chapter 7 Pigments and colors.docx
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chapter 7 Pigments and colors.docx
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chapter7Pigmentsandcolors
Chapter8Pigments
8.1Introduction
Tounderstandcolorantsinfoodssometermsneedtobedefined.Colorreferstohumanperceptionofcoloredmaterials---red,green,blue,etc.Acolorantisanychemical,eithernaturalorsynthetic,thatimpartscolor.Foodshavecolorbecauseoftheirabilitytoreflectoremitdifferentquantitiesofenergyatwavelengthsabletostimulatetheretinaintheeye.Theenergyrangetowhichtheeyeissensitiveisreferredtoasvisiblelight.Visiblelight,dependingonanindividual’ssensitivity,encompasseswavelengthsofapproximately380-370nm.Thisrangemakesupaverysmallportionoftheelectromagneticspectrum.Inadditiontoobviouscolors(hues),black,white,andintermediategraysarealsoregardedascolors.
Pigmentsarenaturalsubstancesincellsandtissuesofplantsandanimalsthatimpartcolor.Dyesareanysubstancesthatlendcolortomaterials.Thetermdyeiscommonlyusedinthetextileindustrial.IntheU.S.foodindustry,adyeisafetid-gradewater-solublecolorantcertifiedbytheU.S.FoodandDrugAdministration(FDA).Thesespecificdyesarereferredtoas“certifiedcolors,”andeachoneisassignedanFD&Cnumber.TheFD&Cdesignationmeansthatthedyemaybeusedinfoods,drugs,andcosmetics.AddedtotheapprovedlistofcertifiedcolorsaretheFD&Clakes.Lakesaredyesextendedonasubstratumandtheyareoildispersible.Thedye/substratumcombinationisachievedbyadsorption,coprecipitation,orchemicalreaction.Thecomplexinvolvesasaltofawater-solubleprimarydyeandanapprovedinsolublebasestratum.AluminaistheonlyapprovedsubstratumforpreparingFD&Clakes.Inaddition,thereareotherdyesorlakesapprovedforuseinothercountries,wherespecificationsareestablishedbytheEuropeanEconomicCommunity(EEC)ortheWorldHealthOrganization(WHO).Colorantsexemptfromcertificationmayalsobeused.Thesearenaturalpigmentsorsunbatancesynthesized,butidenticaltothenaturalpigment.
Colorandappearancearemajor,ifnotthemostimportant,quality,attributesoffoods.Itisbecauseofourabilitytoeasilyperceivethesefactorsthattheyarethefirsttobeevaluatebytheconsumerwhenpurchasingfoods.Onecanprovideconsumersthemostnutrition,safest,andmosteconomicalfoods,butiftheyarenotattractive,purchasewillnotoccur.Theconsumeralsorelatesspecificcolorsoffoodstoquality.Specificcolorsoffruitsareoftenassociatedmaturity---whilerednessofrawmeatisassociatedwithfreshness,agreenapplemaybejudgeimmature(althoughsomearegreenwhenripe),andbrownish-redmeatasnotfresh.
Coloralsoinfluencesflavorperception.Theconsumerexpectsreddrinkstobestrawberry,raspberry,orcherryflavored,yellowtobelemon,andgreentobelimeflavored.Theimpactofcoloronsweetnessperceptionhasalsobeendemonstrated.Itshouldalsobenotedthatsomesubstancessuchasβ-caroteneorriboflavinarenotonlycolorantsbutnutrientsaswell.Itisclearthereforethatcoloroffoodshasmultipleeffectsonconsumers,anditiswrongtoregardcolorasbeingpurely'cosmetic.
Manyfoodpigmentsare,unfortunately,unstableduringprocessingandstorage.Preventionofundesirablechangesisusuallydifficultorimpossible.Dependingonthepigment,stabilityisimpactedbyfactorssuchasthepresenceorabsenceoflight,oxygen,heavymetals,andoxidizingorreducingagents,temperatureandwateractivity,andpH.Becauseoftheinstabilityofpigments,colorantsaresometimesaddedtofoods.
Thepurposeofthischapteristoprovideanunderstandingofcolorantchemistry---anessentialprerequisiteforcontrollingthecolorandcolorstabilityoffoods.
8.2Chlorophyll
8.2.1StructureofChlorophylls
Chlorophyllsarethemajorfightharvestingpigmentsingreenplants,algae,andphotosyntheticbacteria.Severalchlorophyllsarefoundinnature.Theirstructuresdifferinthesubstituentsaroundthepborbinnucleus.Chlorophyllaandbarefoundingreenplantsinanapproximateratioof3:
1.TheydifferinthecarbonC-3substituent.Chlorophyllacontainsamethylgroupwhilechlorophyllbcontainsaformylgroups(Fig.1).BothchlorophyllshaveavinylandanethylgroupattheC-2andC4position,respectively;acarbomethoxygroupattheC-l0positionoftheisocylicrings,andaphytolgroupesterfiedtopmpionateattheC-7position.Phytolisa20-carbonmonounsaturatedisoprenoidalcohol.Chlorophyllcisfoundinassociationwithchlorophyllainmarinealgae,dinoflagellates,anddiatoms.Chlorophylldisaminorconstituentaccompanyingchlorophyllainredalgae.Bacteriochlorophyllsandchlorobiumchlorophyllsareprincipalchlorophyllsfoundinpurplephotosyntheticbacteriaandgreensulfurbacteria,respectively.
Figure1Structureofchlorophyll.
8.2.2PhysicalCharacteristics
Chlorophyllarelocatedinthelamellaeofintercellularorganellesofgreenplantsknownaschloroplasts.Theyareassociatedwithcarotenoids,lipids,andlipoproteins.Weaklinkages(noncovalentbonds)existbetweenthesemolecules.Thebondsareeasilybroken;hencechlorophyllscanbeextractedbymaceratingplanttissueinorganicsolvents.Varioussolventshavebeenused.Polarsolventssuchasacetone,methanolethanol,ethylacetate,pyridine,anddimethylformamidearemosteffectiveforcompleteextractionofchlorophylls.Nonpolarsolventssuchashexaneorpetrolleumetherarelesseffective.High-performanceliquidchromatography(HPLC)todayisthemethodofchoiceforseparatingindividualchlorophyllsandtheirderivatives.Inthecaseofchlorophyllaandb,forexample,theincreaseinpolaritycontributedbytheC-3formylsubstituentofchlorophyllbcausesittobemorestronglyadsorbedonanormal-phasecolumnandmoreweaklyabsorbedonareverse-phasecolumnthanchlorophylla.
8.2.3ColorLossduringThermalProcessing
Lossofgreencolorinthermallyprocessedvegetablesresultsfromformationofpheophytinandpyropheophytin.Blanchingandcommercialheatsterilizationcanreducechlorophyllcontentbyasmuchas80-100%.Thegreateramountofpheophytindetectedinfrozenspinachascomparedtospinachblanchedforcanningismostlikelyattributabletothegreaterseverityoftheblanchtreatmentthatisgenerallyappliedtovegetablesintendedforfreezing.Oneofthemajorreasonforblanchingofspinachpriortocanningistowiltthetissueandfacilitatepackaging,whereasblanchingpriortofreezingmustbesufficientnotonlytowiltthetissuebutalsotoinactivateenzymes.Thepigmentcompositionshownforthecannedsampleindicatedthattotalconversionofchlorophyllstopheophytinsandpyropheophytinshasoccurred.
Degradationofchlorophyllwithinplanttissuespostharvestisinitiatedbyheat-reduceddecompartmentalizationofcellularacidsaswellasthesynthesisofnewacids.Invegetablesseveralacidshavebeenidentified,includingoxalic,malic,citric,acetic,succinic,andpyrrolidonecarboxylicacid(PCA).ThermaldegradationofglutaminetoformPCAisbelievedtobethemajorcauseoftheincreaseinacidityofvegetablesduringheating.Othercontributorstoincreasedaciditymaybefattyacidsformedbylipidhydrolysis,hydrogensulfideliberatedfromproteinsoraminoacids,andcarbondioxidefrombrowningreactions.
8.2.4TechnologyofColorPreservation
Effortstopreservegreencolorincannedvegetableshaveconcentratedonretainingchlorophyll,formingorretaininggreenderivativesofchlorophyll,thatis,chlorophyllides,orcreatingamoreacceptablegreencolorthroughtheformationofmetallocomplexes.
AcidNeutralizationtoRetainChlorophyll
Theadditionofalkalizingagentstocannedgreenvegetablescanresultinimprovedretentionofchlorophyllsduringprocessing.TechniqueshaveinvolvedtheadditionofcalciumoxideandsodiumdihydrogenphosphateinblanchwatertomaintainproductpHortoraisethepHto7.0.Magnesiumcarbonateorsodiumcarbonateincombinationwithsodiumphosphatehasbeentestedforthispurpose.However,allofthesetreatmentsresultinsofteningofthetissueandanalkalineflavor.
Blairin1940recognizedthetougheningeffectofcalciumandmagnesiumwhenaddedtovegetables.ThisobservationledtotheuseofcalciumormagnesiumhydroxideforthepurposeofraisingpHandmaintainingtexture,Thiscombinationoftreatmentsbecameknownasthe“Blairprocess”.Commercialapplicationoftheseprocesseshasnotbeensuccessfulbecauseoftheinabilityofthealkalizingagentstoeffectivelyneutralizeinteriortissueacidsoveralongperiodoftime,resultinginsubstantialcolorlossafterlessthan2monthsofstorage.
Anothertechniqueinvolvedcoatingthecaninteriorwithethycelluloseand5%magnesiumhydroxide.ItwasclaimedthatslowleachingofmagnesiumoxidefromtheliningwouldmaintainthepHatornear8.0foralongertimeandwouldthereforehelpstabilizethegreencolor.Theseeffortswereonlypartiallysuccessful,becauseincreasingthepHofcannedvegetablescanalsocausehydrolysisofamidessuchasglutamineorasparaginewithformationofundesirableammonia-likeodors.Inaddition,fattyacidsformedbylipidhydrolysisduringhighpHblanchingmayoxidizetoformrancidflavors.InpeasanelevatedpH(8.0orabove)cancauseformationofstruvite,aglass-likecrystalsconsistingofamagnesiumandammoniumphosphatecomplex.Struviteisbelievedtoresultfromthereactionofmagnesiumwithammoniumgeneratedfromtheproteininpeasduringheating.
High-TemperatureShort-Timeprocessing
Commerciallysterilizedfoodsprocessedatahigherthannormaltemperatureforarelativelyshorttime(HTST)oftenexhibitbetterretentionofvitamins,flavoran
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