Application of high pressure processing to improve the functional properties of pale,

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InnovativeFoodScienceandEmergingTechnologies

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Applicationofhighpressureprocessingtoimprovethefunctionalpropertiesofpale,soft,andexudative(PSE)-liketurkeymeat

JackyT.Y.Chan,DileepA.Omana,MirkoBetti⁎

DepartmentofAgricultural,FoodandNutritionalScience,UniversityofAlberta,Edmonton,Alberta,CanadaT6G2P5

articleinfoabstract

ImprovementoffunctionalandrheologicalpropertiesofturkeybreastmeatproteinswithdifferentultimatepHsat24hpost-mortem(pH24)wasattemptedusinghighpressureprocessing(upto200MPafor5minat4°C).Pressuresof50and100MPawerefoundtoincreasethewaterholdingcapacityoflowpHmeat.Atthesepressures,higherproteinsurfacehydrophobicityandgreaterexposureofsulfhydrylgroupswereevident.Theseelementsmayhavecontributedtoimprovedwaterretentionpropertiesofthetreatedprotein.Theformationofabettergelnetworkwasalsoevidentat50and100MPaasrevealedbythedynamicviscoelasticbehavior.Applicationofhighpressuresignificantly(Pb0.05)increasedtotalproteinsolubilityinbothlowandnormalpHmeats.AggregationofmyofibrillarproteinsincreasedinlowpHmeatathigherpressure(200MPa)asrevealedbySDS-PAGEprofile.

Industrialrelevance:Amajorconcerninthepoultryindustryisreducedmeatfunctionality,suchaslowwaterholdingcapacity(WHC)inlowpHpoultrymeatleadingtoreducedyieldcausingeconomiclossintheproductionoffurtherprocessedproducts.Analternativetechnologytoreducesaltandimprovewaterretentionpropertiesisbytheapplicationofhighpressureprocessing(HPP)toproducehealthierfoodproducts.

©2011ElsevierLtd.Allrightsreserved.

Articlehistory:

Received16December2010Accepted19March2011

Keywords:PSEDFDTurkey

UltimatepH

FunctionalandrheologicalpropertiesHighpressureprocessing

1.Introduction

Globalturkeyproductionhasbeengrowingsteadilyworldwideoverthepastfewdecades.Traditionallythepoultryindustrymarketwasprimarilyonwholebirdsandcut-upproducts.Morerecently,processorshaveadaptedtochangingtrendsasconsumersincreasetheirdemandformoreconvenient,readytoeatproducts(PoultryMarketplace,2010).Increasingoccurrenceofapale,soft,exudative(PSE)-likemeatconditioninturkeys,similartotheoneobservedinpigs,hasbecomeamajorconcerninthepoultryindustrysinceitaffectsimportantmeatqualityattributesinvolvedintheproductionofvalue-addedproducts.Inpigs,thecombinationofrapidpost-mortempHdeclineandhighcarcasstemperaturecausesproteindenaturationinthemuscle,whichleadstoreducedproteinfunctionality,suchasdecreasedwaterholdingcapacity(WHC)(Santos,Roseiro,Goncalves,&Melo,1994).Theconditioninpoultryseemstobemorerelatedtotheextentofpost-mortemacidification(i.e.lowpHat24h)ratherthanfastpost-mortempHdeclineafterslaughter(Fraqueza,Cardoso,Ferreira,&Barreto,2006).Ithasalsobeenshownthatinbroilers,lowpost-mortemmusclepHledtodecreasedWHCandweakergelformation(Zhang&Barbut,2005).Thus,reducedproteinfunction-⁎Correspondingauthorat:DepartmentofAgricultural,FoodandNutritionalScience(AFNS),603GeneralServicesBuilding,UniversityofAlberta,Edmonton,Alberta,CanadaT6G2P5.Tel.:+17802481598;fax:+17804926739.

E-mailaddress:Mirko.Betti@ales.ualberta.ca(M.Betti).1466-85/$–seefrontmatter©2011ElsevierLtd.Allrightsreserved.doi:10.1016/j.ifset.2011.03.004

alityinlowpHpoultrymeatmayhaveseriousconsequences.ItleadstoanestimatedeconomiclossofmorethanU.S.$200,000,000peryearintheturkeyindustryinfurtherprocessedproducts(Owens,Alvarado,&Sams,2009).ThesuitabilityofPSEmeatcanbeincreasedfortheproductionoftheseproductsbyvaryingprocessingconditionssuchasmarinationwithsaltorphosphatesusingtumblersorinjectorstoincreasethejuiciness(Barbut,2009).However,theadditionoftheseingredientsmayposeaproblemforconsumersduetohealthconsciousnessandademandforadditive-freeproducts.

Highpressureprocessing(HPP)iscurrentlybeingusedbythemeatindustryasapost-processingtechnologytoextendshelflifeandimprovethesafetyofreadytoeatmeatproducts(Jofré,Garriga,&Aymerich,2008).Highpressure(upto1000MPa)canaffectproteinconformationandmayleadtoproteindenaturation,aggregationorgelation,dependingonfactors,suchastheproteinsystem,appliedpressureandtemperature,andthedurationofthepressuretreatment(Messens,VanCamp,&Huyghebaert,1997).Highpressureprocessingcanalsobeusedasameanstoimprovethefunctionalpropertiesofmuscleproteins(Macfarlane,1974).Animportantaspectinmeatprocessingisthesolubilityoftheproteinsasitisrelatedtomanyoftheirfunctionalproperties.Studiesreportedthatlowpressuretreatmentat150MPaand200MPaincreasedtheproteinsolubilityofsheepmyofibrillarproteins(Macfarlane,1974)andchickenmyofibrils(Iwasaki,Noshiroya,Saitoh,Okano,&Yamamoto,2006),respectively.PartialreplacementofadditivessuchasNaClandpolyphosphatesispossibleusinghighpressureprocessing,sinceit

J.T.Y.Chanetal./InnovativeFoodScienceandEmergingTechnologies12(2011)216–225217

hasasimilareffectonmyofibrillarproteinsastheadditives.ArecentstudyconductedbySikes,Tobin,andTume(2009)reportedalargeincreaseinmyofibrillarproteinsolubilityandimprovementinwaterretentionofcookedproductsandtexturalpropertiesoflow-saltbeefsausagebatterswiththeapplicationofhighpressureupto400MPa.Hence,ithasbeenshownthathighpressureprocessingiseffectiveinimprovingthefunctionalpropertiesofproteins,whileallowinglowsaltlevelstobeusedinfoodprocessingofvariousmeatproducts.

PreviousstudieshaveshownthatlowermusclepHisassociatedwithlowerwaterholdingcapacity,asevidentinpaleturkeymeat(Barbut,1993;Fraquezaetal.,2006).HighpressureprocessingmayinducechangesinlowpHturkeybreastmeatproteinstopositivelyinfluencewaterholdingcapacity.Wehypothesizethathighpressureprocessingusedatlowpressurelevels(50–200MPa)forshorttimeasapre-treatmentbeforethermalprocessing,canimprovethewaterretentionpotentialofmeatproteins.Infact,pre-treatmentusinghighpressureprocessing(50–300MPa)beforecookinghasbeenshowntoimprovegelformingabilityinfishsurimi(Hsu&Jao,2007).Toourknowledge,nostudieshavebeenreportedonimprovingproteinfunctionalityofPSE-liketurkeybreastmeatwiththeapplicationofhighpressureprocessing.Hence,theaimofthisworkwastostudytheeffectsofhighpressureprocessingonturkeybreastmeatwithlowandnormalpHat24hpost-mortemanditsrelationshipwithpressureinducedchangesonproteinfunctionality.2.Materialsandmethods2.1.Sampleselection

Turkeyswereslaughteredat106dayofageandtheaverageflockweightwas11.1kg.Atotalnumberof35skinless,bonelessbreastfilletsfromHybridTomturkeyswereinitiallyselectedfromalocalprocessingplant(LilydaleInc.,Edmonton,Alberta,Canada)at24hpost-mortem.Theyconsistedof26paleand9normalfillets.Selectionwasmadeusingcolormeasurementbasedonlightness(L*)valuesasreportedbyZhangandBarbut(2005).Thesampleswerelabeledandplacedindividuallyinplasticbags,packedoniceandtransportedtothelaboratory.L*valuesandultimatepHat24hpost-mortem(pH24)weremeasuredagainonall35breastsamplesinthelaboratory.Eightbreastsampleswerefurtherselectedwithineachclass(paleandnormal),accordingtotheirpH24.Inthisexperiment,twoclassesofmeatwithdifferentpHswereselectedbecauseultimatepHhasbeenindicatedasavalidtooltodifferentiateproteinfunctionality.Insummary,thecolorandpHcharacteristicsarereportedinTable1.TheaverageL*andpHvaluesofthesampleswerewithinthefollowingrange:pale(L*N52,pH≤5.7)andnormal(46bL*b52,pH≥6.0)andreferredtoaslowandnormalpHmeats,respectively.EachfilletwaslabeledaccordingtotheclassandwasmincedindividuallyinaKitchen-Aidfoodprocessor(ModelKFP7500B,KitchenAid,St.Joseph,MI,USA)for2min.Thefoodprocessorwaspre-chilledandoperatedinacoldroom(4°C)topreventthetemperatureofthesamplesexceeding10°Cthroughoutthemixingprocess.Thesampleswithineachclasswerethenmixedhomogeneouslytoobtaintwobattersof

Table1

PhysicalpropertiesoflowandnormalpHmeats⁎.Measurement

ClassofmeatLowpHmeat

NormalpHmeatL*(24h)52.1±0.9a46.4±0.8ba*(24h)2.9±0.3b5.3±0.8ab*(24h)0.84±0.90a−0.22±0.77bpH245.68±0.09b6.08±0.10aMeanswithineachrowwithnocommonsuperscriptdiffersignificantly(Pb0.05).L*=lightness;a*=redness;b*=yellowness;andpH24=ultimatepHat24hpost-mortem.

⁎Resultsarepresentedasmeans±standarddeviations(n=8).

lowandnormalpHmeats.TheaveragepHvaluesoflowandnormalpHmeatbatterswere5.75±0.06and6.01±0.03,respectively.Alltheanalyseswerecarriedoutinfrozenmeatsamplesthatwerevacuumpackagedinpolyethylenebagsandstoredat−30°Cfor3weeksandthawedovernightat4°C.2.2.Colormeasurements

Thecolorimeter,MinoltaCR-400(KonicaMinoltaSensingAmericas,Inc.,Ramsey,NJ07446)wascalibratedusingastandardwhiteceramictile.ColorwasmeasuredontheinternalsideofturkeybreastsinanareafreeofobviouscolordefectstogetauniformcolorreadingwithilluminantD65asthelightsource.L*,a*andb*refertolightness,redness,andyellowness,respectively.2.3.pHmeasurements

Approximately5gofmincedturkeybreastmeatwashomoge-nizedwith45mLofdistilledwaterandthepHofthehomogenatewasdeterminedusingapHmeter(UB-10,UltraBasicpHmeter,DenverInstrument,Bohemia,NY,USA).

2.4.Meatbatterformulationforhighpressureprocessing

Theformulationofbattersusedinthisstudywaschosenbasedonapreliminarystudyconductedinourlaboratory.BatterswerepreparedwithNaCl(0.5%w/w)andvaryingconcentrationsofwater(10,20and30%)andsubjectedtohighpressureat100and200MPa.Resultsshowedthathighpressureat100MPaincreasedproteinsolubilityandimprovedwaterholdingcapacityofbatterswith10%watercomparedtoallotherbatterswithnopressuretreatment(control)andpressuretreatmentat100and200MPa.Thus,theoptimalbatterformulationforthisstudywasdeterminedtocontain10%water.Rawmeatbatterswerepreparedbycomminutingmincedturkeymeat(.5%)withwater(10%)andNaCl(0.5%)usingamotorandpestle.Duringpreparation,thebatterwasmaintainedatatemperatureoflessthan10°C.Thebatterswerethenfilledintocryovials(12.0mm-diameterand24.0mm-height)of2mLcapacityforhighpressuretreatments.2.5.Highpressureprocessing

PressuretreatmentswereperformedusingaU111highpressuremultivesselapparatus(UNIPRESSEquipmentDivision,Warsaw,Poland),whichhasamaximumpressurelimitof800MPaandiscapableofoperatingwithtemperaturebetween0°Cand120°C.Theapparatushasfourpressurechamberstoholdsamples,eachwithamaximumsampleenvelopedimensionof12.4mm-diameterand60.0mm-height.Thepressuremediumusedinthesamplechamberswaspropyleneglycol.Theapparatusisthermostatedbyaheatexchangerconnectedtoanexternalcirculator.Thetemperaturewasmaintainedbyathermostatingcirculatorbath(LaudaProlineRP855LowTemperatureThermostat,GMBH&Co.Lauda-Konigshofen,Germany).Theapparatushasonehighpressuretransducerbetweentheintensifierandthevessels,whichmonitoredthepressureprofileduringtreatmentcycles.Thebattersweresubjectedto50,100,150and200MPaat4°Candwereheldfor5min.Thetimerequiredtoreachpressurewas20,30,38,and43sfor50,100,150,and200MPa,respectively.Allsampleswerethenkeptat4°Cforanalyses.2.6.Proteinsolubility

SarcoplasmicandtotalproteinsolubilitiesweredeterminedaccordingtothemethodasdescribedbyVanLaack,Liu,Smith,andLoveday(2000)withmodifications.Forsarcoplasmicproteinsolubility,2gofmeatwashomogenizedwith40mL0.03Mphosphatebuffer(pH

218J.T.Y.Chanetal./InnovativeFoodScienceandEmergingTechnologies12(2011)216–225

7.4).HomogenateswerecentrifugedusingAvantiJ-Erefrigeratedcentrifuge(BeckmanCoulter,Inc.,PaloAlto,CA,USA)at15,300×gfor15minat4°CandfilteredthroughWhatmanNo.1filterpaper.Theproteinconcentrationofthefiltrate(proteinsolution)wasassessedusingtheBiuretprocedure(Gornall,Bardawill,&David,1949)withbovineserumalbumin(BSA)asstandard.Fortotalproteinsolubility,1gofmeatwashomogenizedwith40mL0.05Mphosphatebuffercontaining0.55MKI(pH7.4).Homogenateswerecentrifuged,filteredandanalyzedforproteinconcentrationasdescribedforthesarcoplas-micproteinsolubility.

2.7.Proteinsurfacehydrophobicity

SurfacehydrophobicityofsarcoplasmicandmyofibrillarproteinswasdeterminedaccordingtothemethodasdescribedbyKim,Park,andChoi(2003)usinghydrophobicfluorescentprobes,1-anilino-8-naphthalene-sulfonate(ANS;8mMin0.1Mphosphatebuffer,pH7.4).SarcoplasmicproteinsolutionwaspreparedasdescribedintheMaterialsandmethodssectiononproteinsolubility.TheproteinconcentrationwasassessedusingtheBiuretprocedure(Gornalletal.,1949)withbovineserumalbuminasstandard.Theproteinsolutionwasseriallydilutedwith0.03Mphosphatebuffer(pH7.4)toafinalvolumeof4mLtoobtainaproteinconcentrationrangingfrom0.008to0.03%.ANSsolution(20μL)wasaddedtothesamplesolutionandtherelativefluorescenceintensity(RFI)ofANSproteinwasmeasuredwithaspectrofluorometer(ThermoElectronFluoroskanAscent,Vantaa,Finland)using355nmand460nmastheexcitationandemissionwavelengths,respectively.ThenetRFIwasobtainedbysubtractingtheRFIofeachsamplemeasuredwithoutANSfromthatwithANS.Theinitialslope(Ho)oftheRFIagainstproteinconcentration(expressedin%),calculatedbylinearregressionanalysis,wasusedasanindexofproteinsurfacehydrophobicity.Myofibrillarproteinsolutionwaspreparedafterseparationofsarcoplasmicproteins.Thesedimentwasfurtherhomogenizedwith40mLof0.05Mphosphatebuffercontaining0.55MKI(pH7.4).Homogenateswerecentrifugedat15,300×gfor15minat4°CandfilteredthroughWhatmanNo.1filterpaper.Therestoftheassaywasthesameasthatofsarcoplasmicproteinsurfacehydrophobicity.2.8.Reactive(free)andtotalsulfhydrylcontent

Reactive(R-SH)sulfhydrylcontentwasdeterminedaccordingtothemethodasdescribedbyKimetal.(2003).25mLoftris-glycinebuffer(pH8.0)containing5mMEDTAwasaddedto2.5gofmeatandwasintermittentlyvortexedfor20mintoobtainahomogenizedmixture.Thehomogenatewasfilteredandto1mLoffiltrate,4mLoftris-glycinebufferandanaliquot(50μL)ofEllman'sreagent(10mM5,5′-dithiobis(2-nitrobenzoicacid))wasaddedtodeterminethereactivesulfhydrylcontent.TheproteincontentofthefiltratewasdeterminedusingtheBiuretprocedure(Gornalletal.,1949).Themixturewaskeptat4°Cfor1hwithoccasionalstirring.Total(T-SH)sulfhydrylcontentwasdeterminedaccordingtothemethodasdescribedbyChoiandPark(2002)withmodifications.To1mLoffiltrate,4mLof10Mureaand50μLofEllman'sreagentwereadded.Themixturewaskeptatroomtemperature(25°C)for1hwithoccasionalstirring.Theabsorbancewasmeasuredat412nmusingaspectrophotometer(V-530,JascoCorporation,Japan).Thesulfhydrylcontentsweredeterminedusingtheextinctioncoefficientof13,600M−1cm−1andexpressedasμmol/gofprotein.2.9.Cookingofsamples

Thesampleswereheatedat95°Cinawaterbathuntiltheinternaltemperatureofthesamplesreached75°C,atwhichthesamplesareconsideredcooked.Thetemperaturewascheckedusingthermocou-plesinsertedinthecenterofthesamples.Aftercooking,thesamples

werecooledatroomtemperaturefor15minandstoredat4°Covernightforexpressiblemoisture(cookedsamples)andtextureprofileanalyses.

2.10.Expressiblemoisture

Thewaterholdingcapacity(WHC)ofthesamplesasmeasuredbyexpressiblemoisture(EM)wasdeterminedaccordingtothemethodasdescribedbyOmana,Moayedi,Xu,andBetti(2010).Rawandcookedsamples(300mg)wereplacedonapre-weighedWhatmanNo.1filterpaperandwereplacedbetweentwoglassplates.Usingthetextureprofileanalyzer(TA-XTExpress,Stablemicrosystems,Ltd.,Surrey,England),inadhesivetestmode,thesamplesweretestedwithatargetforceof1000gfor2min,whichwassufficienttoexpressthewatercontent.Afterthetest,thefilterpaperwiththeabsorbedwaterwasimmediatelyweighed.Expressiblemoisturewasmeasuredasthequantityofwaterreleasedpergramofmeatandwasexpressedasapercentage.

Expressiblemoistureð%Þ=

ðWeightofwetpaper−WeightofdrypaperÞ

Weightofmeat

×100

>

2.11.Sodiumdodecylsulfate-polyacrylamidegelelectrophoresis(SDS-PAGE)SDS-PAGEwascarriedoutbythemethodofLaemelli(1970)using10–20%readygels(Bio-RadLaboratories,Inc.,Hercules,CA)ataconstantvoltagemode(200V)inaMini-PROTEANtetracellattachedtoaPowerPacBasicelectrophoresisapparatus(Bio-RadLaboratoriesInc.,1000AlfredNobelDrive,Hercules,California,USA).20μgofproteinwasloadedforallsamples.ProteinmarkersofhighrangemolecularweightobtainedfromBio-Rad(Bio-RadLaboratories,Inc.,Hercules,CA)wereloadedintoaseparatewellforcomparisonofmolecularweights.GelsafterstaininganddestainingwerescannedusinganAlphaInnotechgelscanner(AlphaInnotechCorp.,SanLeandro,CA)withFluorChemSPsoftware.2.12.Textureprofileanalysis

Textureprofileanalysis(TPA)wascarriedoutusingatextureprofileanalyzer(TA-XTExpress,StableMicroSystems,Ltd.,Surrey,England)underTPAtestmode,bythemethodofOmanaetal.(2010).SampleswerepreparedasdescribedintheMaterialsandmethodssectiononcookingofsamples.Afterovernightstorageat4°C,thesampleswerecutintocylindricalshapes(1.0cm-height)forTPA.Adoublecompressioncycletestwasperformedupto50%compressionoftheoriginalheightwithanaluminumcylinderprobeof5cmdiameter.Theelapsetimebetweenthetwocompressioncycleswas1s.Thetriggerforceusedforthetestwas5gwithatestspeedof5mm/s.Whenthetestwascompleted,thesoftware(TA-XTExpressSoftware)calculatedvaluesforhardness(maximumforceinNrequiredtocompressthesample),springiness(abilityofthesampletorecoveritsoriginalformafterdeformingforcewasremoved),chewiness(workneededtochewasolidsampletoasteadystateofswallowing),cohesiveness(extenttowhichthesamplecouldbedeformedpriortorupture),andresilience(abilityofthesampletoregainitsoriginalpositionaftercompression).2.13.Dynamicviscoelasticbehavior

Dynamicviscoelasticbehavior(DVB)ofrawmeatsampleswascarriedoutbythemethodofOmanaetal.(2010)withmodificationsusingaPhysicaMCRRheometer(AntonPaarGmbH,Virginia,USA)equippedwitha2.5cmparallelplatemeasuringgeometry,inthetemperaturerangeof7°Cto80°C(heating)and80°Cto7°C(cooling),underoscillatorymode.Thegapbetweenthemeasuring

J.T.Y.Chanetal./InnovativeFoodScienceandEmergingTechnologies12(2011)216–225219

geometryandthepeltierplatewasadjustedto1000μm.Themeasurementsweremadeundercontrolledstrain(0.5%)withafrequencyof1.0Hz.Linearviscoelasticregion(LVR)wasdeterminedusingamplitudesweepinarangeofdeformationfrom0.1to10%.Theheatingandcoolingratesusedwere2°C/min.Theresultsofthemeasurementswereexpressedasthestoragemodulus(G′)andlossmodulus(G″).Theratioofthesetwovalues,tandelta(tanδ),wasalsorecordedthroughouttheheatingandcoolingprocesses.2.14.Statisticalanalysis

Eachassaywascarriedoutin4replicationspermeattype.Reportedresultsrepresentanaverageofeachexperimentalassay.Thedatawereanalyzedasa2×5factorialANOVAusingtheMixedprocedureofSAS(SASversion9.0,SASInstitute,Cary,NC,USA,2006).ThemodeltestedthemaineffectsformeatpHgroup(lowandnormal)andtreatments(control,50MPa,100MPa,150MPaand200MPa)aswellastheinteractiontermusingresidualerrors.DifferencesbetweengroupmeansweredeterminedusingHSDTukeydifferencesandwerereportedassignificantatthePb0.05level.3.Resultsanddiscussion3.1.Proteinsolubility

Solubilityofproteinsisanimportantaspectinmeatprocessingbecauseitisrelatedtomanyfunctionalproperties.Proteinsolubilityisagoodindicatorofproteindenaturation(VanLaacketal.,2000).Inthepresentstudy,theeffectofhighpressureontotalproteinsolubilitywasfoundtobedependentontheclassesofmeatandtheintensityofthetreatmentsasindicatedbythesignificantinteraction(Pb0.05)reportedinFig.1A.TotalproteinsolubilityoflowandnormalpHmeatsforthecontrolsampleswas101.9mg/gand103.4mg/g,respectively,andwasnotstatisticallydifferent,whichisconsistentwithourpreviousstudy(Chan,Omana,&Betti,2011).Ingeneral,applicationofhighpressureprocessingcausedasignificantincreaseintotalproteinsolubilityinbothlowandnormalpHmeatscomparedtothatofcontrol(unpressurized)samples.Iwasakietal.(2006)foundanincreaseinproteinsolubilityofchickenmyofibrilsthatweresubjectedtopressuretreatmentat200MPacomparedtocontrolsamples.Itwashypothesizedthatpressuretreatmentcausedswellingofmyofibrilsandledtodisruptionanddispersionintoshortfilamentswhichincreasedsolventaccessibilitytothesmallermodifiedstructures,leadingtoincreasedsolubilization(Macfarlane,1974;Sikesetal.,2009).Asignificant(Pb0.05)andlargeincreaseintotalproteinsolubilityat50and100MPainbothlowandnormalpHmeatscomparedtocontrolsampleswasobserved.However,thispercentageincreaseinproteinsolubilitywasgreaterinnormalpHmeatcomparedtolowpHmeatatbothpressuretreatments(47.0and48.6%vs.35.6and40.3%,respectively).ThismaybethedirectconsequenceofthedifferentpHvaluesinthetwoclassesofmeat.MyofibrillarproteinsinnormalpHmeatarefurtherawayfromtheisoelectricpoint(pI),whichisusuallyatpH5.3(Offer&Knight,1988),thanlowpHmeat,andthusproteinspackmorelooselytogetherduetoelectrostaticrepulsion.Therefore,theapplicationofpressuretothistypeofmeatismoreeffectiveinpromotingdepolymerizationofmyofibrillarproteins(Cheftel&Culioli,1997).Conversely,inlowpHmeat,electrostaticattractionofmyofibrillarproteinsishighduetothecloseproximitytothepI,andthusmoreenergyisrequiredtodepolymerizetheproteins.

Inthecaseofsarcoplasmicproteinsolubility,asignificantinteraction(Pb0.05)betweenclassesofmeatandpressuretreat-mentswasobserved(Fig.1B).SarcoplasmicproteinsolubilityoflowandnormalpHmeatsforthecontrolsampleswas88.1mg/gand86.5mg/g,respectively,andwasnotstatisticallydifferent.However,sarcoplasmicproteinsolubilitydecreasedwithincreasingpressuresin

Fig.1.EffectofhighpressureprocessingontotalandsarcoplasmicproteinsolubilitiesinlowandnormalpHmeats.A:Totalproteinsolubility(TPS).B:Sarcoplasmicproteinsolubility(SPS).Dissimilarsuperscriptsdenotesignificantdifference(Pb0.05).Resultsarepresentedasmeans±standarddeviations(n=4).

bothlowandnormalpHmeats.Pressurizationat200MParesultedinthemaximumreductioninsarcoplasmicproteinsolubilityforbothclassesofmeat.Marcos,Kerry,andMullen(2010)alsofoundadecreaseinsarcoplasmicproteinsolubilityofbeefmusclesthatwerepressurizedat200MPaat10,20and30°C.Thedecreasedproteinsolubilitymaybeduetocrosslinkingofsarcoplasmicproteinsinducedbyhighpressureprocessing,whichwasmoreapparentatprocessingat200MPaasrevealedbyreductioninreactivesulfhydrylgroups(Fig.3A).Forinstance,sarcoplasmicproteinsappearedtobecovalentlylinkedwithahighlycompactstructureduringhighpressureprocessingoffishmeat(Ohshima,Ushio,&Koizumi,1993).3.2.Proteinsurfacehydrophobicity

Proteinsurfacehydrophobicityrelatestotheextentofdistributionofhydrophobicresiduesontheproteinsurface.Theeffectofhighpressureonmyofibrillarproteinsurfacehydrophobicitywasfoundtobedependentontheclassesofmeatandthepressuretreatmentsasindicatedbythesignificantinteraction(Pb0.05)reportedinFig.2A.Therewasnosignificantdifferenceinmyofibrillarproteinsurfacehydrophobicityoflow(352Ho)andnormal(331Ho)pHmeatsforthecontrolsampleswhichwassimilartoourearlierstudy(Chanetal.,2011).InlowpHmeat,pressureat50MPacausedthegreatestincreaseinsurfacehydrophobicity(450Ho),howeveritwasnotstatisticallydifferentthanthatat100and200MPa.ChapleauanddeLamballerie-Anton(2003)haveshownthatpressuresupto450MPainduceathreefoldincreaseinsurfacehydrophobicityofbovinemyofibrillarproteins.Ikeuchi,Tanji,Kim,andSuzuki(1992)alsoobservedanincreaseinsurfacehydrophobicityofrabbitactomyosinthatwaspressurizedat150MPafor5min.Hummer,Garde,García,Paulaitis,andPratt(1998)andTanakaetal.(2000)reportedthatpressurecausesconformationalfluctuationsofaminoacidsidechainsinproteins.Thiscreatespathwaysforwatertoenterintotheinterioroftheprotein,whichfillsthecavities,andthus,causestheopeningof

220J.T.Y.Chanetal./InnovativeFoodScienceandEmergingTechnologies12(2011)216–225

theproteinstructure.Hence,thismayexposehydrophobicgroupsontheproteinsurfaceleadingtohigherhydrophobicity.

InthecaseofnormalpHmeat,therewasnostatisticaldifferenceinmyofibrillarproteinsurfacehydrophobicityofpressuretreatedsamples(Fig.2A).ThisindicatedthatmyofibrillarproteinsinnormalpHmeatweremorestableunderhighpressurecomparedtothatoflowpHmeat.However,at50and100MPa,myofibrillarproteinsurfacehydrophobicitywashigherinlowpHmeatcomparedtothatofnormalpHmeatwhichindicatesmoreproteinunfolding.Hence,theeffectofpressureseemstobemoreevidentinlowpHmeataffectingthestructuralchangesinproteins.Althoughthemechanismisnotimmediatelyclear,somehypothesesareworthconsidering.ItcanbespeculatedthatproteinshaveincreasedconformationalfluctuationsintheaminoacidsidechainsinlowpHmeatduringpressureprocessingprovidingmorepathwaysforwatertodiffuseintotheproteininterior(Tanakaetal.,2000).Asaconsequence,thiswouldcauseanincreasedswellingofproteins,whichleadstomoreproteinunfolding.PressuremayalsoalterthestructureoftheprotectinglayerofwatersurroundingtheproteinsurfacemoreeffectivelyinlowpHmeatinducingconformationalchangesinproteins(Hayakawa,Linko,&Linko,1996).NMRstudiesinmeatseemtosupportthishypothesis,wheretheresearchersinvestigatedwatercompartmentalizationanddistributioninmuscletissue(Bertram&Andersen,2007).EarlierstudyrevealedthatrelaxationtimesweredependentonpHdecreaseinpost-mortemmuscle(Bertram,Whittaker,Andersen,&Karlsson,2003).Theyfoundbroaddistribu-tionsoflongerrelaxationtimesinlowpHmeatthaninnormalmeat,whichsuggestschangesinthewaterlayerssurroundingtheproteins.Furthermore,inlowpHmeat,anincreaseinmyofibrillarproteinsurfacehydrophobicitymayalsobethereasonfordecreasedtotalproteinsolubilityat50and100MPa.

Fig.2.EffectofhighpressureprocessingonmyofibrillarandsarcoplasmicproteinsurfacehydrophobicitiesinlowandnormalpHmeats.A:Myofibrillarsurfaceproteinhydrophobicity(MPH).Dissimilarsuperscriptsdenotesignificantdifference(Pb0.05).B:Sarcoplasmicproteinsurfacehydrophobicity(SPH).Dissimilarsuperscriptsdenotesignificantdifference(Pb0.001).Resultsarepresentedasmeans±standarddeviations(n=4).

Sarcoplasmicproteinsurfacehydrophobicitywasaffectedbythesignificantinteraction(Pb0.001)betweenmusclepHandpressuretreatments(Fig.2B).SarcoplasmicproteinsurfacehydrophobicityoflowandnormalpHmeatsforthecontrolsampleswas86.1Hoand84.1Ho,respectively,andwasnotstatisticallydifferent.Similarfindingswerealsofoundinourpreviousstudy(Chanetal.,2011).InlowpHmeat,pressureappliedat50MPacausedunfoldingofproteinsasindicatedbysignificantlyhigher(Pb0.001)proteinsurfacehydrophobicitycomparedtoallothersamples.However,aspressurewasincreased,surfacehydrophobicitydecreased,whichmayrevealthatproteinstendtoformamorecompactedstructureresultinginlesshydrophobicgroupsexposedonthesurfaceofproteins.InnormalpHmeat,nostatisticaldifferenceinsurfacehydrophobicityofallsampleswasfound(Fig.2B),whichshowedthatsarcoplasmicproteinsinnormalpHmeatweremorestableunderhighpressurecomparedtothatoflowpHmeat.At50and100MPa,sarcoplasmicproteinsurfacehydrophobicitywashigherinlowpHmeatthaninnormalpHmeat.Asexplainedpreviously,lowpHmeatmaybemoresensitivetopressure,whichcauseschangesintheprotectingwaterlayerasevidencedbyNMRstudies.InlowpHmeat,theincreaseinsarcoplasmicproteinhydrophobicitymaynothaveaffectedproteinsolubilityofsarcoplasmicproteinsbecausethesetypesofproteinscontainmainlyhydrophilicgroupswhichhelpinsolubilization.3.3.Reactive(free)andtotalsulfhydrylcontent

Sulfhydrylgroupsareoneofthemostreactivefunctionalgroupsinproteins.Asignificantinteraction(Pb0.001)wasfoundbetweenmusclepHandpressuretreatmentsonreactivesulfhydrylgroups(Fig.3A).Therewasnosignificantdifferenceinreactivesulfhydrylgroupsoflow(39.9μmol/gprotein)andnormal(38.6μmol/gprotein)pHmeatforthecontrolsamples.Thiswasconsistentwithourpreviousstudy(Chanetal.,2011).InlowpHmeat,pressureat50MPacausedthegreatestincreaseinreactivesulfhydrylgroups(43.7μmol/g).Thismaybeexplainedbythegreaterextentofproteinunfoldingasshownbyhighsurfacehydrophobicityvaluesat50MPa(Fig.2A,B),andthus,agreateramountofreactivesulfhydrylgroupswasalsoexposedontheproteinsurfaceatthispressurelevel.InnormalpHmeat,pressureat50MPaalsocausedthegreatestincreaseinreactivesulfhydrylgroups(44.4μmol/g),althoughnotstatisticallydifferent.Furtherincreaseinpressureto200MParesultedinreductionofexposedreactivesulfhydrylgroups.

Therewasnosignificantinteractionintotalsulfhydrylgroupsbetweenclassesofmeatandpressuretreatments.Totalsulfhydrylgroupswerefoundtobedependentonpressureeffectsonly(Pb0.001)andareshowninFig.3B.Controlsampleshadthehighestamountoftotalsulfhydrylgroups(80.2μmol/g).Thetotalsulfhydrylgroupsslightlydecreasedat50MPaandremainedconstantto200MPa.Ko,Jao,andHsu(2003)havealsofoundadecreaseintotalsulfhydrylcontentintilapiamyosinsafterpressurizationat50MParevealingtheformationofdisulfidebonds.3.4.pH

PressuretreatmentofmeatandmeatproductsisknowntoproduceasmallincreaseinpH(Angsupanich&Ledward,1998),whichmayresultfromadecreaseinacidicgroupsduetoconformationalchangesofproteinsassociatedwithdenaturation(Poulter,Ledward,Godber,Hall,&Rowlands,1985).Inthepresentstudy,themeanpHoflowpHmeatcontrolsampleswas5.51andwassignificantlylower(Pb0.001)thanthatofnormalpHmeatcontrolsamples(5.65)(Table2).UnexpectedlyandincontrasttoAngsupanichandLedward(1998),inlowpHmeat,pressureat50and100MPacausedadecreaseinpH.Thismayberelatedtomoreexposureofacidicgroupsontheproteinsurfaceduetoproteinunfoldingasrevealedbysurfacehydrophobicitydata,andhencepHdecreases.Ontheotherhand,pressureat150and

J.T.Y.Chanetal./InnovativeFoodScienceandEmergingTechnologies12(2011)216–225

221

Fig.3.Effectofhighpressureprocessingonreactiveandtotalsulfhydrylcontents.A:Reactivesulfhydrylcontent(R-SH)inlowandnormalpHmeats.B:Totalsulfhydrylcontent(T-SH)atdifferentpressuretreatments.Dissimilarsuperscriptsdenotesignificantdifference(Pb0.001).Resultsarepresentedasmeans±standarddeviations(n=4).

200MPacausedanincreaseinpH.InnormalpHmeat,pressureat50MPacausedanincreaseinpH,withfurtherincreaseinpHat100MPa,afterwhichitremainedstableat150and200MPa.3.5.Expressiblemoisture

Waterholdingcapacity(WHC)isanimportantpropertywhichdeterminesmeatqualityandisoftenusedtoevaluatePSEmeat(Woelfel,Owens,Hirschler,Martinez-Dawson,&Sams,2002).Expressiblemoisture,agoodindicatorofwaterholdingcapacity,isthepercentageoftotalwaterinthemeatthatcanbeexpressedbyappliedforce.Inthisstudy,expressiblemoisturewasconductedonuncookedandcookedsamples.

Therewasasignificantinteraction(Pb0.001)betweentheclassesofmeatandpressuretreatmentsonexpressiblemoistureofuncookedsamples(Fig.4A).TheexpressiblemoistureoflowpHmeatcontrol(unpressurized)sampleswas25.2%andwassignificantlyhigher(Pb0.001)thanthatofnormalpHmeat(16.9%),whichrevealedthatithadlowerWHCcomparedtonormalpHmeat.Thiswasconsistentwithpreviouswork(Barbut,1993;Fraquezaetal.,2006),whichdemonstratedthatlowermusclepHisassociatedwithlowerWHC.InlowpHmeat,theexpressiblemoisturewasdecreasedat50and100MPa,withthelowestleveloccurringat100MPa(18.7%),whichindicatedanincreaseinWHCatthesepressurelevels.ItwasalsofoundthatinuncookedlowpHmeat,pressureat100MPawasthebesttreatmenttoimproveWHC,closetothelevelasthatofnormalpHmeatcontrolsamples.However,at150MPa,theexpressiblemoisturewasincreasedto24.8%,withsubsequentdecreaseat200MPa(19.4%).Pressurizationat150MPacausedthegreatestdecreaseinWHCinlowpHmeatandwasatthesamelevelascontrol.Thismayberelatedtopoorgelformationwithlesswaterentrapmentduetolesshydrophobicinteractions,asindicatedbylowerproteinsurfacehydrophobicityat150MPa.Lakshmanan,Parkinson,andPiggott(2007)alsoobservedthehighestdecreaseinWHCinsalmonwithpressurizationat150MPa.FornormalpHmeat,theexpressible

Table2

EffectofhighpressureprocessingonpHoflowandnormalpHmeats⁎.ClassofmeatTreatmentpH

LowpHmeat

Control5.51±0.01g50MPa5.35±0.01h100MPa5.30±0.01i150MPa5.56±0.01f200MPa5.55±0.01fNormalpHmeat

Control5.65±0.01e50MPa5.67±0.01d100MPa5.83±0.01a150MPa5.78±0.01c200MPa

5.81±0.01bMeanswithincolumnwithnocommonsuperscriptdiffersignificantly(Pb0.001).⁎Resultsarepresentedasmeans±standarddeviations(n=4).

moistureofthesamplesat50and100MPawerenotstatisticallydifferentthanthatofcontrolsamples,althoughthelowestleveloccurredat100MPa(14.2%).At150MPa,theexpressiblemoisturewasincreasedto21.8%,withsubsequentdecreaseat200MPa(17.6%).

Asignificantinteraction(Pb0.001)wasfoundbetweentheclassesofmeatandpressuretreatmentsontheexpressiblemoistureofcookedsamples(Fig.4B).TheexpressiblemoistureoflowpHmeatcontrolsampleswas18.3%andwassignificantlyhigher(Pb0.001)thanthatofnormalpHmeat(15.2%),whichalsorevealedthatithadlowerWHCcomparedtonormalpHmeat.Sikesetal.(2009)havefoundthattreatmentat200MParesultedinsignificantimprovementinWHCasrevealedbylowercooklossinbeefbatterscontaining0.6%saltcomparedtocontrolsamples.Inthepresentstudy,pressureat50MPaseemstobethebesttreatmenttoimproveWHCofcookedlowpHmeat,closetothelevelasthatofnormalpHmeatcontrolsamples.However,pressuresabove50MPacausedaslightincreaseinexpressiblemoisture.InnormalpHmeat,treatmentatallpressurelevelsdidnotcausefurtherdecreaseinexpressiblemoistureascomparedtocontrolsamples.

Fig.4.EffectofhighpressureprocessingonexpressiblemoistureinlowandnormalpHmeats.A:Expressiblemoisture(EM)inuncookedsamples.B:Expressiblemoisturecookedsamples.Dissimilarsuperscriptsdenotesignificantdifference(Pb0.001).Resultsarepresentedasmeans±standarddeviations(n=4).

222J.T.Y.Chanetal./InnovativeFoodScienceandEmergingTechnologies12(2011)216–225

3.6.Sodiumdodecylsulfate-polyacrylamidegelelectrophoresis(SDS-PAGE)SDS-PAGEprofileoftotal,sarcoplasmic,andmyofibrillarproteinsextractedfromlowandnormalpHmeatsisshowninFig.5.Themajorproteinswhichcontributetomostofthefunctionalpropertiesofmyofibrillarproteinsaremyosinandactin.Inthecontrolsamples,theelectrophoreticbandpatternsofproteinsfromlowandnormalpHmeatsintotal(lanes2and3inFig.5A),sarcoplasmic(lanes2and3inFig.5B),andmyofibrillar(lanes2and3inFig.5C)fractionsweresimilar.IntheSDS-PAGEprofileoftotalproteins(Fig.5A),thebandintensitieswerelessathigherpressurelevels,whichweremoreapparentinlowpHmeatthaninnormalpHmeat.ThemostsignificantchangethatoccurredinlowpHmeatwasat200MPa(lane7),wheretheconcentrationofmyosinheavychaindecreasedcomparedtoothersamples.Thismaybeduetoaggregationasindicatedbyreactivesulfhydrylgroupdata(Fig.3A).Thisaggregationismainlyduetointermoleculardisulfidebondformationathigherpressurelevels(Angsupanich,Edde,&Ledward,1999).IntheSDS-PAGEprofileofsarcoplasmicproteins(Fig.5B),thebandintensitieswerelessabove50and100MPainlow(lanes5,6,and7)andnormalpHmeat(lanes10and11),respectively,specificallythebandatapproximately100kDa.Thedecreasedbandintensitiesmaybecausedbyproteindenaturationleadingtoproteindegradationorinsolubilizationofsarcoplasmicproteins(Marcosetal.,2010).Underhighpressure,certainsarcoplasmicproteinscanbecomecovalentlylinkedtogetherratherthanbedegradedintolowermolecularweightcomponentsandthus,theybecomeresistanttoextractionwithSDS,hencetheintensityofbandswasreduced(Ohshimaetal.,1993).IntheSDS-PAGEprofileofmyofibrillarproteins(Fig.5C),thedecreasedconcentrationofmyosinheavychainwasalsoevidentinlowpHmeatat200MPa(lane7).Inlow(lanes6and7)andnormal(lanes10and11)pHmeatsat150and200MPa,thebandintensitiescorrespondingtoα-actinin(95kDa),actin(43kDa),andtropomyosin(36kDa)werelessintensecomparedtoothersamples,whichrevealedinsolubiliza-tionoftheseproteinsunderpressuretreatment.ChapleauanddeLamballerie-Anton(2003)havefoundthedisappearanceofα-actininbandinbovinemyofibrillarproteinfractionafterpressurizationabove300MPa.

3.7.Textureprofileanalysis

Texturalcharacteristicsareimportantintheproductionoffurtherprocessedmeatproductsbecausetheyaffectconsumeracceptability.MoisturecontentoflowandnormalpHmeatsusedfortextureprofileanalysiswasapproximately77%andwasnotstatisticallydifferentincontrolandpressuretreatedsamplesforthetwoclassesofmeat(datanotshown).TextureprofileanalysisoflowandnormalpHmeatsfordifferenttreatmentsispresentedinTable3.Thetexturalcharacter-isticsofcontrolsamplesoflowandnormalpHmeatswerenotstatisticallydifferent.Sikesetal.(2009)observedanincreaseinhardnessofbeefsausagebatterswith0.5%saltthatweretreatedat200MPacomparedtocontrolsamples.Inthepresentstudy,nodifferencewasfoundinhardnessbetweensamplestreatedathigherpressurelevelsandcontrol.Thismaybeduetodifferenttypesofmeatusedinthebatterpreparation.Springinessistheabilityofasampletorecoveritsoriginalformafterdeformationandchewinessistheworkneededtochewasampletoasteadystateofswallowing(Martinez,Salmeron,Guillen,&Casas,2004).Springinessandchewinessgiveanindicationofjuicinessandtenderness,respectively.InlowpHmeat,pressureat200MPacausedanincreaseinspringinesscomparedtothatofcontrolsamples,whileinnormalpHmeat,thespringinessinallpressuretreatedsampleswasnotdifferentfromthecontrolsamples.InlowandnormalpHmeats,pressurizationdidnotcausefurtherincreaseinchewiness,whilepressureabove100MParesultedingreatercohesivenessinsamples.Pressureat200MPacausedthelargestincreaseinresilienceinthetwoclassesofmeat.Acomplete

Fig.5.SDS-PAGEprofileoftotal,sarcoplasmic,andmyofibrillarproteinsextractedfromlowandnormalpHturkeybreastmeat.A:Totalproteinprofile.B:Sarcoplasmicproteinprofile.C:Myofibrillarproteinprofile.Sampleswereloadedinthesameorder.Lane1:standardmarker;lane2:lowpHmeat(control);lane3:normalpHmeat(control);lane4:lowpHmeat(50MPa);lane5:lowpHmeat(100MPa);lane6:lowpHmeat(150MPa);lane7:lowpHmeat(200MPa);lane8:normalpHmeat(50MPa);lane9:normalpHmeat(100MPa);lane10:normalpHmeat(150MPa);andlane11:normalpHmeat(200MPa).MHC:Myosinheavychains;andMLC:Myosinlightchains.

sensorypanelevaluationisneededtofurtherunderstandconsumeracceptanceintermsoftexturalpropertiesofproducts.3.8.Dynamicviscoelasticbehavior

Smallstraintestordynamicrheologicaltestiswidelyusedtomeasurethepropertiesofheat-inducedgelation,suchasinmeatproteinsystems(Westphalen,Briggs,&Lonergan,2005).Rheograms(heatingandcooling)obtainedforlowandnormalpHmeatsarepresentedinFigs.6and7,respectively.Thestoragemodulus(G′)isameasureoftheelasticcomponentofthenetworkandthelossmodulus(G″)isameasureoftheviscouscomponent.Tandelta(tanδ=G″/G′)isameasureoftheenergylostduetoviscousflowcomparedtotheenergystoredduetoelasticdeformationinasingledeformationcycle.Decreaseintandeltavaluesshowstheformationofbetterthree-dimensionalnetwork(Hamann,1992).Intheheatingprocess,themaximumrateofincreaseinG′valueswasinthetemperaturerangeof40°Cto55°C.ThehighestG′valuesoflow

J.T.Y.Chanetal./InnovativeFoodScienceandEmergingTechnologies12(2011)216–225

Table3

EffectofhighpressureprocessingontexturalpropertiesoflowandnormalpHmeats⁎.

Hardness(N)

Classesofmeat(CM)(n=20)LpHNpH

Treatment(T)(n=8)

Control50MPa100MPa150MPa200MPa

Interaction(CMxT)(n=4)LpH

Control50MPa100MPa150MPa200MPaControl50MPa100MPa150MPa200MPaP-Value

13.8±1.6a13.3±2.6a14.2±1.8a,b11.7±3.1b14.2±1.2a13.8±2.1a,b13.7±1.3a,b14.0±1.6a14.0±0.6a14.4±1.2a12.8±2.7a,b13.5±1.7a,b14.3±2.2a9.4±2.8b14.1±1.4a14.8±0.8a13.8±0.9a0.4070.0420.010

Springiness0.±0.07a0.90±0.06a0.86±0.02b0.85±0.02b0.86±0.02b0.96±0.06a0.95±0.06a0.85±0.01b,c0.84±0.01c0.87±0.01b,c0.94±0.07a,b0.97±0.06a0.88±0.02a,b,c0.86±0.03b,c0.85±0.02b,c0.97±0.05a0.92±0.05a,b,c0.811b0.0010.162

Chewiness885±100a870±182a851±94a,b714±176b2±57a967±138a965±76a810±55b,c839±45a,b910±51a,b888±159a,b980±85a,b3±115a,b5±168c874±a,b1046±49a949±75a,b0.625b0.0010.003

Cohesiveness0.71±0.03a0.71±0.02a0.68±0.04b0.71±0.02a,b0.71±0.02a,b0.72±0.02a0.73±0.01a0.67±0.04b0.70±0.01a,b0.71±0.02a,b0.73±0.03a,b0.73±0.01a0.70±0.03a,b0.72±0.03a,b0.71±0.01a,b0.72±0.01a,b0.73±0.01a0.5030.0040.554

Resilience

223

0.46±0.04a0.48±0.03a0.43±0.03c0.46±0.03b0.47±0.02b0.49±0.02a,b0.51±0.01a0.41±0.04c0.45±0.06b,c0.47±0.02a,b0.49±0.03a,b0.51±0.02a0.44±0.03b,c0.48±0.03a,b0.47±0.02a,b0.48±0.01a,b0.51±0.01a0.108b0.0010.313

NpH

SourceofVariationCMT

CM×T

LpH=LowpHmeat;andNpH=NormalpHmeat.

Meanswithincolumnwithineachanalysiswithnocommonsuperscriptdiffersignificantly(Pb0.05).⁎Resultsarepresentedasmeans±standarddeviations.

Fig.6.RheogramsoflowandnormalpHmeatsduringheating(7°Cto80°C).A:Storagemodulus(G′)oflowpHmeat,B:storagemodulus(G′)ofnormalpHmeat,C:lossmodulus(G″)oflowpHmeat,D:lossmodulus(G″)ofnormalpHmeat,E:tandelta(tanδ)oflowpHmeat,andF:tandelta(tanδ)ofnormalpHmeat.

224J.T.Y.Chanetal./InnovativeFoodScienceandEmergingTechnologies12(2011)216–225

Fig.7.RheogramsoflowandnormalpHmeatsduringcooling(80°Cto7°C).A:Storagemodulus(G′)oflowpHmeat,B:storagemodulus(G′)ofnormalpHmeat,C:lossmodulus(G″)oflowpHmeat,D:lossmodulus(G″)ofnormalpHmeat,E:tandelta(tanδ)oflowpHmeat,andF:tandelta(tanδ)ofnormalpHmeat.

(842kPa)andnormalpH(808kPa)meatsoccurredat69°Candwasobtainedinsamplesthatwerepressurizedat100MPa(Fig.6A,B).TheG′valuesoflow(760kPa)andnormalpH(800kPa)meatthatwereobtainedinsamplespressurizedat50MPawerecomparabletothoseat100MPa,whilethelowestG′valuesoflow(495kPa)andnormalpH(583kPa)wereobtainedinsamplesthatwerepressurizedat200and150MPa,respectively.Thisdataalongwithhigherexpressiblemoisturecontentat150MPaconfirmedthatthegelnetworkformedinnormalpHmeatatthispressurelevelhadlessabilitytoretainwaterinthegelnetworkcomparedtootherpressuretreatedsamples.TheseresultssuggestthatlowandnormalpHhadformedastrongerintermolecularnetworkandhadbettergelformingabilityinsamplesthatweresubjectedtopressureat50and100MPa,comparedtocontrolsamplesandsamplestreatedat150and200MPa.InlowandnormalpHmeats,higherhydrophobicitywasevidentat50and100MPa,leadingtomorehydrophobicinteractionswhichresultedinabettergelnetwork.Duringheating,increaseinG′valuesindicatesthatproteinsunderwentorderedaggregationandformationofathree-dimensionalnetworkwithentrapmentofwaterinthematrix.Theforcesresponsibleforgelationarehydrophobicinteractions,disulphidecrossbridgesandhydrogenbonds(Hamann,1992).ThehighestlossmodulusvaluesoflowpHmeat(160kPa)wereobtainedinsamplesat100MPa,whileinnormalpHmeat(152kPa),theywereobtainedinsamplesat50MPa,whichrevealedtheformationofaviscoelasticgelnetwork(Fig.6C,D).Similartostoragemodulusvalues,thelowestlossmodulusvaluesoflowandnormalpHmeatsoccurredinsamplesat200and150MPa,respectively.Duringheating,tandeltavaluesoflowandnormalpHmeatsamplesat100MPawerethelowestcomparedtotheothersamples,whichsuggestthatpressureat100MPaproducedabetterthree-dimensionalnetworkinthetwoclassesofmeat(Fig.6E,F).Inmuscleproteins,tandeltavaluesdenotetwotransitions;firstoneat52°C(duetodenaturationofmyosin)andthesecondoneat70°C(duetodenaturationofcollagen)(Brunton,Lyng,Zhang,&Jacquier,2006;Westphalenetal.,2005).Inthisstudy,onemajortransitionforlowandnormalpHmeatswasevidentat50.4°C(Fig.6E,F)incontrolandpressuretreatedsamples,whichwasthetemperatureofmyosindenaturation.ThisrevealedthatthetemperatureofmyosindenaturationwasindependentofpH24andpressuretreatment.Inthisstudy,themyosindenaturationtemperaturewassimilartotheonefoundbyOmanaetal.(2010),inwhichtheyhavefound50.1°Cinproteinsfromchickendarkmeat.Thesecondtransitionduetodenaturationofcollagenwasnotevidentinthepresentstudy.Thismaybeduetothelowamountofcollagenpresentinbreastmeat.

Duringcooling,G′andG″valuesoflowandnormalpHmeatsincreaseduntilapproximately20°C,afterwhichtheyslightlydecreased,whichindicatetheformationofcross-linksandrearrange-mentofthenetworkstructure(Fig.7A,B,C,D).ThisincreaseinG′andG″valueswasduetotheformationofhydrogenbonds,whichcontributedtothestabilityandelasticityofmyosingelnetworks(Hamann,1988).TandeltavaluesoflowandnormalpHmeatsremainedrelativelyconstant(Fig.7E,F),whichindicatedthatastablegelhadformedandtheformationofhydrogenbondsduringthecoolingphaseaddedstrengthtoboththeelasticandviscouscomponentstothenetwork(Sun&Arntfield,2010).Thedatafromthedynamicrheologicaltestrevealedthatheatingandcoolinghad

J.T.Y.Chanetal./InnovativeFoodScienceandEmergingTechnologies12(2011)216–225225

causedanincreaseingelrigidityandstrengtheningofthegelnetwork,respectively.4.Conclusions

ThisstudyrevealedthatfunctionalandrheologicalpropertiesoflowandnormalpHmeatcontrolsampleswerenotdifferent,exceptlowerwaterholdingcapacityoflowpHmeat.Incookedanduncookedsamples,pressureat50and100MPa,respectively,wasfoundtobethebesttreatmenttoincreasetheWHCoflowpHmeat,closetothelevelofnormalpHmeatcontrolsamples.InlowpHmeat,pressureat50and100MPacausedmoreproteinunfoldingcomparedtootherpressurelevels,asrevealedbyhighersarcoplasmicandmyofibrillarproteinsurfacehydrophobicityvaluesandreactivesulfhydrylgroups,whichmayhavecontributedtoimprovedwaterretentionproperties.ApplicationofpressurecausedanincreaseintotalproteinsolubilityofbothlowandnormalpHmeats.InlowandnormalpHmeats,samplestreatedatlowerpressurelevels(50and100MPa)hadbettergelformingabilityasrevealedbydynamicviscoelasticbehavior.Thisstudyconcludesthatpressureof50and100MPaarethebesttreatmentstoimprovetheWHCoflowpHmeat.Acknowledgments

ThisstudywassupportedbyfundsprovidedbyAlbertaLivestockandMeatAgency(ALMA)(Edmonton,Alberta,Canada).TheauthorswouldliketoextendappreciationtoLilydaleInc.(Edmonton,Alberta,Canada)forprovidingturkeybreastmeat.TheauthorsextendappreciationtoYanXu(UniversityofAlberta,Edmonton,AlbertaCanada)forhisassistanceandexpresstheirgratitudetoDr.GrahamPlastow(UniversityofAlberta,Edmonton,AlbertaCanada)forhisvaluablesuggestions.References

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