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The term "quality" as applied to eggs refers ultimately to their desirability for human consumption. When an egg deteriorates its cooking qualities are altered. It is commonly accepted that an egg of high quality is better for poaching, boiling, and frying than one of inferior quality. At present, for trade use, eggs of quality are defined as those having a relatively high percentage of thick to thin white, and a high percentage of yolk solids. These eggs, when broken on a flat surface, do not tend to flatten out as quickly nor to so great an extent as eggs of poorer quality. See Fig. 1.
Tests for quality. A candler's grading of the egg is based largely on the size of the air cell and the visibility of the yolk. Perry found that yolk color, ascertained after breaking the egg, influences the yolk shadow and the yolk movement, as determined in candling. The dark yolks cause a darker shadow and increase the apparent movement, though the percentage of thick albumen did not influence the yolk shadow and movement. The air cell increases in size with loss of water from the egg. The less the humidity of the air in which the egg is stored, the greater the loss of water. When water is lost to the outside air the total solids of the egg increase. The changes in the cooking quality of the white and yolk may not be so detrimental as when the water loss is prevented. Since the size of the air cell increases with the loss of water, the humidity of storage rooms is usually controlled to prevent a large moisture loss. But other changes, more detrimental to the quality of the egg than water loss, may occur.
Almquist, Givens, and Klose say that the transmission of light varies for different layers of egg albumen, being lowest for the firmest or gelatinous layers. They find the transmission of light to be correlated with the percentage of mucin in the albumen. In addition it varies with the temperature and pH, both of which affect the physical condition of the mucin.
At lower temperatures and at lower pH the translucency of the mucin fibers is lessened.
Wilcke used a torsion pendulum to measure externally the interior viscosity of an egg and from observations worked out an index K, which was a measure of the combined viscosity of the entire contents of the egg. He found that the K value increased with increased weight of the eggs, but that the rations used did not affect the K values of the eggs produced by hens on the rations used in his study. The index, K, is a characteristic of the individual hen.
Standing-up quality of the yolk. Sharp has worked out a system of determining the quality of the egg from the standing-up ability of the yolk. If a fresh egg of good quality is broken out of the shell, the yolk stands up. But as the interior quality deteriorates the yolk flattens out more and more readily until a stage is reached at which the yolk membrane breaks, no matter how carefully handled, when the shell is broken. Both the time the yolk is on the dish and its temperature affect the extent of flattening, the flattening being greater with longer time and a higher temperature. By dividing the height of the yolk by its width a numerical index is obtained that indicates the quality of the egg. The measurements of the yolk, after being freed of the white, taken 5 minutes after it is laid on the Petri dish and at a temperature of 25°C, give an average value of about 0.41 for eggs 3 to 4 hours old. With deterioration of the egg the index becomes less, and breaking occurs when the index falls to about 0.25.
The standing-up ability of the white may also be used to determine egg quality.
Vitelline membrane strength. Another method for determining quality is to measure the strength of the vitelline membrane. The average thickness of this membrane has been reported to be about 64/100,000 of an inch. In a fresh egg its bursting strength is about 0.065 pounds per square inch. With deterioration the strength of the membrane decreases and the yolk breaks easily when the strength has fallen to a little over half this value.
Factors affecting quality of eggs. Fresh-laid eggs vary in proportions and viscosity of the thin and thick white. The yolk membrane also varies in strength. These variations are probably due to feed, the season of the year, the period of the laying cycle, and individual characteristics of the hen. It has already been mentioned that Wilcke found that the rations did not influence the viscosity of the egg in his investigations, but he states his work does not rule out this factor. Lorenz and Almquist report that the percentage of firm white is lowered by higher air temperature during the hours immediately after the egg is laid, resulting in an apparent seasonal variation in internal egg quality. The poorer quality of eggs obtained during summer months is attributed to the higher temperature during this season. The finest quality eggs are claimed to be those laid in the spring, which coincides with the time of greatest production.
Preservation and deterioration of eggs. Sharp states that "as soon as the egg leaves the hen it begins to decline in interior quality and the best we can hope to do is to retard these changes as much as possible. They cannot be stopped, they can only be retarded. An egg a week old may have deteriorated more in quality than an egg properly cared for which is a year old."
Eggs are preserved by (1) storing at low temperatures, (2) by freezing, (3) by drying, and (4) by oil dipping.
Storing at low temperatures. In commercial storage the temperature, humidity, and air currents are controlled, the last to prevent mold growth. A high moisture content in the storage air lessens the amount of the water evaporated from the egg, but encourages mold growth. The storage temperature is usually maintained at 29° to 30°F. In addition chemical control of the atmosphere is frequently practised in the storage rooms. Stewart and Sharp state that at 30°F., if 0.6 per cent carbon dioxide is used, the pH of the egg white will be maintained at 8.0 to 8.1. If the concentration of carbon dioxide is too high, the white becomes turbid, but loses this turbidity after loss of some carbon dioxide from the egg on breaking.
Structure and composition. The shell forms about 11 per cent of the egg and is largely composed of calcium carbonate with some magnesium carbonate, calcium and magnesium phosphates, and organic matter.
Within the shell is the shell membrane, a thin, semi-permeable membrane made up of two layers, the inner and outer. After the egg is laid, its contents cool and shrink. The air cell at the large end of the egg is formed during this shrinkage by separation of the two membranes.
For cookery purposes the white and yolk are the important parts of the egg. The white is clear, transparent, and jelly-like. It composes about 57 per cent of the weight of the whole egg. The layer next the shell is a thin soft white, its relative proportion varying somewhat at time of laying. Fig. 32. Next comes a layer of rather thick, viscous white, containing a larger proportion of mucin than the thin white. Beyond this is a small layer of thin white surrounding the yolk. Schaible, Moore, and Davidson say that the thick, "firm white is of laminated structure, composed of concentric layers containing mucin fibers." If a freshly laid egg is broken into a large quantity of distilled water (the thick white having been slit with scissors on one side to remove yolk) and allowed to stand, the mucin and probably some globulin are precipitated at the edge of the cut surface. Six or more sheets or layers, one over the other, can be distinctly seen. Fig. 2.
Fig. 2. - Showing the laminated structure of firm egg white, yolk removed.
The chalazae are dense cord-like strands of white substance, one on each side of the yolk, which anchor the yolk near the center of the egg. They allow the yolk to revolve. Being dense the chalazae are not broken down readily when the egg is beaten, hence they are often caught on the blades or wires of the egg beater.
The yolk forms about 32 per cent of the whole egg. It is separated from the white by a yolk sac, called the vitelline membrane. The yolk is made up of layers. See Fig. 3. The fat is more concentrated around the germ spot; hence its specific gravity is less and when the egg is turned the yolk rotates so that the germ spot is always uppermost. In eggs of average size the white usually averages about 30 grams and the yolk about 18 grams.
Composition. The composition of the white is approximately 86.2 per cent water; 12.3 per cent protein; 0.2 per cent fat; and 0.6 per cent mineral. 1 he composition of the yolk is about 49.5 per cent water; 33.3 per cent fats, including lecithin and cholesterol; 15.7 per cent proteins; and 1.0 per cent minerals.
Fig. 3. - Diagram showing internal structure of an egg.
Color of shell and yolk. The color of the shell is determined by inheritance, certain breeds laying white, others brown-colored shells. The Barnaveldters lay eggs with nearly chocolate-brown shells.
The coloring matter of the yolk, according to Palmer, Mattikow, and others is xanthophyll, with a small proportion of a carotene-like pigment. The intensity of the yolk color is determined by the amount of xanthophyll in the food of the hen. The color of the yolk may vary from a very pale, almost white yolk, through deeper yellows to orange and finally a deep-red-orange. Some feeds may produce red-colored yolks. Some attempts have been made to work out color charts, giving each shade a number; but most of these are only tentative. It is an advantage for bakers and mayonnaise manufacturers to obtain yolks of uniform color to insure uniformity of color of their products. A small number of manufacturers at present do specify the color of the egg yolks when they place their contract for them. Reaction of eggs. The egg white is alkaline. Sharp found that the pH of the white varies from 7.6 to 9.7, whereas the yolk of fresh eggs averages pH 6.0 or slightly lower. The pH of both the white and yolk increases with age because of the loss of carbon dioxide. Sharp and Powell state that since the loss of some carbon dioxide cannot be prevented before the pH is taken, that a pH of 7.6 is probably too high for freshly laid eggs. With further loss of carbon dioxide the pH may increase to 9.5. Then, probably owing to break down of some of the egg protein, the pH decreases. The pH of the yolk may reach 6.8, but changes more gradually than that of the white. When the whole egg is beaten so that the white and yolk are mixed, a pH intermediate between that of the white and yolk is obtained.
Taken from : Properties of Egg Proteins
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