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Diagram Showing Structure of Egg
(Redrawn from Wilson's Domestic Science in Elementary Schools.) EGGS Structure.
The outer covering of the egg, the shell, is of course for protection. It is very brittle and if examined under the microscope will be found to be quite porous. Because of these two facts it is provided with a very tough lining membrane which helps to keep water in the egg from evaporating and also prevents the loss of the contents of the egg should the shell be slightly cracked.
The white of the egg consists of a thick portion surrounded by a thin, watery portion.
The yolk is enveloped in a very thin membrane and is kept in position in the center by two spiral shaped strings of tissue. On the yolk will be found a small, round, light colored spot. This spot is the embyro or germ from which the little chick is to develop. Just as starch is stored in the seed to nourish the little plant when it first begins to grow, so are the yolk and white of the egg placed there to supply the chick with nourishment until it is ready to leave the shell.
Composition.
Since the contents of the egg is to furnish the only nourishment the chicken receives until it is fully developed and ready to hatch, the egg must contain all the food elements necessary to sustain life.
About one-sixth of the egg is albumin (a form of protein) used in building the different tissues of the chick. Seventyfour per cent is water to supply the water necessary in these tissues and in the circulating liquids. Fat, the most concentrated form of heat producing food, is there in quite large quantity to furnish heat and muscular energy and lastly there are the mineral salts, especially lime, for the bones and other tissues. The white of the egg is made up of albumin, water and mineral matter; the yolk contains water, albumin, fat, minerals and some coloring matter.
Composition of edible portion of hen's egg compared with moderately lean meat (Bailey's Sanitary & Applied Chemistry): Water Fat Protein Hen's Egg. 73.7 10.5 14.8 Lean Meat. 73.0 5.5 21.0 1.0 Mineral Matter 1.0 By comparing the composition of the egg with that of lean meat you will see that eggs make a good substitute for meat. Eight to ten eggs will weigh one pound and contain about the same nourishment that will be found in one pound of beef. As there is no starch or sugar present in the egg, eggs should be served with such foods as rice, bread, potatoes, etc., or made into desserts with sugar, tapioca, etc.
Spoiling of Eggs.
If eggs are kept for any length of time, some of the water will evaporate and bacteria-laden air will find its way into the egg. These bacteria soon cause the changes in the egg that we call spoiling. Sometimes if eggs are kept in a warm, damp place mold will form on the outside and push its way through the shell, ruining the contents of the egg as an article of food. Preservation of Eggs.
To keep eggs from spoiling we must exclude the air from them. This can be done by coating them with a solution of water glass (silicate of soda). To preserve with silicate of soda, dissolve 4 C. of water glass in ten quarts of water, pack the eggs, small end down, in a stone jar, then pour over them the solution EGGS of water glass until the eggs are completely covered. Cover the jar and place in a cool place. Eggs will keep all winter packed in this way.
As bacteria need warmth if they are to grow and cause food to spoil, eggs can be kept from spoiling by being put in cold storage. Thousands of dozens of eggs are put into cold storage every summer and sold in the cities during the winter. Tests for Fresh Eggs. 1. Drop the egg into a bowl of water. If it sinks it is fresh, if it stands on end it is not quite fresh and if it floats it is probably spoiled.