Wed. May 25th, 2022

 Eggs are laid by female animals of many species including birds, amphibians, reptiles and fish. Chicken eggs are the most popular choice of egg consumption. Eggs are an inexpensive source of protein. Used as a reference standard against which all other protein containing foods are assessed.

Structure of an Egg

An egg basically consists of three parts:

  • Shell
  • Egg white
  • Egg yolk
Egg structure

Structure of an Egg

Eggshell: The outer eggshell is made almost entirely of calcium carbonate (CaCO3) and is covered with as many as 17,000 tiny pores.  It is a semipermeable membrane, which allows air and moisture to pass through its pores. The shell also has a thin outermost coating called the bloom or cuticle that helps keep out bacteria and dust.

Outer and inner shell membrane: These two membranes are inside the shell surrounding the albumen (white). The two membranes are made partly of keratin and provide an efficient defence against bacterial invasion. The outer membrane sticks to the eggshell while the inner membrane sticks to the albumen.

Chalazae: Strands that are twisted in opposite directions and anchor the yolk in the center of the egg. The more prominent the chalazae, the fresher the egg.

Exterior albumen (outer thin albumen): The outer thin albumen; a narrow fluid layer next to the shell membrane.

Middle albumen (inner thick albumen): The inner thick white (chalaziferous layer) is a dense, matted, fibrous capsule of albumen around the vitelline membrane of the yolk. An excellent source of riboflavin and protein.  In high-quality eggs, the inner thick albumen stands higher and spreads less than thin white. In low-quality eggs, it appears thin white.

Vitelline membrane: Clear seal which holds the egg yolk

Yellow yolk: The yolk contains less water and protein than the white, some fat, and most of the vitamins and minerals of the egg.  It is also a source of lecithin; an effective emulsifier.  Yolk color ranges from just a hint of yellow to a magnificent deep orange, according to the feed and breed of the hen.

Air cell: An air space forms when the contents of the egg cool and contract after the egg is laid. The air cell rests between the outer and inner membranes at the eggs larger end.  As the egg ages, moisture and carbon dioxide leave through the pores of the shell, air enters to replace them and the air cell becomes larger.

Germinal disk (blastoderm): A small, circular, white spot (2-3 mm across) on the surface of the yolk; it is where the sperm enters the egg. • The embryo develops from this disk, and gradually sends blood vessels into the yolk to use it for nutrition as the embryo develops.

Protein Content of an Egg : Protein content of an egg accounts to about 12.6% by weight of the edible portion Distribution of proteins in an egg Egg white 57% and  Egg yellow 43%.

Protein contents of egg

DescriptionProtein Content (per 100g)Protein Content (%)
Egg,whole,raw,fresh12.56g12.56%
Egg,white,raw,fresh10.90g10.90%
Egg,yolk,raw,fresh15.86g15.86%
Egg,whole,dried48.37g48.37%
Egg,whole,cooked,hardboiled12.58g12.58%
Egg,whole,cooked,fried13.61g13.61%
Egg,whole,cooked,omelet10.57g10.57%

Protein Content of an Egg: Contains all essential amino acids .Rich in essential amino acid leucine . Composition of amino acids in an egg matches the requirement of amino acids by the human body.

Types of Protein Present in Egg White and Egg Yellow Egg White: Egg white consists of about 90% water in which about 10% of proteins are dissolved. Egg white is an alkaline solution which contains approximately 40 different proteins.

Types of Proteins Present in Egg White:

  • 54% Ovalbumin
  • 12% Ovotransferrin
  • 11% Ovomucoid
  • 4% Ovoglobulin G2
  • 4% Ovoglobulin G3
  • 3.5% Ovomucin
  • 3.4% Lysozyme
  • 1.5% Ovoinhibitor
  • 1% Ovoglycoprotein
  • 0.8% Flavoprotein
  • 0.5% Ovomacroglobulin
  • 0.05% Avidin
  • 0.05% Cystatin

Ovalbumin: Ovalbumin is an almost spherical glycophosphoprotein which is the most common protein in egg white and Storage protein.

Ovotransferrin: Composed of 686 amino acids.Consists of 2 forms which contains neither phosphorus nor sulfur. Disulfide crosslinks are present. It has an antimicrobial and iron-binding ability.

Ovomucoid: Ovomucoid is a glycoprotein .Consists of 9 disulphide bonds .It has a stable spatial structure that it is not denatured even upon boiling

Types of Proteins Present in the Egg Yolk: The major proteins in egg yolk are lipoproteins, Livetin, Lipovitellin, and Phosvitin

Phosvitin: A type of phosphoprotein. About 16% of egg yolk proteins. Consists of about 10% phosphorus. It has iron and calcium binding capacity

References:

  • Egg Signals-Piet Simons

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