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. 32.

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.