According to science, egg is an organic vessel which holds an embryo. The only difference being whether the egg is fertilized or unfertilized. The eggs which are used for human consumption are unfertilized eggs. The eggs which are fertilized by a sperm develop into living organisms in favorable conditions. An egg basically consists of 3 parts: A shell; an egg white and an egg yolk.

A Shell

The shell is made up of calcium carbonate and calcium phosphate. The shell is built of 8000-10000 pores, which ensures that oxygen can penetrate and CO2 and other gases can escape. These pores are sealed with the calciferous protein keratin. The shell represents of 10% of weight of egg. The color of shell is majorly dependent on the breed. The shell consists of 3 layers. Outer cuticle made up of keratin, Middle spongy layer and Inner mammary layer.

Shell membranes: Lying between the shell and egg albumen, there are two transparent protein layers. These layers prevent the entry of bacteria into the egg. These layers are partly made up of keratin which is protein in hair.

Air cell: An air cell is formed when the contents of egg contracts after cooling. The air cell usually rests between the outer and inner shell membranes towards the larger end of the egg.

As the age of the egg increases the size of the air cell increases. This is due to considerable amount of moisture loss from the contents of egg. Hence, the size of the air cell is used to determine the freshness of egg.

Egg Albumen/Egg White: The egg white is also known as albumen, which originated from a Latin word “albus” meaning white. Albumen constitutes about 66% of the liquid weight of egg. Four alternative layers of thick and thin albumen are present inside the egg.

From the yolk outward, there are designated as inner thick or chalaziferous white, the inner thin white, the outer thick white and the outer thin white. As the egg ages the albumen tends to thin down because of the change in its protein structure. Hence, it is used to determine the quality of egg. The fresh egg sits upright when broken but the aged egg spreads out.

Chalazae: Ropy strands of egg white which holds the egg yolk in place in the center of the egg albumen. The more prominent the chalazae, the fresher are the eggs.

Egg Yolk: The yolk or the yellow portion of the egg makes up to 34% of liquid weight of egg. It contains all the fat in the egg. In fertilized eggs, the yolk is the site of embryo development.

Vitelline Membrane: It is the clear casing that holds the egg yolk.

Germinal Disc: The channel is leading to the center of the yolk. The germinal disc is slightly noticeable as a slight depression on the surface of the egg yolk. If an egg is fertilized, the sperm passes through the germinal disc to the center of the yolk and an embryo is formed.

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