Food Additives, Second Edition Revised and Expanded

24 Pages·2005·0.78 MB·English

16
Synthetic Food Colorants
JOHNH. THORNGATE III
UniversityofCalifornia,Davis,California
I. INTRODUCTION
Theimpactofthephysicalworldonourlivesis,quiteliterally,coloredbyourperceptions.
Throughthemechanismsofourvarioussensoryreceptorsweconvertsuchenvironmental
stimuliaspressurewavesintosound,chemicalstructureintotasteandsmell,andelectro-
magnetic radiation into sight. Indeed, the last sense is of immense importance to us; hu-
mansaredecidedlyvisualcreatures.Visionnotonlyrepresentsthemostcomplexsensory
system in the human body (Mason and Kandel, 1991), but also dominates the central
processing unit itself, the human brain (Sekular and Blake, 1985).
Of those qualities associated with vision, color has perhaps the most immediate
impact. Lindberg (1938) outlined a number of early psychological experiments which
examinedtherelativeimpactofcolor,anddevisedteststomeasure‘‘colorattitude,’’that
is, the prevalence of response to color over form. With regard to food, color has often
beenstated,usuallyapocryphally,tobeofpreeminentsensoryimportance(Noonan,1972;
IFT, 1986; Newsome, 1990; von Elbe and Schwartz, 1996), although Clydesdale (1993)
notes that color’s true role remains ‘‘elusive and difficult to quantify.’’
Whether or notcolor is indeed the most important sensory characteristic discerned
is academic; color’s practical importance to food acceptability and flavor has long been
recognized. Foods and beverages have been ‘‘artificially’’ colored since ancient times.
Pliny, in Book XIV, Section viii of his Naturalis Historia, mentions that unscrupulous
dealersinNarbonneartificiallycoloredtheirwinesthroughtheuseofsmokeoraloe(Pliny,
1945).Plinyalsonotesthepracticeofaddinggypsum(calciumsulfatedihydrate)towine
ostensibly to ‘‘soften any roughness,’’ although besides modulating the acidity gypsum
would also reduce the red color (Book XIV, Section xxiv). In the same section Pliny
477
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478 Thorngate
mentions the Greek habit of adding seawater to wine, which, as Amerine and Singleton
(1977) note, would increase the color’s brightness.
Scully (1995) discusses the importance of appearance to the medieval cook, for
whom color, especially yellow, was prized. Saffron and egg yolk were the colorants of
preference, although gold leaf was also (infrequently) used. It is interesting to note that
thecolor yellowwasalsocentral toan early edictagainstcoloradulteration;in 1396the
Parisian government prohibited the coloring of butter (NAS/NRC, 1971). Unfortunately
not all coloring agents were harmless; medieval cooks had also learned that verdigris
(copper oxides) imparted an excellent green color (Scully, 1995).
The increasing use of toxic colorants and other adulterants led Fredrick Accum to
publish A Treatise on Adulterations of Food and Culinary Poisons in 1820, in which he
decried, among other practices, the coloring of hedge leaves with verdigris to resemble
green tea and the artificial greening of pickles and candies with copper salts (Accum,
1820).Accum’scriticisms,whileinternationallysensationalistic,didlittletospurimmedi-
atereform(HuttandHutt,1984);ironically,theuseoftoxiccolorantsacceleratedfollow-
ingSirWilliamHenryPerkin’ssynthesisofmauvefromthecoal-tarderivativemethylan-
alinein1856.Thissyntheticpurpledyesparkednotonlythebirthofthemodernorganic
chemicalindustry,butalsothequestforanduseofsyntheticcolorantsbyafoodindustry
whichwasturningincreasinglytoprocessedfoodsinordertofeedaburgeoningindustrial
middle class (Tannahill, 1988).
Thefoodindustryfavoredthesyntheticdyesoveranimal-,plant-,ormineral-based
colorantsduetotheirmoreconsistenthues,dyestrength,andstability(NAS/NRC,1971).
Only four years following Perkin’s initial discovery the French were coloring wine with
thecoaltarderivativefuchsine(nowknownasMagentaI)(Marmion,1979);by1886the
UnitedStatesCongresshadapprovedtheuseofsyntheticcoloringforbutterandby1896
for cheese (Marmion, 1979). By 1900 approximately 80 dyes were being used as food
colorants in the United States (Noonan, 1972).
II. U.S. GOVERNMENT REGULATION
Accum’s book initiated a flurry of investigations and reports on food adulteration in the
United States; Hutt and Hutt (1984) list a number of treatises published in the mid- to
late-1800s which served to prompt state governments increasingly into legislating the
safety of the food supply. Consideration of this issue reached a national climax with the
National Board of Trade’s 1879 competition to award $1000 to the person submitting
thebestdraftofafoodadulterationact;thecompetitionwinner,announcedin1880,was
one G. W. Wigner of England (Hutt and Hutt, 1984). Wigner [cited in Hutt and Hutt
(1984)] outlined seven specific elements which defined food adulteration, the first and
sixth being
If any substance, or any substances, has, or have been mixed with it, so as to reduce, or
lower,orinjuriouslyaffectitsquality, strength,purity,ortruevalue.
If it be colored, or coated, or polished, or powdered, whereby damage is concealed,
oritismade toappearbetterthan itreallyis,orofgreatervalue.
This increased awareness was however insufficient to compel Congress into legislating
adulterationatthenationallevel;althoughbillswereintroducedasearlyas1879Congress
chose instead (not atypically) to further investigate the issue (Hutt and Hutt, 1984). The
positiveresultofthisinactionwasthatCongressbeganappropriatingfundsfortheDepart-
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SyntheticFoodColorants479
mentofAgriculture’sDivisionofChemistry(laterBureauofChemistry)toinvestigate
theadulterationoffoods(in1889)andthespecificuseofcolorants(in1900).Thelatter
actionwas,toquoteMarmion(1979),‘‘Thefirsteffectivesteptakenbythegovernment
tochecksuchpractices.’’Baseduponthisresearch,theSecretaryofAgriculturereleased
aFoodInspectionDecisionin1906,thefirstsuchpertainingtotheuseofcolorants:the
coaltarderivativeMartiusYellowwasdeclaredunsafeforfoods(Marmion,1979).
ConcurrentwiththefoodpreservativesresearchconductedbyDr.HarveyW.Wi-
ley’sPoisonSquadintheBureauofChemistry,Dr.BernhardC.Hessewasinvestigating
thesuitabilityoftheavailablecoaltardyes(some700)asfoodcolorants(NAS/NRC,
1971).Hesse’sliteraturesearchandsubsequentphysiologicalstudies,alongwithWiley’s
work,ledtothepassageofthePureFoodandDrugActof1906,thefirstcomprehensive
federallegislationregulatingadditives(IFT,1986).Hesse’sfindingswerespecificallyin-
corporatedintotheActthroughtheFoodInspectionDecisionof1907,whichrecognized
sevendyesasmeetingtherequisitesuitabilitystandards.ThesearelistedinTable1.
Hesse’scriteriaforassessingthesuitabilityofdyesforfoodusewereincorporated
intothe1906legislation;theseincludedtherequirementsthatalldyesbemanufactured
understrictcontrol,andtestedviahumanandanimalphysiologicalstudiesbeforebeing
certifiedassuitableforfooduse(NAS/NRC,1971).Thecertificationprocesswas,how-
ever,avoluntaryprocessoverseenbytheSecretaryofAgriculture(NAS/NRC,1971);
nevertheless,thedyeindustryquicklysawtheadvantagestomarketingcertifiedcolorants,
andby1908thefirstvoluntarycertifieddyewasavailable(Marmion,1979).Underthis
processtenadditionaldyeswerecertifiedforfooduseintheyearspriorto1938(see
Table2),althoughtwoofthese(SudanIandButterYellow)werealmostimmediately
delisted for causing contact dermatitis in dye workers (NAS/NRC, 1971).
WhilethegroundworkhadbeenlaidwiththeActof1906,thequestionofenforce-
ment remained problematic. In 1907 the Secretary of Agriculture created the Board of
Food and Drug Inspection, charged with conducting hearings on alleged violations, and
in 1908 President Roosevelt commissioned the Referee Board of Consulting Scientific
Experts to rule on specific issues pertaining to food adulteration (Hutt and Hutt, 1984).
The USDA Bureau of Chemistry, however, remained the enforcing agency of record.
(WileyhadchairedtheBoardofFoodandDrugInspection.)In1927theBureauevolved
intotheFood,Drug,andInsecticideAdministration,whichwithinthreeyearsbecamethe
FoodandDrugAdministration.Hutt(1996)notesthatthe1906ActrequiredtheBureauto
Table 1 ColorantsPermittedfor Use inFoods FollowingFIDNo.76,
July1907
Colorindex Year
Commonname 1938 FDAnomenclature number delisted
Ponceau3R FD&C RedNo.1 16155 1961
Amaranth FD&C RedNo.2 16185 1976
Erythrosine FD&C RedNo.3 45430 —
Indigotine FD&C BlueNo.2 73015 —
LightGreen SF FD&C GreenNo.2 42095 1966
NaptholYellowS FD&C YellowNo.1 10316 1959
Orange1 FD&C OrangeNo.1 14600 1956
Source:Marmion(1979).
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480 Thorngate
Table 2 Additional ColorantsPermittedfor UseinFoods,1907–1938
CI Year Year
Common name 1938 FDA nomenclature number listed delisted
Tartrazine FD&C YellowNo.5 19140 1916 —
Sudan I — 12055 1918 1918
Butter Yellow — — 1918 1918
Yellow AB FD&C YellowNo.3 11380 1918 1959
Yellow OB FD&C YellowNo.4 11390 1918 1959
Guinea GreenB FD&C GreenNo.1 42085 1922 1966
FastGreenFCF FD&C GreenNo.3 42053 1927 —
Brilliant BlueFCF FD&C Blue No.1 42090 1929 —
PonceauSX FD&C RedNo.4 14700 1929 1976
Sunset YellowFCF FD&C YellowNo.6 15985 1929 —
Source:NAS/NRC(1971);Marmion(1979).
enforcethelegislationsolelyinresponsetomalfeasance;theActmandatednopremarket
approval process other than the voluntary certification schema.
As early as 1917 the Bureau of Chemistry had noted the shortcomings of the Act
of1906,specificallywithregardstovariouseconomicrepercussionsoffoodadulteration
(HuttandHutt,1984).AbillwaseventuallyputbeforetheSenatein1933whichbecame
uponitspassagetheFederalFood,Drugand CosmeticActof1938.Fromthestandpoint
of food colorants the 1938 Act was notable for making certification mandatory and for
creatingthreespecificcertifiedcategories:colorssuitableforfoods,drugs,andcosmetics
(FD&C);colorssuitablefordrugsandcosmetics(D&C);andcolorssuitableforexternally
applieddrugsandcosmetics(Ext.D&C).Thefifteenfoodcolorantsthenlisted(theorigi-
nal seven of Hesse in addition to the eight which had been successfully listed in the
intervening years) were again subjected to toxicological testing; in 1940 those colorants
wereagaindeemedsuitableforuse,althoughsubjecttothe1938Act’sprovisionsregard-
ingspecifications,usesandrestrictions,labeling,andcertification(referto21C.F.R.§74,
1996).
FollowingpassageoftheFood,DrugandCosmeticAct,andpriortotheamending
legislationof1958,fouradditionaldyesobtainedcertificationasfoodcolorants(seeTable
3). However, inthe early1950s threeincidences involvingexcessively appliedcolorants
inpopcornandcandiespromptedtheFDAtobeginamorerigoroustoxicologicaltesting
program(NAS/NRC,1971;Noonan,1972).Asaconsequence FDAdelistedFD&CRed
Table 3 Additional ColorantsPermittedfor UseinFoods,1939–1956
CI Year Year
Common name FDA nomenclature number listed delisted
Napthol YellowSa FD&CYellowNo.2 10316 1939 1959
OilRedXO FD&CRedNo.32 12140 1939 1956
OrangeSS FD&COrangeNo.2 12100 1939 1956
Benzyl Violet4B FD&CViolet No.1 42640 1950 1973
aPotassiumsalt.
Source:NAS/NRC(1971);Marmion(1979).
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SyntheticFood Colorants 481
No.32, andFD&COrangeNos.1and2in1956.Althoughmanufacturerspetitionedthe
FDA to consider the safety of the colorants in the context of proper usage, FDA instead
chose to interpret the ‘‘harmless and suitable’’ standard of the 1938 Act as ‘‘harmless
perse,’’meaningabsolutelyharmlessatalllevels.Lowercourtchallengestothisinterpre-
tationculminatedinthe1958SupremeCourtcaseFlemmingv.FloridaCitrusExchange,
inwhichtheSupremeCourtruledthattheFDAdidnothavetheauthoritytosetquantity
limitations;thatis,azerotoxicitycriterionwasestablished(Noonan,1972;Taylor,1984).
This led to the subsequent delisting of the four FD&C Yellow dyes, Nos. 1, 2, 3, and 4
in 1959.
Itwasclearthatthe‘‘zerotoxicity’’criterionoftheFlemmingcasewouldeventually
disallow the use of any synthetic colorant; the FDA was being forced to delist colorants
regardless of their true capacity to endanger public health (Noonan, 1972). In response
the color manufacturing industry, through the Certified Color Industry Committee, and
theFDAhelpedtoinitiatelegislationwhichbecametheColorAdditivesAmendmentsof
1960. The Amendments established an alternativesafety standard to the 1938 Act; how-
ever, in order to ensure public health the Amendments called for premarket approval of
allcolorants,nowreferredtoas‘‘coloradditives.’’Coloradditivesencompassedawider
setof colorantsthan the syntheticcoal tarderivatives regulatedbythe 1938 Act;a color
additivewasdefinedas‘‘adye,pigmentorothersubstancemade...or...derivedfrom
a vegetable, animal, mineral or other source and that, when added or applied to a food,
drug, or cosmetic or to the human body or any part thereof, is capable . . . of imparting
a color thereto’’ [21 C.F.R. §70.3(f), 1996]. Both synthetic and natural colorants were
subjecttopremarketsafetyevaluation,butcertification(asaguaranteeofpurity)wasonly
required for synthetic colorants (Newsome, 1990).
Additionally,theColorAdditivesAmendmentsprescribehowtheFDAdetermines
the safety of color additives. With regards to the safety standard, the Color Additives
Amendmentsreplacedthe‘‘harmlessperse’’standardwithageneralsafetyclausebased
upon a premarket evaluation of the color additive’s safety; ‘‘safety’’ was not explicitly
defined in the statute, but was interpreted as ‘‘convincing evidence that establishes with
reasonablecertaintythatnoharmwillresultfromtheintendeduseofthecoloradditive’’
[21 C.F.R. §70.3(i), 1996]. As Taylor (1984) notes, the burden of proof of an additive’s
safety was now incumbent upon the manufacturer, and that proof had to be evidentiary
in nature. The factors which have to be considered by FDA in the safety determination
includeexposure,cumulativeeffects,pertinentsafetyfactors,andavailabilityofpractica-
ble analytical detection methodology (Bachrach, 1984); the approved process for de-
terminingsafetyisdetailedintheFDApublicationToxicologicalPrinciplesfortheSafety
Assessment of Direct Food Additives and Color Additives Used in Food (the so-called
Redbook).
If the color additive is known or suspected of causing cancer, however, the Color
Additives Amendments invoke a Delaney clause similar to that of the Food Additives
Amendment of1958.The Delaney clauseexplicitlystatesthat a coloradditive‘‘shallbe
deemed unsafe, and shall not be listed for any use . . . if the additive is found . . . to
inducecancer’’[21U.S.C.§376(b)(5)(B),1976].Thoughthisisfrequentlymisinterpreted
as yet again meaning ‘‘harmless per se,’’ the Delaney clause was not intended represent
an absolute ban; in 1984 the Supreme Court upheld this reasoning in the case of Scott
v. Food and Drug Administration, regarding the presence of a carcinogenic impurity in
D&C Green No. 5 (Bachrach, 1984). It was argued that the presence of para-toluidine,
a known carcinogen in D&C Green No. 5 should invoke the Delaney clause and cause
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482 Thorngate
the dye to be delisted. The FDA, however, had previously interpreted their regulatory
responsibility thusly:
[Ifdatasuggestthat]thecoloradditive,includingitscomponentsorimpurities,inducescancer
in manor animal, theCommissioner shall determinewhether . .. cancer hasbeen induced
and whether the color additive, including its components or impurities, was the causative
substance. If it is his judgment that the data do not establish these facts, the cancer clause
isnotapplicable;andifthedataconsideredasawholeestablishthatthecoloradditivewill
be safe under the conditions that can be specified in the applicable regulation, it may be
listedforsuch use.(21C.F.R.§70.50,1996;emphasis added)
Thisinterpretation,thebasisfortheso-calledconstituentspolicy,wasupheldbythecourt.
TheprimarydifficultyassociatedwiththeColorAdditiveAmendmentswasnotthe
inclusionoftheDelaneyclause,butratherthefailureoftheAmendmentstoallowprevi-
ously listed colorants as having generally recognized as safe (GRAS) status. As Hutt
(1996)notes,thismeantthatallcoloradditives,regardlessofpreviousstatusortype,were
subjected to the regulatory requirements of the Amendments; this is the basis of Title II
in the Color Additive Amendments, which allowed provisional listing status for then-
current colorants until their safety andsuitability could be ascertained. Unfortunately, as
Hutt(1996)alsonotes,Congressdidnotestablishastatutorydeadlineforthisprovisional
status;thecourtshaveinsteadextendedthetransitionalperiodfromtheoriginal21/2years
untilthepresentday.Muchofthedelaywasduetoincreasedtestingrequirementsimposed
by the FDA, especially with respect to carcinogenicity. Only two new certified colors
have been permanently listed in the years following the passage of the Color Additive
Amendments: FD&C Red No. 40 and its alumina lake. The ten certified color additives
currently permanently listed for use in foods in the United States are presented in Table
4; the lakes of FD&C Blue No. 1, Blue No. 2, Green No. 3, Yellow No. 5, and Yellow
No. 6 continue to have provisional status.
The Color Additive Amendments also saw the creation of the class of uncertified
colors(colorsexemptfromcertification):thenatural-sourceandnature-identical(synthetic
Table 4 Certified ColorAdditivesPermanentlyListed forUse inFoods
CI EEC Year
Common name FDA nomenclature number number (re)listed
Brilliant BlueFCF FD&C Blue No.1 42090 E133 1969
Indigotine FD&C Blue No.2 73015 E132 1983
FastGreenFCF FD&C GreenNo.3 42053 — 1982
Erythrosine FD&C RedNo.3 45430 E123 1969
Allura RedAC FD&C RedNo.40 16035 E129 1971
Allura RedACLake FD&C RedNo.40Lake 16035 — 1994
Tartrazine FD&C YellowNo.5 19140 E102 1969
Sunset YellowFCF FD&C YellowNo.6 15985 E110 1986
Citrus RedNo.2a — 12156 — 1959
OrangeBb — 19235 — 1966
aRestrictedtothecoloringoforangeskins,nottoexceed2ppmbyweight.
bRestrictedtothecoloringofcasingsorsurfacesoffrankfurtersandsausages,nottoexceed150
ppmbyweight.
Source:21C.F.R.74(1996);Marmion(1979);Jukes(1996);vonElbeandSchwartz(1996).
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SyntheticFoodColorants483
Table5 ColorAdditivesExemptfromCertificationPermanentlyListedforUseinFoods
CIEEC
ColoradditivenumbernumberRestrictions a
Annattoextract75120E160b
β-Apo-8′-carotenal40820E160ente15mg b
Beets,dehydrated(beetpowder)E162
Canthaxanthin40850E161gnte30mg b
CaramelE150
β-Carotene75130E160a
Carrotoil
Cochinealextract;carmine75470E120
Cottonseedflour,toasted,partiallydefatted
FerrousgluconateE579Ripeolives
Fruitjuice
GrapecolorextractE163Nonbeverages
Grapeskinextract(enocianina)E163Beverages
Ironoxide,synthetic77499E172Sausagecasings
PaprikaE160c
Paprikaoleoresin
RiboflavinE101
Saffron75100
Titaniumdioxide77891E171nte1% c
Turmeric75300
Turmericoleoresin75300E100
Vegetablejuice
ante—nottoexceed.
bPerpoundorpint.
cByweight.
Source:21C.F.R.§73(1996);Marmion(1979);Rayner(1991);Jukes(1996),vonElbeandSchwartz(1996).
dyeswhichareidenticaltonaturalpigments)colors.TheFDAdoesnotrecognizeany
categoryofcolorantasbeing‘‘natural,’’astheadditionofanycoloranttofoodresults
inanartificiallycoloredproduct(Newsome,1990).Asnotedpreviously,thesecolorants
aresubjecttothesamesafetystandardascertifiedcolors,butdonotrequirechemical
puritycertification.ThecoloradditivesexemptfromcertificationarelistedinTables5
(fordirectconsumables)and6(foranimalfeeds).
III.INTERNATIONALGOVERNMENTREGULATION
OnlyfiveofthecertifiedfoodcoloradditivesallowedforuseintheUnitedStatesare
permittedforusebytheEuropeanEconomicCommunity.AsvonElbeandSchwartz
(1996)note,thedisparaterangeofcolorantsallowedanddisallowedbyvariouscountries
hascreatedaformidabletradebarriertothefoodindustry.Norway,forinstance,does
notallowanysyntheticfoodcolorantsinfoodmanufacture.Table7[aftervonElbeand
Schwartz(1996)]reviews currentregulationsinthe EEC,Canada,Japan,andtheUnited
States.
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484 Thorngate
Table 6 Color AdditivesExemptfrom CertificationPermanentlyListed
for UseinFoods
CI EEC
Color additive number number Animal
Algae meal,dried Chickens
Astaxanthin Salmonidfish
Canthaxanthin 40850 E161g Chickens
Corn endospermoil Chickens
Iron oxide,synthetic 77499 E172 Cats,dogs
Tagetes(Aztec marigold)meal 75125 E161b Chickens
and extract
Ultramarineblue 77007 General
Source:21C.F.R§73(1996);Marmion(1979);Rayner(1991);vonElbeandSchwartz
(1996).
Table 7 Synthetic ColorAdditivesAllowed forUse inFoodWorldwide
CI EEC Countries
Common name FDA nomenclature number number permittinga
Allura RedAC FD&C RedNo.40 16035 E129 C,US
Brilliant BlueFCF FD&C BlueNo.1 42090 E133 C,EEC,bJ,US
Erythrosine FD&C RedNo.3 45430 E127 C,EEC,J,US
FastGreenFCF FD&C GreenNo.3 42053 — US
Indigotine FD&C BlueNo.2 73015 E132 C,EEC,J,US
Sunset YellowFCF FD&C YellowNo.6 15985 E110 EEC,US
Tartrazine FD&C YellowNo.5 19140 E102 C,EEC,e J,US
Amaranth (FD&C RedNo.2)th 16185 E123 C,EECe
Brilliant BlackBN 28440 E151 EECef
Brown FK — E154 EECd
Carmoisine 14710 E122 EECefg
ChocolateBrown HT 20285 E155 EECf
Green S 44090 E142 EECefg
Patent BlueV 42051 E131 EECf
Ponceau4R 16255 E124 EEC,eJ
QuinolineYellow D&C YellowNo.10 47005 E104 EECf
Red2G 18050 E128 EECd
Yellow 2G 18965 E107 EECd
aC—Canada,EEC—EuropeanEconomicCommunity,J—Japan,US—UnitedStates.
bPermittedonlyinDenmark,Ireland,andtheNetherlands.
cPermittedonlyinIreland,andtheNetherlands.
dPermittedonlyinIreland.
eNotpermittedinFinland.
fNotpermittedinPortugal.
gNotpermittedinSweden.
hDelisted.
Source:Rayner(1991);Jukes(1996);vonElbeandSchwartz(1996).
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SyntheticFood Colorants 485
IV. COLOR CHEMISTRY
Theperceptualpropertyofcolorisinduced bytwobroadmechanisms:eitherwhitelight
is selectively interacted with by matter and thus decomposed into its constituent wave-
lengths,orelsenonwhitelightisdirectlyemittedbysomesource(Nassau,1987).Nassau
(1987)proceedstolistfifteenspecificphysicochemicalmechanismswherebycolorispro-
duced; from the standpoint of the food scientist, however, those mechanisms involving
electron transitions between molecular orbitals are the most important. These molecular
orbitaltransitionsarelargelyresponsibleforthecolorassociatedwithorganiccompounds,
whether synthetic or natural in origin.
Itisthebondconjugationwithintheorganicmoleculewhichisresponsibleforcolor;
delocalizationof the π-bondingelectronslowers theirexcitationenergies, allowing them
to absorb light (Nassau, 1987). Extensive conjugation, or the presence of electron donor
and acceptor groups within the molecule, serves to shift the absorption of light to the
lowerenergies(thatis,longerwavelengths)comprisingthevisiblespectrum(wavelengths
of 400 to 750 nm). Those wavelengths absorbed will be dependent on the existence of
molecularorbitallevels separated byenergy, hc/λ, wherehis Planck’s constant,c isthe
velocityoflight,andλthewavelengthoftheabsorbedradiation;theincidentradiationwill
causeanelectrontoshifttothehigherenergy((cid:2)hc/λ)orbital.Thisharmlessinteractionof
photonwithelectrondefineswhatwavelengthsarevisibletous;lowerenergylight(λ(cid:7)
750nm)inducesonlysmallvibrationalchanges(atmostperceivedasheat)whereashigher
energy light (λ (cid:3) 400 nm) will ionize matter (that is, knock out electrons completely)
(Nassau, 1987).
Note, however, what we properly see as color are not the wavelengths of light ab-
sorbed,butrathertheremainderoftheincidentlightreflected(orinthecaseoftransparent
objectsorsolutions,theremainderoflighttransmitted)followingabsorption(Paviaetal.,
1979). These perceptual colors are termed ‘‘complementary’’ to those wavelengths ab-
sorbed (see Table 8).
A. Colorants Subject to Certification
The certified colors discussed previously are all organic dyes, and may be grouped into
thefollowingfiveclassesbasedupongeneralchemicalstructure:monoazo(FD&CYellow
Table 8 RelationshipBetweenAbsorbedColor
andObserved(Complementary) Color
Wavelength Color of Colorof
absorbed(nm) absorbedlight observedlight
400 Violet Yellow
450 Blue Orange
500 Blue-green Red
530 Yellow-green Red-violet
550 Yellow Violet
600 Orange-red Blue-green
700 Red Green
Source:Paviaetal.(1979).
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486 Thorngate
Figure1 Monoazocolorants.
No.6,FD&CRedNo.40,CitrusRedNo.2;seeFig.1),pyrazolone(FD&CYellowNo.
5,OrangeB;seeFig.2),triphenylmethane(FD&CBlueNo.1,FD&CGreenNo.3;see
Fig.3),indigoid(FD&CBlueNo.2;seeFig.4),andxanthene(FD&CRedNo.3;see
Fig.5)(Marmion,1979).Tables9and10summarizetheirrespectivechemicalproperties
andstabilities.Asisevidentfromthetables,thedyeshavevaryingdegreesofstability
dependentupontheirchemicalstructure.Themonoazoandpyrazolonestructuresaresub-
jecttoSO decolorizationthroughHSO (cid:1)additiontothenitrogens,resultinginthecolor-
2 3
lesshydroazosulfonicacids(vonElbeandSchwartz,1996);althoughdatawerenotavail-
ableforCitrusRedNo.2andOrangeB,theirstructuresindicatethattheytoowouldbe
Figure2 Pyrazolonecolorants.
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