Egg Structure and Components

Egg Structure

Shell

• The outer covering, accounting for 9-12% of the egg's total weight.

• Serves as the first line of defense against bacterial contamination.

• Produced by the shell gland (uterus) of the oviduct.

• Has an outer coating called the bloom or cuticle, which:

  • Seals the pores.

  • Reduces moisture loss.

  • Prevents bacterial penetration.

Air Cell

• Empty space between the white and shell, usually at the large end of the egg.

• Barely existent in newly laid eggs.

• Forms as the egg cools and contents contract, separating the inner and outer shell membranes.

Albumen (Egg White)

• Accounts for about 67% of the egg's liquid weight.

• Produced by the oviduct.

• Consists of four alternating layers:

  • Inner thick (chalaziferous) white.

  • Inner thin white.

  • Outer thick white.

  • Outer thin white.

• The outer thin white is a narrow fluid layer next to the shell membrane.

• The outer thick white is a gel forming the center of the albumen.

• The inner thin white is a fluid layer located next to the yolk.

• The inner thick white is a dense, matted, fibrous capsule terminating in the chalazae on each end, which keep the yolk centered.

Chalaza

• Ropey strands of egg white on both sides of the egg.

• Anchor the yolk in place in the center of the thick white.

• Twist keeps the germinal disc on top, regardless of the egg's orientation.

• More prominent chalazae indicate a fresher egg.

Germinal Disc

• Entrance of the latebra, leading to the center of the yolk.

• Appears as a slight depression on the yolk's surface.

• If the egg is fertilized, sperm enter through the germinal disc, and a chick embryo starts to form.

Membranes

• Two types: shell membrane and vitelline membrane.

• Shell Membranes:

  • Inner and outer layers just inside the shell.

  • The air cell forms between these layers due to egg contraction upon cooling.

  • The outer membrane sticks to the shell, while the inner membrane sticks to the albumen.

  • During storage, water loss through evaporation causes the air cell to enlarge.

• Vitelline Membrane: Covers the yolk.

Yolk

• Makes up about 33% of the egg's liquid weight.

• Formed in the ovary.

• Contains a small white spot (2 mm in diameter) called the germinal disc, present even in infertile eggs.

• In fertilized eggs, the germinal disc contains the zygote.

• Serves as a food source for embryonic development.

• Contains all the fat in the egg and a little less than half of the protein.

• Main proteins include:

  • Vitelline (lipoprotein).

  • Phosvitin (high in phosphorus, antioxidant properties).

  • Livetin (high in sulfur).