J.Wangetal./FoodChemistry106(2008)804–810805

extraction,microwave-assistedextraction,andacceleratedsolventextraction(Wang&Weller,2006).Amongthese,ultrasound-assistedextractionisaninexpensive,simpleande cientalternativetoconventionalextractiontech-niques.Theenhancementinextractionobtainedbyusingultrasoundismainlyattributedtothee ectsofacousticcavitationsproducedinthesolventbythepassageofanultrasonicwave.Ultrasoundalsoexertsamechanicale ect,allowinggreaterpenetrationofsolventintothesamplematrix,increasingthecontactsurfaceareabetweensolidandliquidphase;asaresultthesolutequicklydi usesfromsolidphasetothesolvent(Rostagno,Palma,&Barroso,2003).Inaddition,thereisnochemicalinvolvementintheultrasound-assistedextraction,whichcouldpreventpossiblechemicaldegradationoftargetedcompounds(Wang&Weller,2006).

Inthepresentstudy,ultrasound-assistedextractionparameterssuchasthesolvent,extractiontemperatureandextractiontimewereoptimisedusingresponsesurfacemethodology(RSM),byemployinga ve-level,three-vari-ablecentralcompositerotatabledesign(CCRD),inordertoobtaintheoptimalconditionsfortheextractionofphe-noliccompoundsfromwheatbran.2.Materialsandmethods2.1.Materials

WheatbranwasobtainedfromamillingplantinBei-jing,China.Thebranwasmilledandpassedthrougha0.5mmsieve.Thewheatbranwaskeptinasealedplasticbagandstoredat4°Cuntiluse.Allchemicalsandsolventsusedwereofanalyticalgrade.

2.2.Ultrasound-assistedextractionofphenoliccompoundsfromwheatbran

Ultrasound-assistedextractionwasperformedinanultrasoniccleaningbath(SB-5200Dtype,40kHz,250W,NingboScientzBiotechnologyCo.Ltd.,China)withause-fulvolumeof10l(internaldimensions:300Â240Â150mm).Workingfrequencywas xedat40kHz.Sampleswereplacedintoavolumetric ask(100ml),madeuptovolumewiththeextractingsolventandsonicatedfordi er-enttimesattherequiredtemperature.Thetemperaturewascontrolledbycirculatingexternalwaterfromathermostat-tedwaterbath.Aftertheextraction,the askwasremovedfromthebathandcooledtoroomtemperaturebycoolingwater.Thewheatbranextractswere lteredthroughWhat-manNo.1paperundervacuum,andthesolutionwascol-lectedinavolumetric ask,andusedforthedeterminationofthetotalphenoliccontent.2.3.Selectionofextractionsolvent

Fivegramsofaccuratelyweighedwheatbranwereextractedwith100mlof70%(v/v)methanol,70%(v/v)

ethanoland70%(v/v)acetone,respectively,inavolumetric ask(100ml)andkeptforsonicationat50°C.After20min,thesupernatantandthesedimentwereseparatedbyvacuum ltration.Theextractsobtainedwereusedforthedeterminationofthetotalphenoliccontent.Theproce-dureofultrasonicextractionofmaterialwasrepeatedtwiceunderthesameconditions.

2.4.E ectofethanolconcentrationonextractionoftotalphenoliccompounds

Ethanol–watermixtureswereusedasextractionsol-vents.Phenoliccompoundswereextractedfromwheatbranusingdi erentethanolconcentration,rangingfrom20%(v/v)to95%(v/v);thewheatbran(5.0g)wasmacer-atedwiththeextractingsolvents(100ml),andsonicatedfor20minat50°C.Theextractwas lteredundervacuumandthe ltrateswereusedforthedeterminationofthetotalphenoliccontent.

2.5.E ectofextractiontemperatureonextractionoftotalphenoliccompounds

Fivegramsofwheatbranweremaceratedwith70%(v/v)ethanol(100ml),andsonicatedfor20minatdi erenttemperaturesrangingfrom25to75°C.Theextractwas l-teredundervacuumandthe ltrateswereusedforthedeterminationofthetotalphenoliccontent.

2.6.E ectofextractiontimeonextractionoftotalphenoliccompounds

Wheatbran(5.0g)wasmaceratedwith70%(v/v)etha-nol(100ml),andsonicatedfordi erenttimesrangingfrom10to50minat50°C.Theextractwas lteredundervac-uumandthe ltrateswereusedforthedeterminationofthetotalphenoliccontent.2.7.Experimentaldesign

A velevel,threevariablecentralcompositerotatabledesign(Cochran&Cox,1992)wasappliedtodeterminethebestcombinationofextractionvariablesforthetotalphenoliccontentfromwheatbran.Thefactorialdesignconsistedofeightfactorialpoints,sixaxialpoints(twoaxialpointsontheaxisofeachdesignvariableatadistanceof1.68fromthedesigncenter)andfourcenterpoints,lead-ingto18setsofexperiments.ThevariablesXiwerecodedasxiaccordingtothefollowingequation:xi¼ðXiÀi=DXi

ð1Þ

wherexiisthecodedvalueofanindependentvariable,iistherealvalueofanindependentvariable,istherealvalueofanindependentvariableatthecenterpoint,andDXiisthestepchangevalue.Thevariablesandtheirlevels,withbothcodedvaluesandnaturalvaluesinvestigatedinthisstudy,arerepresentedinTable1.Table2liststheactual