Chemistry in the Dyeing of Eggs R M C. Mebane and Thomas R. RybM University of Tennessee at Chattanooga, Chattanooga, TN 37403 The coloring of eggs with synthetic dyes is a common tradition associated with the Easter holiday and is an activity well-suited for a variety of simple experiments and demonstrations. We wish to discuss the dyeing of eggs and report some experiments that can he used to study factors influencing the intensity of dye that adheres to the surface of an eggshell. Since antiquity, the egg has been a symbol of rebirth and renewal and has frequently been associated with celehrations of spring (1).An ancient Persian myth holds that the earth was hatched from an egg a t the time of the spring equinox (2). The egg had symbolic meaning for the Egyptians, Romans, Gauls, and Greeks, as well as the Persians. The egg plays a role in the Jewish Passover meal and represents the life, growth, and fruition that followed their departure from Egypt. It was used by the early Christians as a symbol of the resurrection of Christ. When eggs were dyed red, the color represented the blood of Christ. In addition to their use as an emblem of the resurrection, colored eggs have been used for hundreds of years to celebrate the end of abstinence associated with Lent (3).A variety of games and customs involving the throwing, cracking, rolling, hunting, or exchanging of colored eggs continues to the present day. The first dyes used for coloring eggs were derived from plant and animal sources since synthetic dyes were not developed until 1856 (4). At the turn of the century, many of the same synthetic dyes used in textile dyeing were also used to color foods and eggs. Over the years, government regulations have restricted the use of dyes in foodstuffs. In 1964, a list of dyes suitable for coloring eggs was published (5). There are seven FD&C (food, drug, and cosmetic) certified dyes (see Fig. 1) approved by the FDA, Food and Drug Administration, for use in foods (6).Many commercial foodcolor preparations provide recipes for their use in dyeing eggs, or one can use the solid dye tablets (7) that are available in stores around Easter. I t is interesting to note that in two in vivo studies where laying hens were fed water-soluble and alcohol-soluble dyes, only artificially colored yolks were obtained (8,9). Although the dyeing of eggs is a simple and familiar activity, an examination of the chemistry associated with this process can lead to insight into the structure and use of synthetic dyes, surface interactions and modifications, properties of proteins, and effects of ions and acid on the formation of salt linkages. The activities described in this paper are quite suitable for junior and senior high school students as laboratory experiments, science fair projects, or simply as activities to explore some applied, "real world" chemistry. Experiments
T o study factors affecting the adsorption of dye on eggs, experiments were conducted to examine the effect of pH and acid variation, dye concentration variation, solvent ionic strength variation, and surface modification. Typical instructions (10) for the dyeing of eggs call for the addition of vinegar to a water-dye solution. Either hard-boiled or unboiled eggs are then dipped into solution until the desired color is obtained. Let us examine some of the questions raised hy this seemingly simple activity. What is the effect of pH on dye adsorption? A stock solution was prepared by adding 90 mL of French's red food
color (water, propylene glycol, and 2.5% dye-FD&C red dves nos. 3 and 40. and FD&C blue no. 1) to 1290 mL of water. Twelve samplea, each containing80 ml. of stock solution, were prepared and adjusted by addition of HCl(aq) or NaOH(aq) to span a pH range from I to 12 by increments of 1. One hard-boiled (unboiled eggs work as well) egg (medium-size hen eggs were used in all experiments, was added to each sample beaker and allowed ro srand for LO min ar room temoerature. 20 O C . Formation of bubbles of carbon dioxide was'observed only a t the lowest pH. Each egg was removed after 10 min.. d i.. n ~ e dfour times in a beaker of water. and allowed to dry a t room temperature. A significant variation of color intensitv was observed raneine from dark red a t nH 1to a light pinLat pH 12 (see Fig.>).kthough the colo; of the dve solutions was not chaneed with DH. as the OH was lowered, the amount of dye i h e r i n g to the eggshell increased and the color of the eee became darker. What is the effect of varying the concentration of uinegar? Three different stock solutions were prepared by adding 30 mL of either French's red, green (2.5% FD&C yellow no. 5 and FD&C blue no. 11, or yellow (4%FD&C yellow nos. 5 and 6 ) food color to 430 mL of water. Samples, each containing 80 mL of stock solution, were placed in beakers, and to each s a m ~ l eeither . 0. 1. 3. 5. or 10 mL of vineear (white distilled, 4.6% acetic acidj i e r k added. Eggs were died in each of these 15 solutions followine indicated -the nrocedure . previously. The color of the eggs became darker as the amount of vinegar was increased (see Fig. 3). The trend of
FObC R e d No. 40 Allurn Red
FDbC R e d No. 3 Erythro5lne
COzNa SOjNa F O ~ C Y e l l o w NO. 5
FObC Y e l l o w No. 6 sunset Y e l l o w
1aTtra21ne
\SO~NH~ FDbC Blue No. I ~ r i l l i a n Blue t FCF
FOhC Blue No. 2 lndlgo c a r m i n e
F a s t Green FCF
Figure 1. F M C cenifisd food colors.
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F i w e 3. Variation o f h amount ofvinegsr(h0mbit to ,right 10,5.3,1. O m l of -1" in 80 mL of slack dye soUlon).
Flaure 4. VarLnion of the mcenbstlon of stock dve solution (first row. first sample an len Is stock dye solullon--from lell to right me mncemratlon Is sequernlslly decreased by a factor of 2 and cominuer dscreaslng on me mp row. lefl to right).
darker eggs with more vinegar was most evident with red, while the vellow and blue tended to level off in the range - of 3 to 5 mL of vinegar. What is the effect of varying the concentration of dye? A stock solution was prepared using red food color as indicated previously. Starting with this stock solution, 12 solutions kere nreoared hv successive dilutions so that each new solution was one-half the concentration of the previous one. Vineear (10 mL) was added to each of these 80-mL solution samges,'and one egg was placed in each of these solutions and dyed following the previously indicated procedure. The coloring of the eggs was proportional to the concentration of solution (see Fig. 4), and the resultant eggs ranged from a dark red to a very faint pink. Does pretreatment of an egg in vinegar haue a n effect on the adsorption of dye? It was apparent from the first two experiments that the presence of acid is important and makes a significant con&bution to the amount of dye adhering to the surface of an egg. To further understand the role of acid in the dyeing process, an egg was soaked in vinegar (pH
3) for 10 min and subsequently dyed for 10 min in 80 mL of neutral red stock solution. Treated and untreated eggs were found to adsorb dye to the same extent. Does pure calcium carbonate adsorb dye to the same extent as eggshells? Knowing that an eggshell is mostly calcium carbonate, the following experiment was performed to examine a model system. Pieces of pure calcium carbonate (marble chips) were placed in a red stock dye solution containing vinegar for 10 min. Very little dye adhered to the surface, and the effect was dramatically different from that observed for eggshells. The formation of bubbles was apparent for the marble chips where it was not observed for the eggshells at the same pH. Is there something on the surface of an eggshell that makes an egg different from calcium carbonate? An examination of the literature on eggs revealed that there is a thin layer of protein, the cuticle, that surrounds and protects the eggshell. In a pretreatment experiment designed to remove the cuticle, an egg was soaked in a 5%EDTA solution adjusted to pH 8.0 (11).The egg was removed after 90 min and
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liehtlv scrubbed under water to remove the cuticle. The ckcie-free egg and an untreated egg were dyed using 10 mL of vinegar in 80 mL of red stock solution. The untreated egg turned dark red; however, the cuticle-free egg was dramatically different-it turned pink, not red. Does the presence of sodium chloride in the dye solution haue a n effect on dveine? T o test if the bindine of dve to the eggshell & ionic in nature, the ionic strength of-the dye solution was varied with sodium chloride. Additions of increasing amounts of sodium chloride to the standard dye solutions (80 mL of stock dye solution and 10 mL of vinegar) resulted in decreasing amounts of dye adsorbing on the eggs. Concentrated salt solutions were observed to reduce the amount of dye adsorbed on the eggs significantly in either acid or neutral pH. Nonionic solutes such as sucrose and isopropyl alcohol had minimal effects on color and intensity of dyeing.
An understanding of the mechanism by which food rolor bind* to eaeshells is ~ussibleonlv after consideration of hoth the chemical compcktion of eggshells and the structure of the dyes in food colors. The chicken eggshell consists of approximately 95% calcite (12). which is one of the crvstalline forms of calcium carbonate, and a sparse protein matrix that hinds the calcite crystals together (11,13). As mentioned ahove, covering the surface of the eggshell is a thin coating of material known as the cuticle. The cuticle is approximately 90% protein (11, 14). The structures of the food colors usedin this study (FD&C certified vellow nos. 5 and 6. red nos. 3 and 40.and hlueno. 1) are among [he appruved food rolur dyes shown in Figure 1 These f w d colors are classified as anionic or acid dves uwine to their basic sulfonate groups (carhoxylate in the case of FD&C red no. 3). Not surorisinalv. these molecules can behave as direct dyes and Ehemi&ly bond to surfaces with cationic sites through salt linkages-much like the interaction between anionic direct dyes and cationic sites (-NH3+ groups) of the protein structure in wool or silk (15,161. Since the eggshell contains protein material bound to and into its crystalline structure, it is reasonable that the primary interaction between the food color dyes and the surface of the eggshell is due to the formation of salt linkages. Consistent with this line of reasonine is the observation that the amount of dye adhering to the-eggshell increases with increased amounts of acid. More -NH?+ - erouns - . are available on the polypeptide chain for salt formation as the pH decreases. (Fia. 5 deoicts the interaction between an anionic dve and - N H ~ +grbups of a polypeptide.) Under neutral o; basic conditions, fewer hasic amino groups of the polypeptide chains are protonated, and, thus, fewer cationic sites will be available for ionic interaction with the anionic dves. As expected, more faintly dyed eggs are observed at higher pH. 'That the prutein surrounding the eggshell is integral in the dyeing process is verified h y t h e oh&vation t h a t eggs that have their protein cuticle removed dye much fainter than eggs with their cuticle intact. Further support of this saltlinkage model of interaction between dye and eggshell protein is the observation that little dve is adsorhed on marble chips. even under conditi~mithat result in substantial coloring crf rggshells. This is of particular significance since marble, which is pure calcium carhunate in the cakite modificatiun, has a suriace churre. In fact, at pH 9.5 calcite hasa zero charge, or an isoelectr$ point ( i 7 ) . ' ~ h u s ,the electrostatic interaction between calcium carbonate and anionic dyes, even a t low pH, is not as pronounced as when protein surrounds the calcium carbonate, as in eggshells. The addition of sodium chloride to the dye solutions retards the adsorption of dyes onto eggshells. The presence of
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E
,-
D C 0
HO
-
D
a
s a l t linkage
0 D
FD&C
yellow no. 6
Figure 5. Salt linkage between polypeptideand FD&C yellow no. 6
sodium chloride in solution diminishes the electrostatic attraction between the dve molecules and the eeeshell and may decrease the concekration of dye in solut& We observed that addition of solid sodium chloride to a red dve solution resulted in the precipitation of some of the red dye, which could he collected by filtration. In fact, in a common synthesis of FD&C red no. 2, the addition of sodiumchloride to the reaction mixture is used to isolate the dve bv oreciniConclusion We have shown here that the adsorption of foodcolor dyes on eggs is sensitive to the amount of acid present in the dye solution. The color of the eggs becomes darker as the amount of acid is increased. Eggs dyed in the absence of acid are fairly light, and eggs dyed in basic dye solutions are even lighter. The protein of the cuticle surroundiue the eeeshell is necessary todye eggs dark. Based on our stud;, it isinteresting to note that the amount of vinegar used in the recipes of commercial food color preparations for dyeing eggs produces solutions with nearly the optimal amount of acid. Extension and modification of these experiments can easily be worked out. For example, it should be possible to determine quantitatively the amount of dye that is adsorbed on the egg by visible spectroscopy (19). The dyeing of eggs is an activity well-suited for student experimentation because the process of dyeing eggs is a familiar activity and the needed materials are readily available. These safe activities will provide the student with unique opportunities for observing chemical phenomena and will demonstrate the relevance of experimentation to everyday life (20,21). Teachers interested in a more complete description on how to incorporate these activities into their courses can obtain supplementary material by writing the authors. Literature Cited
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Emdyeing recipe on package: The R. T French Co.: Rochester, New York. 11. Baker, J. R.;Balch.D.A.Biochem.rl 1362.82.352. 12. McGee. H. On Food and Cookins: Scrihner's: New York. 1981.o61.
1979; ~ 2 1 1 . 19. McKone. H. T. J. Ckem. Educ. 1977.M. 376. 20. Tomorrom;Arneriesn Chemical Soeiety: Washington, DC, 1984; p 30. 21. Mebane,R. C.; Rybo1t.T. R. Aduentur~sluithAlomsond Moiacuier: Endow: Hillaide,
Nd. 1985.
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