Fri. Sep 30th, 2022
structure of egg

The egg has an eggshell, albumen and yolk.

Eggshell: It is the outer covering of an egg (constitutes 9 to 11 % of egg wt.) which consists of pores. The air gets in through the pores for the embryo to breathe. There are approximately 7,500 pores per egg. The size of the pores is big at the broader end of the egg. The outer surface of the shell is covered with a cuticle which seals the pores and checks the outer temperature and prevents carbon dioxide to escape from the egg. The shell also consists of the inner and outer shell membrane. The air cell is formed between the shell membranes usually at the broader end of the egg. The outer shell membrane is the air cell membrane.

Albumen: The white portion or albumen of an egg (constitutes about 58 to 60 % of egg wt.) has three distinct layers of outer thin, middle dense and inner thin liquid. It also consists of chalazae which are attached to the chalaziferous layer. The chalaziferous layer is around the yolk.

Yolk: The yellow portion or yolk of an egg (constitutes around 31 % of egg wt.) has the following layers:

The yolk consists of a germinal disc, latebra, concentric rings of yolk material and vitelline membrane.

  1. Latebra: The white yolk which connects the germinal disc with the centre of the yolk.
  2. Light yolk layer: Where no dietary pigment is available.
  3. Dark yolk layer: The colour is dark yellow due to the periodic deposition of carotenoid pigment.
  4. Yolk membrane: It is also called the vitelline membrane which surrounds the yolk.

Blastodisc/Germinal disc: The germinal disc is known as blastoderm in fertile eggs and blastodisc in infertile eggs. It is located in the cone-like portion of the latebra, known as the “Nucleus of Pander” which is connected with its neck to the centre of the yolk.

blastoderm and blastodisc
1. Fertile – Blastoderm is visible / 2. Infertile – Blastodisc is visible

Air cell: There is no air cell in a fresh egg at the moment it is laid. The moment the contents contract following cooling, a slight vacuum is created drawing air through its pores and giving rise to an air cell between the two shell membranes. Size varies with the duration of holding the egg and the species of the bird.

egg components
Components of Egg

The three main components of the egg are:

  1. Shell       10%
  2. Yolk        28%
  3. Albumen    62%

The albumen consists of different layers:

Chalaziferous layer        –              2.7%

Inner thin layer               –              16.8%

Middle dense or thick layer –     57.3%

Outer thin layer              –              23.2%

As usual, the egg is composed of water and solids. There are two types of solid materials viz., organic and inorganic. Proteins, lipids and carbohydrates compose the organic part. All these are presented in the following table.

Table 2.1: Composition of Hen’s egg

ConstituentsEntire egg (%)Egg Content (%)Yolk (%)Albumin (%)Shell (%)Shell Membrane i%)
Organic matter23.525.650.211.33.370.00

Inorganic matter (i.e. in shell):

Calcium Carbonate        98.43%

Magnesium Carbonate 0.84%

Tri calcium Phosphate 0.73%

The values given in the table show that eggs can act as an important source of protein, fat and minerals by providing a well-balanced source of nutrients for persons of all ages. Its rich protein content and low caloric value are mostly sought in these modem days to curb obesity and other health problems. Species of birds, diet, age of birds, the season of laying, and storage and processing influence the composition of egg.

Eggs are considered a delicacy and recognized as an important protective food since ancient times. Eggs are rich in easily digestible fat, high-quality protein, a good source of vitamins and a valuable source of minerals. Eggs are palatable and useful in special diets and indispensable in cookery practices.

Traces of carbohydrates (1.0 per cent) are present in egg content which reacts with egg protein during cooking causing Maillard reaction (browning) specifically in egg powder manufacture. The lipid content in eggs is 11.87 per cent of egg content. 32.8 per cent of yolk. Types of lipids are phospholipids (32.8 per cent), sterols (4.9 per cent) and neutral fats (62.3 per cent). Fatty acids are palmitic, stearic, linolenic, linoleic and arachidonic acids.

The egg is a rich source of quality protein, an important source of unsaturated fatty acids (oleic), iron, phosphorus, trace minerals, vitamin A, E, K and B complex vitamins, especially vitamin B. As a source of vitamin D, egg ranks second to fish liver oils. Eggs are low in calcium (present in the shell) and have no vitamin C. Due to their low caloric value and easy digestibility, the egg is included in the diet of older people. Egg protein and emulsified egg fat – (egg yolk) are most suitable for babies and growing children. Eggs find a place in all diets including infertile eggs for vegetarian people.

In the egg, all the essential amino acids are present. As you know that essential amino acids are required for the body and have to be provided in the diet, it can not be synthesized in the human body. The amino acids which are essential to human beings are given below:


The nutritional value of protein depends upon its amino acid composition and proportion. Table 2.2 gives daily requirement of amino acids in human diet which can be met by including 2 eggs in the diet.

Table 2.2: Amino acid requirements for man

Amino AcidsDaily Minimum Requirement for an Average Man (in mg.)Quantity in Two Eggs (in mg.)
Threonine500. 605
/’Recommended daily allowance for a moderately active manQuantity in two eggs% of daily requirement supplied by two eggs
Energy, Calories30001806
Proteins, gm70.013.2- 14.020
Fat, gm50.011.022
Carbohydrate, gm570.0
Calcium, gm0.80.068
Phosphorus, gm0.90.2430
Iron, mg12.03.2026
Iodine, mg0.10.0110
Vitamin A, I.U.50001000­ 150020-30
Vitamin D, I.U40010025
Vitamin C, mg.75.0
Vitamin B|, mg.1.50.128
Vitamin B2, mg.2.00.3216
Niacin, mg20.01.206

The egg proteins are easily digested and its biological value is high as compared to other food as shown in Table 2.4

Table 2.4: Biological Value and Protein Efficiency Ratio

 DigestibilityBiological ValueProtein Efficiency Ratio
Fresh water fish9788.52.0

Thus, findings in different tables reveal that egg can act as a good source of protein, vitamin A, vitamin D, riboflavin, folic acid, vitamin B|2 and various essential amino acids (threonine, valine, leucine, isoleucine, lysine, methionine, phenylalanine and tryptophan). Two medium-sized eggs can meet up to 50 % to 100% dietary requirements of these amino acids. Minerals obtained from two medium-sized eggs also fulfil around 10% of our daily requirement of calcium and iron. So, at the existing situation, eggs can be considered as the cheapest source of pure quality protein available on the market.

Egg white proteins (Albumen proteins):

All egg white proteins are globular. Globular proteins are important for foaming properties of egg white. Different egg white proteins are as follows:

  1.  Ovalbumin: It is the main protein constituent. It is 55 per cent of the protein of egg white and is a phosphoglycoprotein.

The carbohydrate components of albumen are mannose and glucosamine. Ovalbumin in solution denatures by mechanical agitation but is resistant to thermal denaturation.

  • Conalbumin: It constitutes 13 per cent protein of egg albumen. Conalbumin complexes with iron making iron unavailable to the system. Conalbumin is easily heat coagulated.
  • Ovomucoid: It constitutes 10 per cent of the egg white protein. Ovomucoid is a trypsin inhibitor.
  • Ovomucin: It is responsible for thickness of albumen and largely present in thick albumen and is insoluble in water but soluble in dilute salt solution.
  • Lysozym: It acts as antibacterial by dissolving the cells of bacteria (lysis). Lysozyme is heat resistant.
  • Avidin: It binds biotin and makes the vitamin (biotin) unavailable to the system. Egg Yolk proteins

Phosvitin: It is rich in phosphorus. This accounts for 80 per cent of the protein of yolk. It binds tightly ferric iron and forms a soluble complex and thus is the iron carrier of yolk.

Lipovitellins: These are high density proteins separated into two fractions. The a and p lipovitellins, contain 40 per cent neutral lipid and 60 per cent phospholipids.

Livetin: It has three components – a, P and y – livetin. These differ in their molecular weight.

Low density lipoprotein (LDL): Egg yolk consists of 74 per cent neutral lipids and 25 per cent phospholipids.


Yolk pigments are yellow or dark depending upon amount and type of pigment present in the feed. Colour of yolk is due to carotoenoid and xanthophyll pigments. Carotenoids are converted to vitamin A in the body.


Egg shell is the major source of calcium present in the egg. Calcium is present significantly in egg yolk but less in albumen. Other minerals present in yolk and albumen are sodium, potassium, magnesium, sulphur and chlorine. The mineral content of eggs depends on the mineral content of the feed of chicken.

The distribution of minerals in different portions of the egg is shown in table 2.5.

MineralsQuantity in Yolk (in mg)Quantity in Albumen (in mg)Quantity in Shell (in mg)
Iodine0.003 – 0.0080.008 – 0.0020.001 -0.003
Zinc0.70 – 1.000.007Occasionally Present


The yolk is a good source of Vitamin A and Vitamin D. Egg is also a good source of B complex vitamins. The distribution of vitamins in the egg is shown below (Table 2.6)

Table 2.6: Distribution of Vitamins in an Egg Content

VitaminsQuantity in YolkQuantity in Albumen
Vitamin A, (l.U.)600.27
Vitamin D, (l.U.)25.70
Thiamine, (mg)0.05. . .
Riboflavin, (mg)0.066. . .
Niacin, (mg)Traces0.033
Pantothenic acid, (mg)0.875- 1.2200.025-0.890
Choline chloride (mg)320

You are now aware of the chemical composition of the egg. The knowledge of the chemical composition and physico-chemical properties of albumen and yolk should be useful for interpreting the changes that occur during shell egg storage and during pasteurisation, drying and freezing.

The alteration in the egg components may lead to loss of functionality of albumen (foaming power) and yolk (emulsifying ability). Both of these functional properties are significant in the quality of food products such as egg noodles, mayonnaise, and cakes where the egg is a useful ingredient.

Diverse biological properties, including antimicrobial activity, protease inhibitory action, vitamin-binding properties, and antigenic or immunogenic characteristics have also been attributed to specific egg components.

The physico-chemical properties of albumen and yolk vary due to their inherent chemical composition. These properties can be studied under the following categories:

The viscosity of egg albumen is dependent on the age of the bird, mixing treatment of the albumen, temperature and rate of shear. It has been reported that unmixed albumen is pseudo plastic at 32°C. With a constant shear rate, the albumen viscosity decreased with time and approached equilibrium in a few minutes. It also has been found that mixed albumen also displayed pseudo-plastic behavior at 10°, 20° and 30°C.

Egg yolk is apseudo plastic Non-Newtonian fluid. The particulate matter (granules) i n the yolk must be responsible for Non- Newtonian characteristic since plasma (yolk without the granules) is essentially a Newtonian fluid. Amarked reduction in apparent viscosity is noted when albumen is added to yolk.

Since proteins and phospholipids are capable of lowering the surface and interfacial tensions, albumen and yolk have low surface tensions. The surface tension of a 12.5% solution of albumen at pH 7.8 has been reported to be 49.9 dynes/cm at 24°C.

Egg yolk is, itself, an emulsion, a dispersion of oil droplets in a continuous phase of aqueous components. In addition, the yolk is an efficient emulsifying agent. The emulsifying substances, also known as surface-active agents, are mixtures or complexes of egg yolk protein fraction such as livetin and lipovitellins. Lecithin present in egg is a natural emulsifying agent.

The pH of albumen from a newly laid egg is between 7.6 and 8.5. During the storage of shell eggs, the pH of albumen increases at a temperature dependent rate to a maximum value of about 9.7. The rise in the albumen pH is caused by a loss of carbon dioxide from the egg through the pores in the shell. The pH of albumen is dependent on the equilibrium between dissolved carbon dioxide, bicarbonate ions, carbonate ions and protein. The pH of yolk in freshly laid eggs is generally about 6.0, but during storage of eggs, the pH gradually increases to 6.4-6.9.

The pH of egg is largely influenced by storage temperature, carbon dioxide present inside the egg and in the external environment. With an increase in the concentration of carbon dioxide in the environment, the concentration of bicarbonate ions ofalbumen increases as the carbonate concentration decreases.

The albumen of a fresh egg is a clear mass with a yellowish tint. But for the presence of two chalazae, which have a cloudy appearance, the rest of the albumen should be free from any muddy appearance. On coagulation the albumen assumes a milky white colour and therefore it is also called as egg white.

The yellow-orange colour of yolk has been attributed to the presence of fat-soluble carotenoids in the lipid portion of lipoproteins. The majority of carotenoids in yolk are hydroxy compounds called xanthophylls with minor amounts of carotenes. The factors that influence the degree of yolk pigmentation include the chemical structure ofthe xanthophylls, the presence of antioxidants in feed, and the fat content of feed. In addition, it has been shown that the genetic capability to absorb and deposit

xanthophylls in yolk varies among individual hen within a single strain. Highly pigmented yolks are required for the manufacture of bakery products, pasta and mayonnaise.

The freezing point of egg yolk is about -1°C (30.2°F), yet, gelation does not occur until a temperature of -6 °C (21.2°F) is attained. Freezing causes only minor changes in raw egg white. Depending on the moisture or solids content of egg products, the thermal properties like specific heats, latent heats and freezing point may vary.

One of the most important factors influencing the acceptability of eggs is their flavour. A fresh egg has a mild flavour and odour. The development of characteristic egg flavour takes place during cooking. Loss of flavour or development of off-flavours may be due to feed ingredients, storage and handling of eggs both before and after cooking. Egg researchers have reported the presence of carbonyls (Acetone, acetaldehyde, formaldehyde, z – pentanone, z – butanone and diacetyl) in fresh egg white. The basic compounds recovered were ammonia, methylamine, dimethylamine and putrescine. The dominant odour of cooked egg white is sulfur, mostly due to the release of H2S gas. In low concentrations, H2S positively affects flavour, while high concentrations make eggs unpalatable.

Structure of Eggshell

The shell of an egg is formed in two layers:

  1. Mammillary layer – a sponge-like layer composed of soft calcite crystals (CaCO3). This layer is the inner layer.
  2. Palisade layer – formed of columns of hard calcite crystals; the longer the columns the stronger the shell. This layer is the outer layer of the egg.

The calcium for the eggshell comes from the diet, a special bone called medullary bone (found in the cavity of long bones)and the skeleton. The hen uses approximately 2.5 grams of calcium in the formation of one normal egg. She cannot absorb sufficient calcium from her diet each day (approximately 2.0 grams per day) to supply this need and hence, it becomes necessary for her to utilise skeletal calcium to make up the shortfall. This is particularly so at night when most of the shell is being formed but the hen is unlikely to be eating. In addition to the calcite, the shell also contains small quantities of sodiumpotassium and magnesium.

The carbonate ions which go with the calcium to form the calcium carbonate of the egg’s shell, come from the blood and the shell gland. If anything should interrupt the supply of carbonate, thin-shelled eggs will result. This occurs in hot weather when hens pant to remove excess heat energy. The increased respiratory rate removes carbon dioxide from the blood thus reducing the carbonate ions available for eggshell formation.

Carbonic anhydrase is the enzyme which catalyses the conversion of carbon dioxide and water into carbonate ions. Zinc is the co-enzyme of carbonic anhydrase and any conditions resulting in Zn deficiency can lead to problems associated with eggshell formation.

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