Eggs

Composition of egg

Percentage distribution of weight

Percentage composition of egg white and yolk

Egg white is composed of thin and thick portions. 20-25% of the total white of fresh eggs (1-5 days old) is thin white. The chief constituents of egg white besides water are proteins. Different type of proteins are present in egg white.

Ovalbumin

This constitutes 55% of the proteins of egg white. This is a phospho glycoprotein and is composed of three components A1, A2, and A3, which differ only in phosphorus content.

Conalbumin

This constitutes 13% protein of the egg albumin. It consists of two forms neither of which contains phosphorus nor sulphur.

Ovamucoid

It is a glycoprotein. This constitutes about 10% of the egg white proteins.

Ovomucin

This protein is responsible for the jelly like character of egg white and the thickness of he albumin. It contains 2% of the egg white. Its content in he thick layers of albumin is about 4 times more than in thin layers. It is insoluble in water but soluble in dilute salt solution.

Lysozyme

3.5% of the egg white protein is lysozyme. This is an enzyme capable of lysing or dissolving the cell of wall of bacteria. It is composed of 3 components A, B and C. It binds biotin and makes the vitamin unavailable.

Avidin

Avidin is 0.05% of the egg white protein. It is denatured by heat and cooked eggs do not affect the availability of biotin.

Ovoglobulin

It is a protein consisting of two components G₁ and G₂ and both are excellent foaming agents.

Ovoinhibitor

0.1% of egg protein is made up of ovoinhibitor. It is another protein capable of inhibiting trypsin and chymotrypsin.

Egg yolk

Solid content of yolk is about 50%.

Percentage composition of egg yolk on dry weight basis

The major proteins in egg yolk are lipoproteins, which include lipovitellins and lipovitellinin. The lipoproteins are responsible for the excellent emulsifying properties of egg yolk, when it is used in such products as mayonnaise.

Fatty acid composition of egg yolk

Nutritive value of egg/100g

Evaluation of egg quality

• Candling

The quality of the egg in the shell is evaluated by candling. The egg is held up to an opening behind which is a source of strong light. Candling will reveal

A crack in the shell.

The size of the air cell.

The firmness of albumin.

The position and mobility of yolk and

The possible presence of foreign substances like blood spots, moulds and developing embryo.

• Floating in water

If the egg sinks it is considered as good. Poor quality egg floats (due to increase in air cell). It shows that the egg floating in water has lost in weight due to dehydration.

• Haughs unit

Good quality egg has 72 haugh units and as the quality deteriorates it comes down to 36-60.

• White index

The height of the thickest portion of the white is divided by the diameter of the egg gives white index.

• Yolk index

Measurement of the height of the yolk in relation to the width of the yolk gives the yolk index.

• Grading

The interior quality of the egg deteriorates from the time it is said to until it is consumed. With proper care, however, this decline in quality can be minimised.

• Deterioration during storage

Fertile eggs get deteriorated more rapidly than infertile eggs. Deterioration takes place as physical and chemical changes takes place.

Egg white becomes less viscous and spreads rapidly.

The size of the air cell and volume increases.

Loss of water, carbon dioxide, protein breakdown,egg flavour deteriorates.

Bacterial decomposition takes place.

• Effect of heat on the egg proteins

Upon heating the egg proteins are denatured and then gradually aggregate to form a three-dimensional get network. Ovalbumin the main protein in egg white is a globular protein denatured by heat. The range in temperatures over which coagulation takes place varies with the rate of heating. Heating on egg much beyond this temperature shrinks and toughens the coagulum of the white.

Preservation of Eggs

• Freezing

Egg breaking, separation and pasteurization and freezing are the steps involved. Eggs ae pasteurized to kill all Salmonella organisms.

• Cold storage

Eggs are usually stored at –1.5⁰C to 0⁰C which is just above their freezing point. Only eggs of high quality should be stored. They remain in desirable condition only if the storage room is well controlled as to humidity, 85-90% circulation of air and free from objectionable odours. Eggs may retain quality as long as 6 months in cold storage. Before being placed in cold storage, eggs may be dipped in light mineral oil.

• Drying

It is a satisfactory method for preserving eggs, either whole or as separated yolks or whites. Spray dried egg white and egg yolk have long shelf lives.

• Soft and hard cooked eggs in the shell

This requires 3-5 minutes of cooking in boiling water. An addition of 2 minutes should be allowed if eggs are at refrigerator temperature or should be cooked at 85⁰ C for 4-6 minutes. Eggs hard cooked in the shell needs to be cooked until the white changes to an opaque tender gel and the yolk becomes pale yellow and has a mealy rather than an pasty consistency. Eggs should be cooked at 85⁰ C for 18 minutes or at 100⁰ C for 8-10 minutes.

• Fried eggs

The shape of a fried egg depends on the thickness of its white and on the temperature at which it is cooked. The ideal temperature to prepare fried egg is around 126-137⁰C.

• Custards

In custards egg acts as thickening agent. A true custard consists only of eggs, milk, sugar and flavouring custards are of two types, the stirred or soft custard.

• Egg white foams

The foaming of egg white plays an important role in many foods because it makes the products light in texture and contribute leavening. Egg white foam is a colloid consisting of bubble of air surrounded by albumin that has undergone some denaturation on the liquid air surfaces.

• Omelettes and souffles

Omelettes may be plain or foamy. The plain omelette is made without separating the yolk from white. The whole egg is beaten together with flavouring materials. The foamy omelette is prepared by beating the whites and yolks separately. For making an omelette, the heat should be kept low so that the mass cooks slowly and can be heated uniformly through out without toughening the bottom layers.

• Souffles

They are similar to foamy omeletts, except that have a thick white sauce base and contain additional ingredients such as grated cheese, vegetable pulp or ground meats. Dessert souffles are sweet and may contain lemon, strawberry and chocolate. Souffles are usually baked though they can be steamed. Crepes are thin, tender pan cakes containing a relatively high proportion of egg. They can be filled with a variety of items, including fish, meat, poultry, cheese, vegetables or fruits. Crepes can be served with sweet, dessert type fillings.

• Shirred eggs

They are cooked and served in the same dish. The dish is coated with butter or margarine, the eggs are broken into it and the dish is set directly on the range until the whites are coagulated. The dish is then transferred to a moderate oven a finish cooking. Care must be taken not to over cook and toughen shirred eggs.

• Role of egg in cookery

There are few ingredients in food preparations that are useful in so many different ways as are eggs. Used alone or in combination with other foods, they may become the major protein dish for a meal.