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Research ArticleReceived: 5 January 2011 Revised: 23 June 2011 Accepted: 16 August 2011 Published online in Wiley Online Library: 14 October 2011

(wileyonlinelibrary.com) DOI 10.1002/jsfa.4657

Possibilities of enhancing the colour of egg yolkPetr Dvorak,a∗ Pavel Suchy,b Eva Strakovab and Vladimır Koprivaa

Abstract

BACKGROUND: This study attempts to compare two possibilities of enhancing the colour of egg yolk. One of them is basedon the ecological rearing of laying hens on natural green grass whereas the other uses a feeding dose supplemented withnatural pigments in laying hens reared in individual cages. Is it possible to distinguish these two technologies using yolk colourdetermination in the CIELAB system?

RESULTS: Yolk colour parameters such as L∗, a∗, and b∗ in the group of grazed hens are significantly different (α = 0.001) fromthose observed in hens reared in cages. The yolk colour shows a darker, redder and more yellow colour. The greatest differencewas seen in the red colour parameter, a∗, that increased more than twice. Visually, this means a shift towards a more orangecolour. Compared to grazing in the meadow (�E∗ = 13.257), the addition of artificial pigments in the feed resulted in a moresignificant increase in the parameter �E∗ (CIE total colour difference), with the greatest value of �E∗ being observed with theuse of both pigments (�E∗ = 24.265).

CONCLUSION: Grazing increases the parameter a∗ whereas the values of the parameter C∗ab remain relatively low. The

parameter �E∗ is significantly lower in the case of grazing as compared to the supplementation of the feed with pigments.However, colourity parameters cannot be used as a specific standard to identify a particular grazing technology as their valuesvary during the laying period.c© 2011 Society of Chemical Industry

Keywords: egg; CIELAB; food colour; yolk

INTRODUCTIONConsumers prefer foods of optical and sensory quality.1 The colouras a qualitative feature of egg yolk is usually associated with thehigh quality of eggs and egg products. Generally, consumersthink that the richer colour of egg yolk indicates better qualityand freshness. Consumers in some European countries, such asGermany, Belgium, the Netherlands and Spain, prefer an eggyolk of orange colour [i.e. values varying in a range of 13–14 inthe Roche Colour Fan (RCF)]. Consumers in France, England andFinland prefer a more yellow colour of egg yolk (RCF 11–12),whereas in Ireland and Sweden a light colour of egg yolk (RCF8–9) is accepted.

Where possible, the term ‘colour’ should only be used to describea subjectively perceived colour. The term ‘chromatism’ is used todescribe the properties of a light source and to characterise colourproperties (spectral composition).2 The term ‘colourity’ describesthe colour properties of objects. Colourity depends on the spectralcomposition of the source of radiation and spectral reflectance orpermeability of a particular material, which results in changes inboth the spectrum and the intensity of radiation.

Unlike the subjectively perceived colour, colourities can becompared objectively using the instruments that record thereflection or absorption, which we reported earlier in this journal.3

Such instruments include spectro-colorimeters with definedparameters. The results are expressed using the InternationalColorimetric System (CIELAB, Commission Internationale del’Eclairage, 1986).4 The complementary colour model is based onthe differences between the three pairs of elementary colours:red–green (parameter a∗), yellow–blue (parameter b∗), andblack–white (parameter L∗). The specific lightness, L∗, as the

third characteristic, is a function of reflectance which is the ratioof the intensity of reflected light to the intensity of incident light.The scale ranges from 0 (black) to 100 (white). Other parametersare determined by calculation. �E∗ (CIE total colour difference)is a distance between two points. The chroma, C∗

ab, (specificcolour fullness) is a value that indicates a difference between therespective colourity and the grey colour. The hue, hab, (the specificangle of a particular colour shade) is expressed by the name of aparticular colour (red, yellow, etc.).

Egg producers search for suitable feeds to obtain egg yolk ofa dark yellow colour. One possibility is to use the alga Chlorellavulgaris.5 The tomato cream and carotenoids can also enhance thecolour of egg yolk.6

Natural pigments can be replaced with synthetic pigmentsthat have been permitted as additives of feeding mixtures forlaying hens.7,8 Apart from synthetic pigments, there is also anumber of other natural pigments that can be divided into yellowand red pigments. They are usually characterised by using thexanthophyll equivalent concentration.9 The addition of carophyll

∗ Correspondence to: Petr Dvorak, Department of Biochemistry, Chemistry andBiophysics, Faculty of Veterinary Hygiene and Ecology, University of Veterinaryand Pharmaceutical Sciences Brno, Palackeho 1–3, 612 42 Brno, Czech Republic.E-mail: dvorakp@vfu.cz

a Department of Biochemistry, Chemistry and Biophysics, University of Veterinaryand Pharmaceutical Sciences Brno, Czech Republic

b Department of Nutrition, Animal Breeding and Animal Hygiene Faculty ofVeterinary Hygiene and Ecology, University of Veterinary and PharmaceuticalSciences Brno, Czech Republic

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red or carophyll yellow (at a level of 10–20 mg kg−1 of feed) willchange the colour of fat contained in egg yolk.10

Apart from a diet, there are also other factors that can havea considerable effect on the quality of eggs. The yolk colour isalso affected by the technology of hen farming.11 Authors havereported that the colour of yolk in eggs from laying hens rearedusing the outdoor rearing system is darker than the yolk in eggsfrom hens reared in cages.

This study attempts to compare two approaches to obtainingthe rich colour of egg yolk: the ecological grazing of laying hens inthe meadow, and the addition of natural pigments in the feedingdose for laying hens reared using the standard rearing system inindividual cages. Is it possible to distinguish these two technologiesusing yolk colour determination in the CIELAB system?

MATERIALS AND METHODSThe experiment was performed in the accredited animal enclosureof the Department of Nutrition, Animal Husbandry, and AnimalHygiene, Faculty of Veterinary Hygiene and Ecology, Universityof Veterinary and Pharmaceutical Sciences in Brno. During theexperiment, hens were housed in individual cages and were fedad libitum with a commercial complete feeding mixture (crudeprotein 195.4 g, fat 52.2 g, fibre 17.0 g, starch 433.3 g, ash 151.5 g,Ca 44.4 g, P 6.5 g, Mg 2.3 g and ME 11.09 MJ). Water was suppliedcontinuously ad libitum using automatic feeders.

The first experiment was performed with a total of 60 Isa Brownhens aged 140 days that were divided into two groups, eachcontaining 30 hens. Thirty hens were grazed on natural greengrass whereas the other 30 hens were housed in three standardcages, each containing 10 hens.

The second experiment was performed with a total of 40 IsaBrown hens aged 448 days that were divided into four groups,each containing 10 animals. The first group was a control groupthat received a feeding mixture which was not supplementedwith pigments. The other three groups were experimental groupsin which hens received feeds supplemented with commercialpigments supplied by Lohmann Animal Health GmbH & Co.(Cuxhaven, Germany). The feeding mixture in the first experimentalgroup was supplemented with the yellow pigment avizant yellow20S at a level of 0.5 g kg−1 of feeding mixture, the feeding mixturein the second group was supplemented with the red pigmentavixanthin 100 at a level of 0.07 g kg−1 of feeding mixture,and the third experimental group received the feeding mixturesupplemented with both pigments at the levels specified above.Avizant yellow 20S contains natural xanthophylls obtained fromthe extract of Mexican marigold (Tagetes erecta).

Avixanthin 100 contains a minimum of cantaxanthin concen-tration of 100 g kg−1 of microgranulate. The preparatory periodlasted 7 days and was followed by the collection of eggs foranalysis (30 eggs were collected in each group).

Determination of egg yolk colourThe egg yolk was separated from the white, placed on a Petridish (50 mm in a diameter), and covered by a thin food foil. Thecolour of the yolk was determined in the CIELAB system usingthe portable colour-guide sphere spex spectrophotometer (BykGardner, Geretsried, Germany), excluding gloss, using a sphericalgeometry d/8◦, D65 as a source of light, the standard observer’sangle set to 10◦, and the diameter of the opening being 8 mm.The instrument was calibrated to the food foil used, prior to

Table 1. Comparison of colour parameters of egg yolk for cage-basedrearing technology using a standard feeding mixture and for outdoorgrazing in the meadow

Parameter

Technology L∗ a∗ b∗ C∗ab hab

�E∗comparedto control

Cage

Mean 48.02 6.00 36.91 37.39 89.74 0.000

SEM 0.547 0.161 1.253 – – –

Grazing

Mean 43.25 14.58 45.82 48.08 80.39 13.257

SEM 0.768 0.371 0.537 – – –

t-test (α = 0.001)

t(47) = 3.397 5.064 21.234 3.803 – – –

measurement. All measurements were performed three times andthe mean value of all three measurements was calculated andused.12

In order to compare the results, further parameters of theCIELAB system were calculated from the mean values of L∗, a∗ andb∗ coordinates.

The distance between the two points, �E∗ (CIE total colourdifference) was calculated according to the equation �E∗ =[(�L∗)2 + (�a∗)2 + (�b∗)2]1/2. The chroma, C∗

ab, is a valuethat indicates the difference between the respective valueof colourity and the grey colour, according to the equation

C∗ab = (a∗2 + b∗2

)1/2. The hue (the specific angle of the colourshade), hab, is described by the name of a particular colour. Itsvalue is calculated according to the equation hab = tan−1 (b∗/a∗).

Data obtained for each parameter (L∗, a∗, b∗) were processedstatistically (Table 1 and Table 2). The effect on yolk colourparameters of the pigment added in the feeding mixture wastested using the single-factor ANOVA (MS Excel). Differencesbetween the cage technology and outdoor grazing were testedusing the t-test. Since we assumed great differences in bothexperiments, the null hypothesis was determined for the level ofsignificance being α = 0.001.

RESULTSBy comparing both (control) groups of hens reared in cageswithout the supplementation of feeding mixtures with pigments(Tables 1 and 2), we have found differences in parameters b∗and C∗

ab. Differences in the parameter L∗ do not exceed thevariability inside the groups. Similarly, the parameter a∗ showed anon-significant decrease.

Parameters of yolk colour, such as L∗, a∗, b∗, for a group of grazedhens differed significantly (α = 0.001) from those for a group ofhens reared in cages (Table 1). Yolks were darker, redder andmore yellow. The greatest difference was found for the red colourparameter, a∗, which increased more than two-fold. Visually, thismeans a shift towards a more orange colour.

Colour parameters of yolk in a group of hens fed with feedingmixtures supplemented with colouring additives are listed inTable 2. The parameter L∗ decreased upon the addition of naturalpigments, which indicates that the yolk turned dark. By analogy,an increase in parameters a∗ and b∗ correlates with the use of aparticular type of pigment. Surprisingly, the level of the parameter

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Table 2. The effect of natural pigments (Avizant yellow 20s orAvixanthin 100 or their mixture) added in the feed on the colourparameters of egg yolk

Parameter

Sample L∗ a∗ b∗ C∗ab hab

�E∗comparedto control

Control

Mean 51.63 5.43 59.01 59.26 94.16 0.000

SEM 0.713 0.192 1.588 – – –

Yellow (Avizant)

Mean 50.88 7.24 76.82 77.16 94.02 17.917

SEM 0.706 0.293 1.156 – – –

Red (Avixanthin)

Mean 44.29 20.59 68.95 71.95 81.53 19.558

SEM 0.524 0.924 1.274 – – –

Yellow + red

Mean 40.85 25.62 67.07 71.80 76.77 24.265

SEM 0.902 0.901 1.615 – – –

ANOVA(α = 0.001)

F(3,116) = 5.798 51.911 218.300 26.071 – – –

a∗ was higher than that in the control with the use of the yellowpigment, and the level of the parameter b∗ increased with thered pigment. Interesting results were also obtained with the useof both pigments. The parameter a∗ (red colourity) was higherthan that in the group in which only the red pigment was added.In contrast, the parameter b∗ (yellow colourity) decreased incomparison with the group in which only the yellow pigment wasadded. This finding indicates that the final colour of the yolk isaffected by the red pigment rather than by the yellow pigment, atrespective concentrations of both pigments. Another interestingresult is that the parameter C∗

ab is similar in experimental groups(Table 2) but differs significantly from the control group. Thisindicates a significant increase in colourity as compared to greycolour. Compared to the control, the parameter hab remainedunchanged upon the use of the yellow pigment. The yellow shadewas maintained because both parameters a∗ and b∗ increasedsimultaneously.

The parameter �E∗ increased more significantly with the useof artificial pigments than in the case of grazing in the meadow.�E∗ was 13.257 for grazing (Table 1) whereas for the addition ofpigments, �E∗ increased, with the highest value being observedwith the use of both pigments �E∗ = 24.265 (Table 2).

DISCUSSIONIt follows from the comparison of the egg colour in the groupof hens reared in cages (Table 1) with that in the control group(Table 2) that there were differences in parameters b∗ and C∗

ab.Such differences can be explained in terms of the reduced layingintensity at the end of the laying period (Table 2). Pigmentsprobably accumulated to a larger extent in the yolk at the samefeeding dose and rearing technology.

The technology of hen farming is one of the factors thatenhances the yolk colour. Experiments have revealed that thecolour of egg yolk is more intense at outdoor grazing than atcage rearing.11 Our study also provides objective evidence that

the colourity of egg yolk increases when laying hens are grazed ina meadow. Grazing increases the parameter a∗ (Table 1) whereasthe parameter C∗

ab remains relatively low. The parameter �E∗ isalso significantly lower than in the case of the supplementation ofa feed with pigments. It follows from our findings that variation incolourity during grazing is a natural result in comparison with theuse of feeds supplemented with pigments. The parameter �E∗

increased significantly, which is associated with a more significantchange in the colour shade. Some consumers may perceive thischange in colour shade as unnatural, which may have a negativeeffect.

However, colourity parameters cannot be used as a specificstandard to identify a particular grazing technology as their valuesvary during the laying period (see above). Baiao et al.9 comparedthe colour parameters of egg yolk upon the supplementationof feeds with oxycarotenoids in cage-based and litter-based henfarming in commercial egg production and recommended theuse of the product of parameters a∗ and b∗ for evaluation. Itfollows from their results that the optimum ratio between theyellow pigment and the red pigment added in the feed is 1 : 1.Yellow xanthophylls contain up to 25% of astaxanthins, 20% oftrans-zeaxanthin, and 80% of trans-lutein. The intensity of egg yolkcolour is proportional to the content of xanthophylls in a diet oflaying hens.13

The intensity of colourity particularly depends on the parameterL∗. However, the yellow pigment avizant yellow 20S added at adose of 50 g per 100 kg of a feeding mixture (used in this study) didnot result in a conclusive increase in this parameter. Similarly, thehue parameter hab, expressed by the name of a particular colour,was maintained (Table 2).

The distribution of vitamin E and antioxidants in the feedingdose of laying hens is another specific issue.14 The addition ofother substances may interfere with the effect of natural pigmentspresent in a feeding dose on the colour of yolk.β-Apo-8′-carotenoicacid ethyl ester pigmented the yolk more efficiently than eitherof two natural saponified marigold extracts.9 Authors have alsoreported the effect of a marigold and pepper extract on the ratioof yellow and red carotenoids and on their natural accumulationin the yolk. RCF values varied in a range of 12.9–13.3.9

Our previous study15 demonstrated a positive effect of litter-based rearing on egg colour parameters, as compared to cage-based rearing. During the 7-month monitoring, the parameter b∗

increased from the third month. The only difference in the feedingdose was that laying hens have access to litter.

The effect of colour pigments, particularly yellow xanthophylls,in poultry is not manifested only in the colour of egg yolk. Theycan also accumulate in breast and abdominal fat tissue. Theaddition of zeaxanthin and cantaxanthin in a feeding mixture forbroilers shows optical activity and optical inactivity, with opticallymore active isomers being accumulated in fat more than opticallyinactive isomers. The respective ratio between optically active andoptically inactive isomers is as follows: 97.8% : 2.2% in SME-10, and16.0% : 84.0% in SME-25. The accumulation of pigments in fat istherefore more significant with SME-10.16

In contrast, some technologies of egg processing, for exampleirradiation, may cause the yolk to lose its colour.12 The monitoringof the additions of some substances is also important. For example,the polysaccharide chitosan has no negative effect on the colourof thermally treated yolk.17

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CONCLUSIONChanges in the feed and the technology of the nutrition of layinghens are clearly reflected in the parameters of yolk colour. Grazingincreases the parameter a∗ (a change from yellow to orange colour)whereas the chroma parameter, C∗

ab, remains relatively low. Theparameter �E∗ (difference in colour) is also significantly loweras compared to the addition of a pigment in the feed. However,colourity parameters cannot be used as a specific standard toidentify a particular grazing technology as their values vary duringthe laying period.

ACKNOWLEDGEMENTThis study was funded by the grant no. MSM6215712402 from theMinistry of Education, Youth and Sport of the Czech Republic.

REFERENCES1 Ueda R, Okamoto N, Araki T, Shibata M, Sagara Y, Sugiyama K, et al,

Consumer preference and optical and sensory properties of freshcod roe. Food Sci Technol Res 15:469–478 (2009).

2 Drdak M, Objectification Evaluation of Colour Some Foodstuffs, 1stedition. VEDA, Bratislava, pp. 18–44 (1985).

3 Dvorak P, Dolezalova J and Suchy P, Photocolorimetric determinationof yolk colour in relation to selected quality parameters of eggs.J Sci Food Agric 89:1886–1889 (2009).

4 Commission Internationale de l’Eclaraige, Colorimetry, 2nd edition,Publication No. 15.2. CIE, Vienna (1986).

5 Gouveia L, Veloso V, Reis A, Fernandes H, Novais J and Empis J,Chlorella vulgaris used to colour egg yolk. J Sci Food Agric70:167–172 (1996).

6 Knoblich M, Anderson B and Latshaw D, Analyses of tomato peel andseed by-products and their use as a source of carotenoids. J Sci FoodAgric 85:1166–1170 (2005).

7 Anonymous, Act No. 91/1996 Coll., on Animal Feed (Zakon e. 91/1996Sb., o krmivech) (1996).

8 Anonymous, Decree of the Ministry of Agriculture No. 451/2000 Coll.implementing the Act on Animal Feed, in the wording of Act No.91/1996 Coll. on Animal Feed as amended by Act No. 244/2000Coll. (Vyhlaska ministerstva zemıdılstvı e. 451/2000 Sb., kterouse provadı zakon e. 91/1996 Sb., o krmivech, ve znını zakona e.244/2000 Sb.) (2000).

9 Baiao NC, Mendez J, Mateos J, Garcia M and Mateos GG, Pigmentingefficacy of several oxycarotenoids on egg yolk. J Appl Poult Res8:472–479 (1999).

10 Anderson DM, Maclssac JL, Daniel MA, MacKinnon TL and Budgell KL,Evaluating the effect of crab meal, Carophyll Red (R) and CarophyllYellow (R) in laying hen diets on egg yolk pigmentatiun andproduction performance. Can J Anim Sci 88:637–640 (2008).

11 Van Den Brand H, Parmentier HK and Kemp B, Effects of housingsystem (outdoor vs cages) and age of latiny hens on eggcharacteristics. Br Poultry Sci 45:745–752 (2004).

12 Dvorak P, Kunova J, Strakova E, Suchy P and Kunova V, Changes in thecolour and the acidity number of egg yolk upon irradiation. EurFood Res Technol 221:348–352 (2005).

13 Klasing KC, Comparative Avian Nutrition. CAB International,Wallingford (1998).

14 Leeson S and Summers JD, Commercial Poultry Nutrition, 3rd edition.University Books, Guelph, Ontario (2005).

15 Dvorak P, Suchy P, Strakova E and Dolezalova, J. Variation in egg yolkcolour in different systems of reading laying hens. Acta Vet Brno79:(Suppl.): 13–19 (2010).

16 Perez-Vondreil AM, Hernandez JM, Llaurado L, Schierle J and Brafau J,Influence of source and ratio of xanthophyll pigments on broilerchicken pigmentation and performance. Poltry Sci 80:320–326(2001).

17 Caner C and Cansiz O, Chitosan coating minimises egg-shell breakageand improves egg quality. J Sci Food Agric 88:56–61 (2008).

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