美国油脂检测标准AOCS Ca 5a-40

Page1of2AOCSOfficialMethodCa5a-40Reapproved2009FreeFattyAcidsDEFINITIONThismethoddeterminesthefreefattyacidsexistinginthesample.SCOPEApplicabletoallcrudeandrefinedvegetableoils,marineoilsandanimalfats.SAMPLINGANDANALYSISOFCOMMERCIALFATSANDOILSAPPARATUS1.Oilsamplebottles—115or230mL(4or8oz),or250mLErlenmeyerflasks.REAGENTS1.Ethylalcohol,95%—USSDformulas30and3Aarepermitted(seeNotes,1).Thealcoholmustgiveadefinite,distinctandsharpendpointwithphenolphthaleinandmustbeneutralizedwithalkalitoafaint,butpermanentpinkcolorjustbeforeusing.2.Phenolphthaleinindicatorsolution—1%in95%alcohol.3.Sodiumhydroxidesolution—accuratelystandardized.SeeAOCSSpecificationH12-52.SeeTable 1fortheappropriatenormalityofthesodiumhydroxidesolution,dependingontheexpectedfreefattyacidconcentrationrangeinthesample.PROCEDURE1.Testsamplesmustbewellmixedandentirelyliquidbeforeweighing;however,donotheatthesamplemorethan10°Coverthemeltingpoint.2.UseTable 1todeterminethetestportionweightforvariousrangesoffattyacids.WeighthedesignatedsamplesizeintoanoilsamplebottleorErlenmeyerflask(seeNotes,2).3.Addthespecifiedamountofhotneutralizedalcoholand2mLofindicator.4.Titratewithstandardsodiumhydroxide,shakingvigorouslyuntiltheappearanceofthefirstpermanentpinkcolorofthesameintensityasthatoftheneutralizedalcoholbeforetheadditionofthesample.Thecolormustpersistfor30seconds.CALCULATIONS1.Thepercentageoffreefattyacidsinmosttypesoffatsandoilsiscalculatedasoleicacid,althoughincoconutandpalmkerneloilsitisfrequentlyexpressedaslauricacidandinpalmoilintermsofpalmiticacid.(a)Freefattyacidsasoleic,%=mLofalkaliM28.2mass,goftestport××iion(b)Freefattyacidsaslauric,%=mLofalkaliM20.0mass,goftestport××iion(c)Freefattyacidsaspalmitic,%=mLofalkaliM25.6mass,goftestport××iion2.Thefreefattyacidsarefrequentlyexpressedintermsofacidvalueinsteadofpercentagefreefattyacids.TheacidvalueisdefinedasthenumberofmilligramsofKOHnecessarytoneutralize1gofsample.Toconvertpercentagefreefattyacids(asoleic)toacidvalue,multiplythepercentagefreefattyacidsby1.99.PRECISIONPrecisiondataforrefined,bleachedanddeodorizedoilsareshowninTable 2.PrecisiondataforcrudeoilsareshowninTable 3.NOTES1.Isopropanol,99%,maybeusedasanalternatesolventwithcrudeandrefinedvegetableoils.2.Capbottleandshakevigorouslyfor1 minifoilhasbeenblanketedwithcarbondioxidegas.REFERENCESSeeJ.Assoc.Off.Anal.Chem.59:658(1976)regardingtheruggednessofthismethod.Page2of2SAMPLINGANDANALYSISOFCOMMERCIALFATSANDOILSCa5a-40•FreeFattyAcidsTable 1Freefattyacidrange,alcoholvolumeandstrengthofalkaliaFFArange(%)Testportion(g)Alcohol(mL)Strengthofalkali0.00to0.256.4±0.2500.1M0.2to1.028.2±0.2500.1M1.0to30.07.05±0.05750.25M30.0to50.07.05±0.051000.25or1.0M50.0to1003.525±0.0011001.0MaFFA,freefattyacid;N,normality.Table 2Theaverage,expectedbetween-laboratoryvariation(standarddeviationofreproducibility,SR)forthedeterminationoffreefattyacidsinrefined,bleachedanddeodorizedoilsaApproximateFFAvalue(%)0–0.050.05–0.10.1–1.01.0–2.0SR0.0070.0100.0460.073RSD(CV,%)33.9312.739.904.75R(95%)=2.8×SR0.020.030.130.20aValuesobtainedfromtheAOCSLaboratoryProficiencyProgram.Table 3Theaverage,expectedbetween-laboratoryvariation(standarddeviationofreproducibility,SR)forthedeterminationoffreefattyacidsincrudeoilsaApproximateFFAvalue(%)0.1–1.01.0–2.0SR0.0770.156RSD(CV,%)14.579.84R(95%)=2.8×SR0.220.44aValuesobtainedfromtheAOCSLaboratoryProficiencyProgram.