The structure of the egg

An egg basically consists of three parts:

  1. a shell
  2. an egg white
  3. an egg yolk

An egg from a hen consists of approximately 2/3 egg white and 1/3 egg yolk. 

Æggets Opbygning UK

The eggshell

The shell is built of 8-10,000 pores, which ensures that oxygen can penetrate and COand other gases can escape. The shell represents about 10 % of the weight of the egg and consists mainly of calcium carbonate and calcium phosphate. The shell thickness and thus the strength depends on egg size, breed, the age of the hen, and feed composition. 

Only the outer layer of the shell is coloured. The colour of the shell can be white or brown – depending on the breed. A white hen lays white eggs and brown hens lay brown eggs. There are also white hens that lay brown eggs but this breed is currently not being used for production in Denmark.

The egg white

The egg white represents approx. 60 % of the weight of the egg and consists of 88 % water and 12 % dry matter, primarily protein. The white is divided into three parts: an inner and an outer liquid layer, and in between those a liquid layer with a thicker consistency. The white prevents external bacteria from penetrating the yolk.

The pale yellow-green colour of the white is due to the presence of riboflavin (vitamin B2). In completely fresh eggs there are lots of small air bubbles, which can give the egg white a dull milky appearance. This is because of carbon dioxide that has not yet leaked out through the shell. The older the egg, the more transparent the egg white.

The egg yolk

The yolk has a much lower water content than the egg white, just under 50 %. The yolk represents about 28 % of the weight of the egg and consists of approx. 2/3 fat and 1/3 protein. The fat content consists primarily of triglycerides, cholesterol, and the phospholipid lecithin. The amount of fat and cholesterol and the composition of the fat is influenced by the diet of the hen. The yolk is held in place by two screw-shaped egg white strands – the chalazae.

The diet of the hen determines the colour of the egg yolk. If the hen is mainly fed by yellow and orange-pigmented food, this is the colour that is more prominent in the egg yolk. A relatively colourless feed, gives almost colourless yolks. In conventional feed, the manufacturer often adds a small amount of plant supplements that provide the yolk with the colour that consumers like best – namely golden or lemon-coloured. 

The structure of the egg

Shell membranes

Under the shell there is an outer and an inner shell membrane. The outer membrane, which is immediately inside the shell, is the most resistant. Besides serving a packaging role, the shell and the shell membranes also have a biological function; namely to regulate evaporation and air circulation, but also to prevent penetration of microorganisms. A colourless wax membrane called the cuticle surrounds the outer shell. It is highly alkaline and therefore acts bacteriostatic. This wax membrane dissolves by washing, which is why in Denmark washing the eggs before sorting is not allowed. 

Air cell

When the egg leaves the hen, it has a temperature of 39 °C. When it is cooled, there is a contraction of the contents and air can penetrate through the shell. In the heavy end of the egg, the outer and the inner shell membrane are split and the air cell is formed here. The older an egg is, the larger the air cell, as water continuously evaporates from the egg during storage. 

Bacterial retardant properties

The egg white protects the yolk, be it among other reasons because of the enzyme lysozyme, which splits the beta-(1,4)-glycoside bond in the cell wall of gram-positive bacteria, wherin the bacterial cell is destroyed. The protein ovotransferrin is also bacteriostatic, in that the binding of iron to ovotransferrin limits the possibilities of certain bacteria's growth.