电子鼻在乳制品检测中的应用中英对照.docx
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电子鼻在乳制品检测中的应用中英对照.docx
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电子鼻在乳制品检测中的应用中英对照
Theelectronicnoseappliedtodairyproducts
Abstract
Thestate-of-the-artandcurrenttrendsinthedevelopmentof“aroma”analysiswithelectronicnosesarereviewedwithspecialreferencetoapplicationstodairy-products.Someofthereportedproblemswithelectronicnoseshaverecentlybeenreduced,e.g.thecorrection/reductionofsignaldrift,theinfluenceofhumidityandtemperature.Newpromisingandreproduciblesensormanufacturingtechniquesarebeingimplemented,e.g.electro-sprayforQMBsensorproduction.Thedevelopmentofmoreselectiveandsensitivesensors,especiallyofQMBandconductingpolymer(CP)type,shouldimprovetheirapplicability.Interestingnovelsamplingtechniques,suchasSPMEorSBSE,offermorepossibilitiesfortheanalysisofsemi-volatilecompoundswhicharegenerallymoreodoriferous.However,standardcalibrationproceduresandreferencematerialsarenotyetavailable.Althoughtheyarenormallylesspowerfulthanhumannoses,electronicnosesoffersomesignificantadvantagesintheanalysisofvolatiles,forexample,ininstrumentalclassificationsbasedonhedonicorsensoryanalysesandinpotentiallyautomatedon-linemonitoringofvolatiles.Severalgroupshaveexploredtheapplicationofdifferentelectronicnosesintheinvestigationofvariousaspectsofdairyproducts.ThepresentreviewincludesasexamplestheevaluationofSwissandCheddarcheesearoma,theassessmentoftheripeningofPecorinoToscanocheese(ewe’s),thedetectionofmouldinParmesancheese,theclassificationofmilkbytrademark,byfatlevelandbypreservationprocess,theclassificationandthequantificationofoff-flavoursinmilk,theevaluationofMaillardreactionsduringheatingprocessesinblock-milk,aswellastheidentificationofsinglestrainsofdisinfectant–resistantbacteriainmixedculturesinmilk.
Sincethefirstapplicationsofsolidstategassensorsinarrays,sometwentyyearsago,“electronicnoses”haveundergoneagreatdealofdevelopment.Aroundathousandarticlesonthissubjecthavebeenpublishedoverthelast4years,mainlyinrelationtothefoodandbeverageindustry[1],butalsoconcerningenvironmental,agricultural,andmedicaltopics,intheautomotiveindustry,etc.However,thenumberofstudiesdedicatedtodairyproductsisstillverylimited,probablyduetothecomplexityoftheirmatrices.Theaimofthepresentpaperistoreviewrecentexploratorystudiesofelectronicnosesappliedtodairyproducts,inordertoperceivetheprospectsandtrendsinthisfield.Traditionallyinthefoodindustry,monitoringofproductsintermsofqualityandcontrolofproductionprocesses(e.g.mixing,heating,drying,cooking,baking,extruding,fermenting,etc.)areperformedviaphysicochemicalmeasurements,i.e.pH-value,colour,concentrationofgivenchemi-calsorbiomoleculesgenerallydeterminedbyspectroscopy(e.g.FTIR,NIR,UV-Vis,etc.)[2]andthisdespitetheextremeimportanceofaromaasanindicatorofqualityandproductconformity.Thiswasmainlyduetothelackofreliableodourassessinginstrumentsandthepracticalimpossibilityofemployingsensorypanelstothecontinuousmonitoringofaroma.Electronicnoseshavethepotentialtofulfilthistask.Comparedtosensorypanelsthemainadvantageofelectronicnosesisthatoncecalibratedtheycanperformodourassessmentonacontinuousbasiswithaminimalcost.Furthermore,onceestablishedthistechniquedoesnotrequiretrainedpersonnellikeasensorypaneldoes,isnotsubjecttoindividualbreakdownorvariationofsensitivity[3],isnotoverloadedundernormaloperationandtakescomparativelyverylittletime.
Beforetheadventofelectronicnosestheonlypossibleinstrumentalanalysisof“aroma”(themixtureofvolatilespresentintheheadspaceofaproduct)wastheidentification/quantificationofindividualchemicalcompounds,afteraseparationstep(e.g.GC–MS,GC–FID,etc.).However,therelationshipbetweenthissequentialanalysisandtheperceptionoftheglobalaromaofaproductisnoteasilyestablishedsincetherulesgoverningthecombinationofindividualchemicalcompoundsinthegenerationofodoursarenotyetfullyunderstood[4–6].
Itshouldbekeptinmindthatinstrumentalanalyses,whetherclassicalsuchasGC–MS,etc.orbyelectronicnoseareperformednotonlyonodorousvolatilesbutalsoonnon-odorouscompoundsoccurringintheheadspace.Thiscanbeinterestingwhenanalysinghazardousnon-odorouscompounds(e.g.carcinogens,toxins,solvents)butalsoimpliesthatinstrumentallyperformedclassifications/analysesmightnotbebasedonaromarelevantmolecules.Furthermore,hedonicassessmentcannotbeperformedbyanyinstrument.Classificationmodelshavetobedefinedbasedontheresultsofsensorypanelspriortoperforminganalyseswithodoursignificance.
2.Theelectronicnoseconcept
Thename“electronicnose”comesfromacertainparallelofthemeasurementconceptoftheinstrumentandthatofthemammalianolfactorysystem.Inthelatter,uponbeingsniffedthroughthenose,orthroughtheretro-nasalpathwaywhenaproductistasted,volatilecompoundsreachtheol-factoryepitheliumwhichisanareaofapproximately5cm2locatedintheuppernasalcavity.There,theinteractionsofodorantswiththeappropriatechemosensoryreceptors,olfactoryneurons(∼107belongingto∼103differentclasses[6])produceelectricalstimuliwhicharetransmittedtothebrain[3,6–9].Apatternrecognitionprocessassistedbythememorythentakesplaceusingallthedatainordertoidentify,classify,orperformanhedonicanalysis[9].Evidenceexistsshowingthatasingleolfactoryneuronrespondstoseveralodorantsandthateachodorantissensedbymultipleolfactoryneurons[10].Inthesameway,electronicnosesbasetheanalysisonthecross-reactivityofanarrayofsemi-selectivesensors.Hence,productswithsimilararomagenerallyresultinsimilarsensorresponsepatterns(similar“fingerprints”)whereasproductswithdifferentaromashowdifferencesintheirpatterns(different“fingerprints”).Thesamplingstepiscarriedouteitherbytakinganaliquotofthesampleheadspace,withasyringe,andinjectingitintothedetector,orbycarryingtheheadspacewithagasstreamintothedetector.Sometimesthecarriergasisbubbledthroughthesampletostripoutcompounds.Theinteractionofvolatileswiththearrayofsensorsprovokesaseriesofsignalswhicharethenprocessedbythecomputerviaapatternrecognitionprogram.
Aspecialtypeofsystemisslowlyappearinginthemarket,thesocalledportable[4,11,12].Thesearesmallinstrumentswherethesensorsarrayisconfinedtoachip.Theanalysisproceedsbyplacingtheinstrumentnearthesample.Portablescanbeusefulinsimpleandwelldeterminedcases,andwheninterferencefromthesurroundingsareminororconstant.
Justlikethehumanolfactorysystem,electronicnosesdonotneedtobespeciallydesignedtodetectaparticularvolatile.Infact,theycanlearnnewpatternsandassociatethemwithnewodoursviatraininganddatastoragefunctionsashumansdo.However,trainingofelectronicnosesbasedonsensorypanelclassificationsisrequiredinordertoobtainodour-meaningfulclassifications.Oftenthesensitivityofelectronicnosesissimilartothatofhumannosesbuthumansarespeciallygiftedinsensingspecificcompounds(e.g.thiols,biogeniccompounds,pyrazines,thiazoles,somealdehydes[13]).Thebiologicalsensitivitycangodowntopptlevelswitharesponsetimeintheorderofmillisecondswhereasinstrumentsbarelygounderppblevelswitharesponsetimeintheorderofseconds(Table1)[2,14].
3.Overviewofgassensors:
technologyand
characteristics
Thenon-selectivityofsolidstatesensors(metaloxidesensors,MOS)wasconsideredaseveredrawbackofthistechnologyintendedasanalyticaltool.Backintheearly1980stheideaofassemblingarraysofsuchsensorswithdifferentsensitivitiesandselectivitieswasputintopractice.Thus,althoughboththequalitativeandquantitativeinformationobtainedfromeachsensorwashighlyambiguous,theircombinationresultedinsomesortof“fingerprint”ofthesample.Andwiththehelpofstatisticalprogramstheclassificationofsamplesintogroupscouldbeachieved.
Oncetheconceptofassemblingarraysofnon-selectivesensorshadbeendeveloped,variousdetectionprinciplesweretested,someofthemalmostaccidentallyasinthecaseofMOSFET[15].Afewofthemhavegivenconsistentresultsandcanbefoundonthemarket.Linkstoproducers,aswellastouniversitygroupsperformingR&Dinthisfield,canbefoundamongothersatthewebaddress:
http:
//www.nose-network.org/review/.Othertypesofdeviceshavealsobeentried-outsuchaselectrochemicalsensors,opticalfibrescoatedwithdye-impregnatedpolymers,biosensors,etc.Severalgoodpapers[3,8,10,14,16–21]provideinterestedreaderswithamoreextensiveinsightintodifferentgassensortechnologies.Abriefdescriptionofsomeofthecommerciallyavailablesensorsfollows.
3.1.MOS
Metaloxidesensorsconsistofametal-oxidesemiconductingfilm(e.g.SnO2,TiO2,ZnO,ZrO2)coatedontoaceramicsubstrate(e.g.alumina).Mostoftenthedevicealsocontainsaheatingelement.Oxygenfromtheairisdissolvedinthesemiconductors’lattice,settingitselectricalresistancetoabackgroundlevel(stablewhenatequilibrium).Duringthemeasurement,thevolatilemolecules(mainlynon-polar)areadsorbedatthesurfaceofthesemiconductorwheretheyreact(oxidation/reduction)withthedissolvedoxygenspeciescausingafurthermodificationoftheresistance(orconductivity)ofthedevice.Thislastchangeistakenastheresponseofthesystemtothatparticularsample(Fig.1)
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