The U.S. Department of Agriculture grades eggs based on standards developed in the Agricultural Marketing Act of 1946, which divides the quality of eggs into three grades in descending order of quality: "AA," "A" and "B." These grades allow consumers to know the quality of the product they purchase. The program is voluntary for distributors of eggs.
For an egg to reach "AA" quality, the air cell cannot exceed a depth of 1/8 inch and can be bubbly or free with unlimited movement. The air cell is the space inside the egg that develops when the egg cools after being laid and is located between the inner and outer membranes. When the egg is twirled in front of a candling light the white of the egg must be firm and clear and the yolk only slightly defined. The yolk should not have any easily observed defects.
The air cell of an "A"-quality egg cannot exceed a depth of 3/16 inch. Like with the "AA" quality egg, the air cell can be free or bubbly and show unlimited movement. When twirled in front of the candling light, the "A"-quality egg white should be reasonably firm and clear. The yolk should not have any obvious defects and the outline of it should be somewhat defined.
The shell of the "B"-quality egg can have some staining, unlike the shell of the higher grades. These stains cannot cover more than about 3 percent of the surface of the shell. Similarly to the other eggs, the air cell here can be bubbly or free with unlimited movement. The air cell of a B quality egg can be more than 3/16 inch in depth. The yolk of this egg can be easily visible in front of the candling light due to a watery and weak egg white. The yolk can have present defects, but not to the point that they make the egg inedible.
Eggs that do not make the grade receive other classifications. For example, the USDA calls an egg "dirty" if it has stains, dirt or other foreign material that cover more than about 3 percent of the egg, or more than about 6 percent if the stain is scattered. Even lower in quality is the "check," which contains either a broken shell or a cracked shell. Although the egg is not pristine, the membranes of the shell remain intact, which prevents its contents from leaking.