Eggs Composition and Structure

Richard E. Austic

Department of Animal Science, N.Y.S. College of Agriculture and Life Sciences, Cornell University, Ithaca, New York, U.S.A.

INTRODUCTION

The avian egg is one of the richest and most balanced sources of nutrients among all of the foods available to mankind. Its biological function is to support the development of the embryo from fertilization to the emergence of the newly hatched chick. The structure of the egg is such that it maintains an aseptic ''milieu'' for embryonic development. It protects the embryo from physical trauma, allows for the exchange of respiratory gases between the embryo and the environment, and provides the embryo with all of the nutrients that are needed for growth and development. Eggs are generally similar among species of birds, but they can differ in some aspects of their physical and chemical composition. This article briefly describes the physical structure, chemical composition, and nutrient content of the egg of the chicken, Gallus gallus domesticus.

EGG STRUCTURE

The physical features of the egg are illustrated in Fig. 1. The relative proportions of the parts as reported by Shenstone[2] are shown in Table 1.

Most of these features can be observed by visual examination of the broken-out egg, but all major compartments of the egg also have unique microstructures that can be seen with the aid of the electron microscope. The genetics, age, and diet of the hen, size of the egg, and environmental factors influence the relative proportions of albumen and yolk in the newly laid egg.[3]

Yolk

The yolk is an ovum, a reproductive cell complete with a cell membrane. The cell is so large that it would not remain as a discrete body if it were not for the vitelline membrane that surrounds it. This membrane is a bilayered extracellular structure that is secreted by the ovarian follicle and the oviduct.[2]

The surface of the yolk in the fresh egg appears uniform except for the presence of the blastodisc. The blastodisc of the unfertilized egg is easily observed as a small white spot, approximately 2 mm in diameter, on the surface of the yolk.[4] It is larger, 3 4 mm, in the fertilized egg[2] because the embryo usually has progressed to the gastrulation stage by the time of oviposition. The latebra is a small sphere of white yolk in the center of the yolk extending with a narrow neck of this unpigmented yolk to the blastodisc. The concentric rings that are shown in Fig. 1 are not visible to the naked eye. They have been observed in specially stained eggs and are believed to represent alternating layers of yellow yolk that is deposited during the day when the hen is consuming feed and white yolk that is formed at night when the hen is not eating. Carote-noid pigments are responsible for the yolk's yellow color.[4] These pigments are present in ingredients such as yellow corn, corn gluten meal, and alfalfa meal in poultry feeds.[5]

Egg yolk is composed of particles that are suspended in a liquid phase containing low-density lipopro-teins, livetins, several vitamin-binding proteins, yolk transferrin, and salts.[2,4] The largest particles are spheres of white and yellow yolk that range in diameter from 50 to 100 mm. These contain inclusions that are numerous in yellow spheres but limited to one or two inclusions per white sphere. Yolk contains an abundance of small particles, known as insoluble yolk globules, which range in size from less than one to several microns in diameter and are enveloped in multilamellar membranes. Yolk also contains granules that are about 1 mm in diameter and contain lipovitel-lins, low-density lipoproteins, and phosvitin. Phosvitin binds calcium, magnesium, and trace minerals and has antioxidant properties. About 90% of the iron in yolk is bound to this protein.[2,4]

Albumen

Albumen, or egg white, is the clear fluid surrounding the yolk. Although it might appear quite uniform in structure, it actually contains four compartments: a thick gelatinous region (the firm albumen), bordered medially by the inner thin albumen and surrounded peripherally by outer thin albumen. The chalaziferous layer is the fourth compartment. It consists of a thin layer of mucinous fibers that surround the yolk and

Fig. 1 The parts of an egg. (From Ref.[1].)

are anchored in the firm albumen in the large and small ends of the egg. These anchoring regions, the chalazae, hold the yolk in the center of the egg. They appear as whitish appendages to the yolk in the broken-out egg.

More than a dozen albumen proteins have been identified.[2,4] All are sources of amino acids for the developing chick embryo. Many have unique properties such as inhibiting proteolytic enzymes (e.g., ovo-mucoid and ovomucin), causing lysis of gram-positive bacteria (lysozyme), binding certain B-vitamins (biotin-, thiamin-, and riboflavin-binding proteins), and binding iron (conalbumen). The gel structure of firm albumen stems from the presence of at least two glyco-proteins, alpha- and beta-ovomucin, possibly in association with another protein, lysozyme.[4] Egg whites gradually become thinner during storage and eventually lose their gel structure. This may be related to the gradual loss of carbon dioxide from the egg and the resulting increase in pH of the egg white.

Shell Membranes

The inner and outer shell membranes surround the egg white. The inner membrane is about one-third the thickness of the outer membrane. It separates from the outer membrane in the large end of the egg after oviposition. This occurs as the egg cools and its contents contract after oviposition. Air enters through pores in the large end of the egg to form the air cell, which further increases in size over time owing to the loss of moisture from the egg. The outer shell membrane contains the sites of mineral crystal formation during the process of egg formation. Electron

Table 1 Proportions and solids contents of egg structures'1

Percent of whole egg

Range of values

Percent solids

Shellb

10.5

7.8 13.6

99

Yolk

31

24.0 35.5

52.5

White (whole)

58.5

53.1 68.9

11.5

Outer thin white

13.5

11.2

Firm white

33.3

12.4

Inner thin white

9.9

12.6

Chalaziferous layer

1.2

15.7

Chalazae

<0.5

ndc

a(Adapted from Ref.[2].) bWith membranes and cuticle. cNot determined.

a(Adapted from Ref.[2].) bWith membranes and cuticle. cNot determined.

microscopy has revealed that the egg shell is embedded in the outer shell membrane.[6]

Egg Shell

The egg shell contains about 2g of calcium and consists of columns of calcium carbonate crystals that extend outward from the outer shell membrane. An organic matrix is deposited in the areas of crystal growth during egg-shell formation. The matrix represents only 2% to 3% of egg shell weight but is believed to be important in the growth of the crystalline structure of the egg shell.[4]

Several thousand channels exist from the outer surface of the egg shell to the shell membranes. These channels, or pores, represent spaces between the columns of calcium carbonate crystals.[6] They are more numerous and greater in diameter in the large end of the egg. The outer surface of the mineralized portion of the egg shell is covered with a proteinaceous coat, the cuticle. The cuticle provides the glossy sheen that is normally visible on the newly laid unwashed egg. It probably functions to plug the openings of the pores on the surface of the egg shell to prevent the entry of microbes into the egg.

NUTRIENT CONTENT OF EGGS

The egg is a rich source of nutrients.[7'8] Eggs contain about 6.5 g of protein (Table 2).

The protein is highly digestible and contains an excellent balance of amino acids. Egg protein has been used traditionally as a standard protein of high biological value, against which proteins from other sources are measured. Yolk and albumen contribute about 40% and 60% to the total protein of the egg. The edible part of the egg contains about 5 g of lipids. These are found almost exclusively in the yolk in the form of

Table 2 Proximate analysis and energy content of the egga

Amount per egg

Table 2 Proximate analysis and energy content of the egga

Amount per egg

Egg

Egg

Whole

Component

white

yolk

egg

Water (g)

28.9

8.89

37.92

Protein (g)

3.6

2.7

6.29

Lipid (g)

0.06

4.51

4.97

Carbohydrate (g)

0.24

0.61

0.39

Ash (g)

0.21

0.29

0.43

Energy (Kcal)

17

55

74

aAn egg weighing approximately 57 g with 50 ml edible contents (33 g of egg white and 17 g of yolk). (From Ref.[8].)

aAn egg weighing approximately 57 g with 50 ml edible contents (33 g of egg white and 17 g of yolk). (From Ref.[8].)

lipoproteins. About 70% of the lipid is triglyceride and about 30% is phospholipid.[4] Phoshatidylcholine (lecithin) and phosphatidylethanolamine (cephalin) account for about three-quarters and one-fifth of the phospholipid fraction, respectively. Yolk phospholipids include small quantities of lysolecithin, phosphatidyl-serine, sphingomyelin, and phosphatidylglycerol.[2,4] Eggs contain cholesterol, about 212 mg in a 60 g egg. Small amounts of free glucose are present in the egg, but most of the carbohydrates of the egg exist as components of glycoproteins.

Yolk lipids are rich in monounsaturated fatty acids, particularly oleic acid, and contain substantial quantities of polyunsaturated fatty acids. The fatty acid composition of eggs reflects the hen's feed. The typical feed is based on corn meal, soybean meal, and a small amount of supplemental animal, vegetable, or animal vegetable fat blend as the sources of fatty acids. The polyunsatu-rated fatty acids in these ingredients are predominantly of the omega-6 series (e.g., linoleic and arachidonic acids). Eggs enriched in fatty acids of the omega-3 series (e.g., linolenic, eicosapentaenoic, docosapentaenoic, and docosahexaenoic acids) are obtained by the inclusion of flax seed, canola seed, fish oils, some species of micro-algae, or other ingredients containing high levels of these fatty acids.[9] Eggs enriched in this manner typically contain 400 to 500 mg of fatty acids of omega-3 series. Recent research has demonstrated that eggs can be enriched in other lipid-soluble factors such as lutein, beta-carotene, lycopene, conjugated linoleic acid, and oleic acid.[9,10]

Eggs contain 12 of the 13 vitamins that are required by man: they lack only vitamin C (ascorbic acid). Chickens, like most birds and mammals, can synthesize this vitamin from glucose, and therefore it is not necessary for it to be present in the egg for the development of the chick. The fat-soluble vitamins (A, D, E, and K) are present only in yolk where they associate with the lipoproteins of yolk. The remaining vitamins and 12 important minerals are present in both yolk and albu-men,[7,8] although not necessarily equally distributed among both egg compartments.

The US Department of Agriculture[8] reports the composition of whole egg, yolk, and albumen based on a running average of values submitted to their database. Currently, the vitamin content of whole eggs is as follows (in mg per egg): folate: 0.024, niacin: 0.035, pantothenic acid: 0.719, riboflavin: 0.239, thiamine: 0.035, vitamin B6: 0.071, vitamin B12: 0.00065, vitamin E (alpha-tocopherol): 0.97, and vitamin K: 0.0003, and (in IU per egg) vitamin A: 487, and vitamin D: 34.5. The mineral content (in mg per egg) is: calcium: 26, copper: 0.051, iron: 0.92, magnesium: 6, manganese: 0.019, phosphorus: 96, potassium: 67, selenium: 0.0158, sodium: 70, and zinc: 0.56. According to Naber,[7] the biotin, inositol, and choline contents of whole egg are 0.0097, 8.19, and 437mg per egg, respectively. The chloride content is approximately 91 mg per egg.

CONCLUSIONS

The egg is composed of structures that serve to protect and nourish the developing embryo. It is a source of all of the essential nutrients except vitamin C. Recent research has demonstrated that the egg can be enriched in several nutrients by altering the composition of the diet of the laying hen.

REFERENCES

1. U.S. Department of Agriculture (USDA). Egg Grading Manual; Agriculture Handbook No. 75; Consumer and Marketing Service: Washington, DC, 1969.

2. Shenstone, F.S. The gross composition, chemistry, and physicochemical basis of organization of the yolk and white. In Egg Quality: A Study of the Hen's Egg; Carter, T.C., Ed.; Oliver and Boyd: Edinburgh, 1968; 26 58.

3. Marion, W.W.; Nordskog, A.W.; Tolman, H.S.; Forsythe, R.H. Egg composition as influenced by breeding, egg size, age and season. Poultry Sci. 1964, 43, 255 264.

4. Burley, R.W.; Vadehra, D.V. The Avian Egg; John Wiley & Sons, Inc.: New York, 1989.

5. Scott, M.L.; Ascarelli, I.; Olson, G. Studies of egg yolk pigmentation. Poultry Sci. 1968, 47, 863 872.

6. Solomon, S.E. Egg and Eggshell Quality; Wolfe Publishing Limited: London, 1991.

7. Naber, E.C. The effect of nutrition on the composition of the egg. Poultry Sci. 1979, 58, 518 528.

8. U.S. Department of Agriculture, Agricultural Research Service. USDA National Nutrient Database for Standard Reference, Release 17; Nutrient Data Laboratory Home Page, 2004; http://www.nal.usda.gov/fnic/foodcomp (accessed May 2005).

9. Gonzalez-Esquerra, R.; Leeson, S. Alternatives for enrichment of eggs and chicken meat with omega-3 fatty acids. Can. J. Anim. Sci. 2001, 81, 295 305.

10. Surai, P.F.; Sparks, N.H.C. Designer eggs: from improvement of egg composition to functional food. Trends Fd. Sci. Tech. 2001, 12, 7 16.

0 0

Responses

  • seth
    What is the Composition of a newly laid egg?
    4 months ago
  • john
    What are the structure and components of eggs?
    3 months ago
  • fedra costa
    What are the physical structures and composition of eggs?
    3 months ago
  • ROSA
    What are the physical structure and composition of eggs?
    3 months ago
  • Rudibert
    How to identify amino acids in chicken egg whites odf?
    2 months ago

Post a comment