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The egg is a biological structure intended by nature for reproduction. It provides complete protection and serves as an important source of nutrients/food for the developing chick. The egg is also one of the most nutritious foods for humans as well.
STRUCTURE OF AN EGG
Very first line that inhibits the entry of any micro-organism that acts as defence line is the SHELL, which may be white or brown in color and is mainly composed of calcium carbonate (CaCO3). It has approximately 8,000-10,000 tiny pores which allow the developing embryo to breathe.
Second defence line is the SHELL MEMBRANES, which are two in number; one sticks to the shell and other is surrounding the white albumen. It is composed of protein fibers.
GERMINAL DISC appears on the surface of the yolk and is the site where the fertilization of egg occurs.
ALBUMEN is also known as egg white, comprising of two layers; thick albumen and thin albumen. It is composed of high quality proteins, water and some minerals. Thick albumen firmly surrounds the yolk.
CHALAZA is the twisted, whitish; cord like structure on the opposite side of the yolk that keeps the yolk in the center.
VITELLINE MEMBRANE is the yolk membrane surrounding the egg yolk.
Major source of fat and cholesterol (minerals and vitamins) is the EGG YOLK. Its color ranges from light yellow to deep orange depending on the feed that is given to a hen.
AIR CELL is formed towards the broader end of the egg after the egg is laid. It is for respiration of the developing chick.
COMPOSITION OF AN EGG
Some standard dimensions of an average egg are as follow;
Surface area (cm2)
Long circumference (cm)
Short circumference (cm)
Long axis (cm)
Short axis (cm)
Whole egg consists of 9-11% eggshell, 28-29% yolk and 60-63% albumen. Following table shows the chemical composition for whole egg;
VALUE/ 100 g
VALUE/ 100 g
VALUE/ 100 g
VALUE / 100 g
VALUE / 100 g
Fatty acids, total saturated
Fatty acids, total monounsaturated
Fatty acids, total polyunsaturated
Now the approximate composition of whole egg, eggshell, albumen and yolk is given below;
EGG COMPONENT (% of total)
0.4- 0.9 %
BIOACTIVE COMPONENTS OF AN EGG
Bioactive components of an egg are divided into two parts;
Bioactive components in egg white
Bioactive components in egg yolk
BIOACTIVE COMPONENTS IN EGG WHITE
Egg white is the common name for the clear liquid in the egg (also known as albumen). It is formed from the layers of secretions of anterior part of oviduct. Its primary function is to protect the yolk and provide nutrition to the developing embryo. It is rich in proteins.
Major proteins, which act as bioactive components in egg white, include; Ovalbumin, ovotransferrin, ovomucin, lysozyme, ovoinhibitor, ovomacroglobulin (ovostatin), cystatin, and avidin
It is a major protein in albumen upto 54 % of egg white. It is a glycoprotein, composed of 385 amino acids with a molecular mass of 45 KDa. Ovalbumin may serve as a source of amino acids for the growing embryo. It biologically acts as anticancer, antihypersensitive, antimicrobial and antioxidant agent.
This glycoprotein is also known as conalbumin. It makes upto 13 % of protein content in egg albumen. OTf is composed of 686-residue single polypeptide chain of amino acids. Its molecular mass is 78-80 KDa. Ovotransferrin exhibits antimicrobial property, depending on its ability to sequester ferric ions (Fe3+), important for bacterial growth. OTf is responsible for transferring ferric ions from hen (oviduct) to growing embryo.
It is sulphated glycoprotein of egg white. It is composed of 2087 amino acids with a molecular mass of 230, 900 Da. It is also present in chalaza and vitelline membrane. It accounts about 2-4 % of egg white proteins. In thick egg white (thick albumen), ovomucin is 2-4 times more than in thin egg white (thin albumen). It acts as gelling agent of egg albumen. Thinning of egg white during storage occurs because ovomucin complex degrades.
It is also known as N - acetylmuramideglycanohydrolases. Lysozyme is a polypeptide of 129 amino acids, having a molecular mass of 14.3 KDa. It is 3.5 % of the total egg white proteins. It acts as defence mechanism until the embryo starts generating its own immunoglobulins.
ovoinhibitor is a serine proteinase inhibitor with a molecular mass of 48 KDa. It inhibits a wide spectrum of proteinases.
Ovomacroglobulin is a 640 KDa glycoprotein. It has a broad spectrum activity against proteases and strong anticollagenase activity in egg albumen.
Cystatin has a molecular mass of 12.7 KDa and is composed of 115 amino acids. It possesses antibacterial activity, thus prevents growth of streptococcus, Salmonella typhimurium.
It is a tetrameric glycoprotein composed of 128 amino acids with a molecular weight of 15-15.8 KDa. It constitutes of 0.05 % of the total egg white proteins.
BIOACTIVE COMPONENTS IN EGG YOLK
Egg yolk is a part of egg that feeds the growing embryo. Water transfer from albumen to yolk takes place during egg storage. Yolk makes upto 36 % of the weight of whole egg. Yolk consists of several proteins, fatty acids.
Protein composition of egg yolk is as follows;
NAME OF PROTEIN
Low density lipoproteins
Plasma and granules
High density lipoprotein
Egg yolk riboflavin binding protein
The LDL of egg yolk is subdivided into VLDL (very low density proteins), because of its high lipid content( 85-89 %) and low specific gravity (0.89). LDLs are spherical particles (17-60 nm in diameter with a mean of about 35 nm) having a lipid core in a liquid state (triglycerides and cholesterol esters) surrounded by a monofilm of phospholipid and protein (in this circumstance called apoprotein)
High density lipoproteins are composed of 75-80% proteins and 20-25% lipids (phospholipids, triglyceride and cholesterol). Lipovitelline (HDL) is a globular protein, subdivided into α-lipovitelline and β-lipovitelline.
Phosvitin is a glycoprotein having a higher phosphorus content, (nearly 10%) and is one of the most highly phosphorylated natural (i.e., nonsynthetic) proteins. Phosphorus is present as phosphoric acid bound to seryl residues; about 96% of seryl residues are phosphorylated. The phosphoserines
are arranged in a singular way, forming blocks that can carry up to 15 consecutive residues (Byrne et al. 1984; Fig. 1). This confers to the protein a very large and central hydrophilic area surrounded by two small hydrophobic areas at the N- (9 residues) and C- (3 residues) terminal parts.
ribofl avin - binding protein in yolk comprises ofglycoprotein and riboflavin. Ribofl avin - binding protein (RBP) is present in both egg yolk and egg white; however, the egg yolk RBP differs from the egg white RBP by a deletion of 11 - 13 amino acids at the C terminus due to limited proteolytic cleavage during oocyte uptake.
Livetins are major components of water - soluble egg yolk protein. There are three types: Ÿ€ - livetin (MW 80 kDa), Ÿ€ - livetin (MW 40 - 42 kDa), and Ÿ€ - livetin (MW 180 kDa). Livetins can also be transferred from blood serum and are identifi ed as serum albumin189
In general, the lipid content total egg yolk weight is nearly 30%; thus the 20 g of yolk in the average egg contains approximately 6 g of lipids. Neutral lipid (65%), the phospholipids (30%), and cholesterol (4%) are the major components of egg yolk lipids. The role of neutral lipid in the hatching process is mainly as an energy - supplying source, while the phospholipids and cholesterol are important ingredients promoting the formation of somatic cell structures and the cell membrane of cranial nerve cells (phospholipid bilayer) of the chick.
The IgG protein is generally found in hen blood serum is known to accumulate in egg yolk to provide an acquired immunity to offspring. An antibody in egg yolk has been identifi ed as IgY.
Hens,s egg is full of all essential nutrients that are needed by the developing embryo and human as well(for use in commercial, industrial, cosmetics , medicines etc…)