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Research ArticleReceived: 30 July 2008 Revised: 15 April 2009 Accepted: 5 May 2009 Published online in Wiley Interscience: 16 June 2009

(www.interscience.wiley.com) DOI 10.1002/jsfa.3668

Photocolorimetric determination of yolk colourin relation to selected quality parametersof eggsPetr Dvorak,a∗ Jana Dolezalovaa and Pavel Suchyb

Abstract

BACKGROUND: Determination of egg yolk colourity and colour was carried out by means of a new photocolorimetricmethodology based on the CIELAB system. Measurements (after gloss elimination) were performed using a Color Guide SphereSpex photocolorimeter. For comparison, a standard method according to the Yolk Colour Fan (Roche) was used.

RESULTS: Yolk colourity was measured in 252 eggs from layers of Isa Brown breed (aged 51–80 weeks). The colourity parameterL∗ ranged from 43.42 to 68.51. The parameter a∗ ranged from 0.05 to 13.49 and the parameter b∗ ranged from 22.38 to48.18. The normal (Gaussian) distribution curve for the individual parameters was validated. The resulting number of 8700combinations of yolk colour parameters was given by multiplication of possible results in the experiment while taking intoaccount the instrumental accuracy. All results according to the Yolk Colour Fan (Roche) scale represented a range of threecolour stripes.

CONCLUSION: The photocolorimetric determination is more precise than the Yolk Colour Fan (Roche) method. No correlationbetween yolk colourity parameters and yolk cholesterol concentration was found. The highest correlation (r = −0.919) wasobserved between yolk colourity parameter b∗ (yellow colour) and total egg weight.c© 2009 Society of Chemical Industry

Keywords: eggs; CIELAB; colourity; colour; cholesterol

INTRODUCTIONAn important biophysical parameter of egg yolk is its colour,which is often related to egg quality. The resulting perceptionof colour depends on the chemical and physical properties ofthe yolk, the parameters of the light source (wavelength andintensity) and the actual ability of the observer to perceivecolour.

Wherever possible, the term ‘colour’ is used only in its primarymeaning, i.e. as a quality of visual perception. For expression of theproperties of light and objects it is recommended to use terms suchas ‘chromaticity’ and ‘colourity’. Chromaticity expresses the colourproperties of light and is determined by the spectral compositionof the radiation emitted by the source.1 Colourity expresses thecolour properties of objects and is determined by the spectralcomposition of the source radiation and the spectral reflectivity orpermeability of the material; as such, it is affected by changes inthe spectral composition and intensity of light.2 From the aboveit is clear that egg yolk colour as perceived by the consumer isthe result of a physical interaction between light and the yolk thatis observed by sight and interpreted by the brain. Thus coloursare perceived individually. Despite this subjective perception,colourity can be compared objectively, but standard conditionsmust be determined for this purpose. To monitor colour changes,it is thus necessary to use instruments that record reflection orabsorption.2,3

In current practice the subjective determination of egg yolkcolour by sight is used most frequently, by means of comparison

with the Yolk Colour Fan (Roche) scale.4 This scale comprises 15colour stripes ranging from very light yellow through orangeto red. The colour shade is given by the sequence numberof the stripe. The scale was determined using colorimetry andassessed in terms of the 3D CIE (Commission Internationalede l’Eclaraige) colour diagram.5,6 However, this determinationis not sufficiently accurate. The surface colourity of foods canalso be measured by means of portable instruments usingspectrocolorimetry with defined parameters (geometry, lightsource, observer angle, opening diameter, etc.).2 The results areexpressed in the international colorimetric system of CIELAB.6 Thecalibration is based on a black standard as L∗ = 0 and a whitestandard as L∗ = 100.

The balanced CIELAB system is determined by three mutuallyperpendicular axes L∗, a∗, b∗ defined by the following equations

∗ Correspondence to: Petr Dvorak, Institute of Biochemistry, Chemistry andBiophysics, Faculty of Veterinary Hygiene and Ecology, University of Veterinaryand Pharmaceutical Sciences Brno, Palackeho 1–3, 61242 Brno, Czech Republic.E-mail: dvorakp@vfu.cz

a Department of Biochemistry, Chemistry and Biophysics, Faculty of VeterinaryHygiene and Ecology, University of Veterinary and Pharmaceutical SciencesBrno, 61242 Brno, Czech Republic

b Department of Nutrition, Animal Breeding and Animal Hygiene, Faculty ofVeterinary Hygiene and Ecology, University of Veterinary and PharmaceuticalSciences Brno, 61242 Brno, Czech Republic

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(Xn, Yn, Zn are trichromatic elements of the normalised lightused):

L∗ = 116(Y/Yn)1/3 − 16

a∗ = 500[(X/Xn)1/3 − (Y/Yn)1/3]

b∗ = 200[(Y/Yn)1/3 − (Z/Zn)1/3]

The coordinates a∗ and b∗ represent the regions of the spectrumwith wavelengths corresponding to colours from green (−a) tored (+a) and from blue (−b) to yellow (+b) respectively. The thirdcharacteristic, specific lightness L∗, placed vertically, is a functionof reflectance, i.e. the ratio of the reflected light intensity to thefalling light intensity. The scale of L∗ values represents an intervalfrom 0 (black) to 100 (white). The complementary colour systemis thus based on the differences between three elementary colourpairs: red/green, yellow/blue and black/white.

In the CIELAB system it is also possible to calculate, fromthe chromatic coordinates a∗ and b∗, parameters that give abetter idea of colour changes: chroma (C∗

ab) and hue angle (H◦ab).

C∗ab determines the distance from the centre of the chromatic

plane a∗b∗, while H◦ab specifies the angle in degrees between the

connecting lines of points representing the individual colours andthe origin. The origin is positioned on the positive half-axis a∗ (red,0◦), while the other half-axes correspond to b∗ (yellow, 90◦), −a∗

(green, 180◦) and −b∗ (blue, 270◦).Other colour systems (CIE L∗u∗v∗ and Hunter Lab xyY and XYZ)

have also been used in assessing the colour of meat.7 However,the CIE L∗a∗b∗ system is regarded as most suitable for determiningthe colour of egg yolk.

The aim of this study was to test a modern photocolorimetricevalution of egg yolk colour and to estimate correlations amongCIE L∗a∗b∗ parameters and some other selected parameters ofeggs originating from a standardised breed.

MATERIALS AND METHODSWe assessed the yolk colourity and colour in 252 eggs fromlayers of Isa Brown breed (aged 51–80 weeks). The hens werebred in cages with bedding in the accredited experimentalworkplace of the Department of Nutrition, Animal Breeding andAnimal Hygiene of VFU Brno in accordance with the conditionsspecified in the technological instructions for breeding of Isa Brownhybrids. During the monitoring period, all birds were supplied withcommercially manufactured complete feeding mixtures N1 (untilthe 5th month of laying) and N2 (starting from the 5th month oflaying to the end of monitoring). We used standardised eggs from58 to 62 g in weight.

Before the determination of yolk colourity and colour the eggswere left in a fridge for 24 h at a temperature of 8 ◦C in order toachieve better separation of the yolk from the white. Then indi-vidual egg yolks were separated from the white, placed on Petridishes (50 mm diameter) and covered with thin elastic cling foil.

Colourity was determined objectively in the CIELAB system bymeans of a portable Color Guide Sphere Spex photocolorimeter(BYK Gardner Company, Geretsried, Germany). Gloss was elim-inated by positioning the instrument perpendicular to the padthat was placed on the foil in order to protect the instrumentagainst penetration by the yolk.

The instrument was first calibrated for the cling foil used. Thiswas done by placing the instrument perpendicular to the blackstandard and then perpendicular to the white standard. The

calibration was checked on the green standard. The standardswere covered with the cling foil used. To perform the actualmeasurement, the instrument was placed perpendicular to thePetri dish filled with the egg yolk and the pad. All egg yolkmeasurements were done automatically three times. The resultingvalue was the average of the three measured values.8,9 The resultsfor parameters L∗, a∗, b∗ were divided into categories. The absolutefrequencies of individual categories were processed in the formof bar graphs. The Gaussian distribution was checked by theχ2 test.

Subjective assessment was performed by means of visualcomparison of egg yolks with the Yolk Colour Fan (Roche)scale.

We also determined total egg weight, yolk weight, whiteweight, eggshell weight and yolk cholesterol concentrationspectrophotometrically using the modified Bio-La-Test.10 Thedependence of these parameters on the L∗, a∗, b∗ parameterswas determined using correlation coefficients. All statisticalcalculations were done with Statistica software (StatSoft, Tulsa,Oklahoma,USA).

RESULTSThe results obtained by the objective and subjective methods werecompared. Each method showed a different number of resultingvalues of determination.

The parameter L∗ ranged from 43.42 to 68.51, with the highestfrequency of 73% corresponding to values in the range 56–64(Fig. 1). The parameter a∗ ranged from 0.05 to 13.49, with thehighest frequency of 81% corresponding to values in the range2–8 (Fig. 2). The parameter b∗ ranged from 22.38 to 48.18, with thehighest frequency of 72% corresponding to values in the range30–41 (Fig. 3). The normal (Gaussian) frequency distribution wasconfirmed. Thus the highest frequency of the values was aroundthe arithmetic mean of the particular colourity parameter. Themore the values differ from the arithmetic mean, the lower is thefrequency of their occurrence (Figs 1–3). The wide range of valuesshows the high sensitivity of the photocolorimetric method. Theresulting number of combinations is given by multiplication of thepossible resulting parameters of yolk colourity in our experiment.The number of combinations is 8700.

All results according to the Yolk Colour Fan (Roche) scalerepresented a range of three stripes of values 5, 6 and 7.

The following values (mean ± standard deviation) weredetermined: total egg weight, 60.85 ± 1.079 g; eggshell weight,7.31±0.128 g; white weight, 37.41±0.942 g; yolk weight, 16.15±0.305 g; yolk cholesterol concentration, 11.3669±0.22399 g kg−1.

The coefficients of correlation between yolk colourity parame-ters, egg weight parameters and yolk cholesterol concentrationare given in Table 1. The correlation coefficients with negativevalues represent inverse proportionality. The most significant cor-relations were between yolk colourity parameter b∗ and total eggweight (r = −0.919) and between parameter b∗ and white weight(r = −0.918). The highest closeness of dependence was shown bythe mutual relation between yolk colourity parameters L∗ and b∗

(r = 0.927).

DISCUSSIONThe photocolorimetric determination of colour in the CIELABsystem is better because it gives information on both colourhue and lightness. The continuous quality of the individual

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Figure 1. Frequency distribution of yolk colourity parameter L∗ (n = 252).

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Figure 3. Frequency distribution of yolk colourity parameter b∗ (n = 252).

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Table 1. Correlation coefficients from mutual comparison of yolkcolourity parameters, egg weight parameters and yolk cholesterolconcentration

Parameter L∗ a∗ b∗

Total egg weight −0.746 −0.715 −0.919

Yolk weight −0.632 −0.860 −0.844

Eggshell weight −0.666 −0.831 −0.861

White weight −0.812 −0.475 −0.918

Yolk cholesterol −0.303 0.430 −0.122

L∗ 1

a∗ 0.205 1

b∗ 0.927 0.477 1

parameters enables using the t test or variance analysis forstatistical comparison of averages in groups with a sufficient rangeof sets and for confirmation of the normal (Gaussian) distributioncurve. The Yolk Colour Fan (Roche) scale can determine onlythe sequence number of a stripe, so it is only possible to processdata statistically using tests for discontinuous quantities (χ2 testor test of independence in contingency tables). In this casethe variation range (maximum–minimum value) is representedby the difference in the three stripes. The significantly highersensitivity of CIELAB colorimetric methods is most evident inthe much wider variation range. The number of combinations ofindividual parameters is 2509 for L∗, 1344 for a∗ and 2580 forb∗. For the final calculation, where we took into considerationcorrections for possible instrumental inaccuracy, we used thevalues of the variation range rounded to one decimal point,which decreased the sensitivity of the determination significantly.Even so, the number of 8700 combinations is many timeshigher than for the Yolk Colour Fan (Roche) scale. Statisticalcomparison of the two methods is not possible, because wecannot compare the quantitative (CIELAB) and qualitative (Roche)results owing to the fact that the latter data had only threevalues.

The highest values of the correlation coefficients in Table 1represent the close negative relations between the parameter b∗and total egg weight (r = −0.919) and white weight (r = −0.918).We can assume that the deposition of yellow pigment in eggyolk is related to the current metabolism capability of the hen.The parameter a∗ is most highly and negatively correlated withyolk weight (r = −0.860). Thus, as the parameter a∗ reflectsthe representation of red colour in the spectrum, with growingegg yolk weight the proportion of orange colour preferred bythe consumer decreases. The lowest correlations were foundbetween the parameters of yolk colourity and the concentrationof cholesterol in the yolk. This confirms previous data indicatingthe independence of the concentration of cholesterol and thedeposition of colour pigments in egg yolk.10

The new method of assessing egg yolk colourity presented inthis paper can determine exactly the impacts of life parameters,

especially feeding of hens. The effects of hen feeding on thecolour of eggs are generally known. Environmentally friendlybreeding of hens by means of grazing on meadows producesegg yolks with a significant proportion of orange colour. In orderto imitate this colour, many manufacturers of feeding mixturesuse artificial colour pigments. Unlike the Roche method, ourpreliminary results11 showed that the photocolorimetric methodenables differentiation of the origin of eggs.

CONCLUSIONBased on a comparison of the results obtained by the two methods,it is evident that the photocolorimetric determination is muchmore exact. The method is not distorted by subjective perceptionof colour. Instrumental determination of the colourity is thusmore beneficial, since it provides much information about yolkcolour, is not very demanding in terms of time and space and isabsolutely objective. The determination of colour stripes of theYolk Colour Fan (Roche) scale is influenced by light and subjectiveperception and does not provide any other information apart fromthe sequence number of the stripe.

ACKNOWLEDGEMENTThis study was funded by grant MSM6215712402 from the Ministryof Education, Youth and Sport of the Czech Republic.

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4 Voilleumier JP, The Roche Yolk Colour Fan – an instrument formeasuring yolk colour. Poultry Sci 48:767–779 (1969).

5 McLaren K, CIELAB hue-angle anomalies at low tristimulus ratios.Colour Res Appl 5:139–143 (1980).

6 CIE, Colorimetry (CIE Publication No. 15.2) (2nd edn). CommissionInternationale de l’Eclairage, Vienna (1986).

7 Esteban MG, Ansorena D, Gimero O and Astiasaran I, Optimization ofinstrumental colour analysis in dry-cured ham. Meat Sci 63:287–292(2003).

8 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).

9 Dvorak P, Strakova E, Kunova J and Kunova V, Variation in egg yolkcolour with the composition of the feeding mixture for laying hens.Acta Vet Brno 76:77–82 (2007).

10 Ingr I and Simeonova J, Rapid determination of cholesterol in egg yolkby the Bio-La-Test. Vet Med Prag 28:97–104 (1983).

11 Kunova J, Mikundova M, Kracmarova E and Kunova V, Colordetermination of egg yolk by CIELAB. VIth Conf. of YoungResearch Workers (with International Participation), Brno, pp. 28–30(2004).

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