Underneath 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 protective role, the shell and the shell membranes also have a biological function; namely to regulate evaporation and air circulation, but also to prevent microorganisms from entering the egg. A colourlesswax membrane called the cuticle surrounds the outer shell. It is highly alkaline and therefore acts as a acteriostatic agent – i.e. it prevents bacteria from reproducing.
When the egg leaves the hen, it has a temperature of 39 °C. As it cools, its contents contract, allowing air to penetrate through the shell. In the heavy end of the egg, the outer and the inner shell membrane split apart, and the air cell is formed here. The older the egg is, the larger the air cell, as water continuously evaporates from the egg during storage.
The egg white makes up approx. 60 % of the eggs weight. It consists of 88 % water and 12 % dry matter, primarily protein. The egg white is made of three parts: an inner and an outer liquid layer and in between those a more viscous liquid part. The egg white is heat regulating, adjusts the humidity and supplies nutrition to the chicken foetus. It also prevents external bacteria from penetrating the yolk.
Bacterial retardant properties
The egg white protects the yolk, for example due to its contents of the enzyme lysozyme, which splits the beta-(1.4)- glycoside bond in the cell wall of gram-positive bacteria; this destroys the bacterial cell. The protein ovotransferrin is also a bacteriostatic agent, in that the binding of iron to ovotransferrin limits the possibilities or certain bacteria’s growth.
Colour of the egg white
The egg white’s pale yellow-green colour is due to the presence of riboflavin (vitamin B2). Completely fresh eggs contain lots of small air bubbles that 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.
Alkaline pH value
Egg white is one of the few alkaline food products that exist. In a freshly laid egg the pH value is approx. 7.6, but it increases during the first 1-2 days to 8.6 and then to 9.7. The rate at which the pH value increases depends on the storage conditions. The higher the temperature, the faster the increase in pH value.
The yolk is a single cell that represents about 28 % of the weight of the egg. The nucleus, called the embryonic disk, can be seen as a small white spot on the outside of the yolk when the egg is opened. The yolk is surrounded by a protective membrane, the vitelline membrane, which becomes weaker the older the egg is.
The yolk has much lower water content than the egg white, just under 50 %. About 2/3 of its dry matter is fat and 1/3 is 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 are influenced by diet.
The yolk is held in place by two screwshaped strands of egg white – the chalazae, which are formed from egg white proteins. The chalazae are most visible in fresh eggs and can be hard to spot in a boiled egg.