Eggs have been used as a food for centuries. As long ago as 1400 BC, natives in Southeast Asia kept poultry, and throughout history eggs have not only been eaten, but have been involved in rituals and used as currency all over the world.
Production of egg
Traditionally, eggs were produced by free range farming. This meant that hens were allowed to roam loose in the farmyard, eating grain and other food from the ground. The eggs were laid in a hen house. The sale of free-range eggs has increased again in recent years as concern about animal welfares have grown.
As the demand for eggs grew, large scale production in the form of battery farms was developed. Thousands of hens are kept in cages in large hen houses which are artificially lit and heated. The hens remain in the cages at all times and the eggs they lay are collected, usually on a conveyor belt, graded, and checked for quality.
Another form of large-scale eggs production involves keeping large numbers of hens together in huge, heated sheds, but not in cages. The hens lay their eggs in nest boxes. This is called deep litter, barn, or perch Ery farming.
Four grain eggs are produces by hens that receive a totally vegetarian cereal diet They are usually barn produced.
Types of egg
Most eggs eaten are hens' eggs, but duck, goose and quail's eggs can be eaten, providing they are very fresh.
Eggs are graded according to size
Large
Medium
Small
Eggs are also graded according to quality.
1. Extra: These eggs have been packed in the previous seven days and are of high quality.
2. Class A: These eggs are of good quality and are the grade usually sold to the consumer.
3. Class B: These eggs are of lower quality and may have dirty shells.
4. Class C: These eggs are usually sold to cake manufacturers as they have weak or damaged shells.
Al eggs carton should state the week in which they were packed during the year, and the supplier. Some producers now stamp a use by date on each egg.
Structure and composition
Eggs are composed of three main parts:
1. Shell
2. Egg white
3. Egg yolk
88.5% of an egg is edible.
Other parts include:
True shell
Inner membrane
Cuticle
Thin white
Nucleus of yolk
Yolk membrane
Thick white
Air space
Shell
The shell consists of an outer cuticle, a true shell, and inner membranes. The true shell forms 11.5% of the whole egg. It consists of:
97% calcium carbonate
3% protein
The shell is porous and therefore allows the developing chick to obtain oxygen. The pores also allow bacteria and Oduors to enter, and water and carbon dioxide to escape. The membranes that line the shell inside act as filter to bacteria to protect the inside.
At one end of the egg, the membrane separates into an air space, to supply the chick with oxygen.
The color of the shells varies according to the breed of the bird and does not influence the nutritional value of egg to anyway. The shell is strong, but older birds tend to produce weaker shells.
Egg white
Egg white has two visible layers:
the thick white
the thin white
The main protein in egg white are ovalbumin and mucin.
Egg Yolk
The yolk is covered by a membrane to separate it from the white and support it. The yolk forms about 30% of whole egg. It consists of:
16% protein
33% fat
50% water
fat soluble vitamins A, D, E and K
The color of egg yolk is related to the diet of hen and is due to the presence of carotene. The nutritional value of the egg is not affected by the color of the yolk.
The yolk is supported by the chalazae which are attach to the egg white and help to keep the egg away from the shell where it could pick up bacteria. The yolk is more vulnerable to bacterial attack when it is older, as the yolk membranes weaken.
Testing an egg for freshness
Before eggs are packed, they are checked for defects by being passed over a strong light. This process is known as candling because wax candles were originally used as a light source.
As an egg gets older, several changes take place:
1. The size of the air space increases.
2. The yolk membranes weaken.
3. Moisture is lost through the shell.
4. Bacteria enter through the shell.
5. The thick white becomes thinner.
Importance of egg in the diet
Eggs are good source of high biological value protein and are easily digested by most people.
As they are used in so many ways in food preparation, they make a valuable contribution to the diet.
There is no carbohydrate in eggs and the iron in the yolk may be unavailable to the body.
They are also a poor source of calcium because it is concentrated in the eggshell which is not eaten.
Uses of egg in food preparation
1. Trapping air
Both the egg white and whole egg can trap air, because of the ability of the ovalbumin to stretch. This ability is utilized in cake making. Eggs are used as raising agent.
2. Thickening
Eggs are used to thicken custards, sauces and soups because of the coagulation of the egg protein.
3. Emulsifying
Egg yolk contain lecithin which is an emulsifier and enables oil and water to be mixed to an emulsion without separating. this is made use of in mayonnaise and cake making
The effect of heat on egg
Ovalbumin in the egg white starts to coagulate at 60 C, until the whole white is solid and opaque. The proteins of egg yolk start to coagulate at 70 C and continue until the yolk is dry and hard. If overcooked, the protein become tough and difficult to digest.
If an egg is boiled for some time, a green, black ring of iron sulfide forms around the yolk. This is due to the reaction of sulphur in the egg white with iron in the egg yolk. This reaction is prevented by cooling the egg rapidly as soon as it has been cooked.
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