersandmixingtimes.Inthefirst(sampleA)theAlveographmixerwasusedwithstandardconditions(250gofflourwasmixedwithwaterfor7mintoformthedough).Inthesecond(sampleM)aprototypemixerwasusedwheretheingredientswerekneadedforonly15sbutathigh-speed(1500rpm).Inthiswayhighamountsofenergyweretransferredtothedough.Theprototypemixerhadaparallelpipedshape(12!8!12cm)withtwoverticalarmsoperatedbya1.5kWmotor(Gamars.r.l.,VE-Italy).Sodiumchloride,04.5%,drybasis(d.b.)wasadded,foreachdifferentkneadingconditionandmixertype(Table1).Beforerheologicalanalysisalldoughswererestedfor30minatroomtemperatureinaplasticcontainer.Doughtemperaturesattheendofkneadingwere26288CforsampleAandw358CforsampleM；althoughtheuseofprototypemixerrapidlyThermomechanicaltestsweremadeusingacontrolledstress-strainrheometer(MCR300,Physica/AntonPaar；Messtechnik,Ostfildern,Germany),usingparallel-plategeometry(25mmplatediameter,2mmplategap).Theupper,serrated25mmplatewaslowereduntilthethicknessofsamplewas2mmandexcesswastrimmedoff.Theexposedsurfacewascoveredwithathinlayerofmineraloiltopreventmoisturelossduringtesting.Thesamplewasrestedanother15minintherheometer,beforeeachmeasurement,allowingrelaxationofstressesinducedduringsampleloadingtorelax.Allmeasurementswereperformedataheatingrateof0.88C/minatfixedfrequencyof1Hzwiththeoscillationamplitudesmallenoughtoensurelinearviscoelasticity.Thedataarereportedasmeansofmeasurementsmadeonthreesamples,whereeachsamplewasobtainedfromaseparatelypreparedbatchofdoughforeachformulationandforthedifferentmixersused.Significantdifferencesinstoragemodulus(G0)at1HzweredeterminedbyLeastSignificantDifferenceanalysiswithP%0.05.AllstatisticalanalyseswereperformedwiththeStatSoftVersion6.3.Resultsanddiscussion3.1.FlourchemicalandphysicalpropertiesThechemicalcompositionandrheologicalpropertiesoftheflourareshowninTable2.AnalysisofAlveographdatacategorisestheflourusedasweak,andasseenbytheP/Lratio,theglutenisricheringliadinsthaningluteninsTheresistanceofaglutendoughtoextensiondecreasesandextensibilityincreaseswithanincreasinggliadintogluteninratio(Grasbergeretal.,2003；Kimetal.,1988；Uthayakumuranetal.,2000).3.2.Thermo-mechanicalpropertiesofdoughsThesameamountsofsalt(0,2.5,3.5,4.5%d.b.)wereaddedtobothsamples(A；M)tochecktheeffect