Hens' eggs are sold in myriad qualities and sizes that are ranked according to USDA standards. While inspection for egg safety and wholesomeness is mandatory, grading for quality is voluntary. If an egg producer chooses to have eggs graded, it pays the USDA for the service.
There are three U.S. grades for eggs. In descending order, they are AA, A and B. Eggs are graded both by shell condition and interior quality and they are grouped by size.
Eggshells are inspected for cleanliness, texture, shape and lack of cracks. Small probes tap an egg multiple times and record the sound that’s made. A high-pitched ring means the eggshell is intact, while a thump indicates a crack. Grades AA and A eggs must have clean shells, while small stains are allowed on Grade B shells.
In the old days, an egg was held up to a candle, illuminating its interior for inspection. While candles are no longer used, the process of illuminating an egg for inspection still is called “candling.” Today, eggs are rotated over high-intensity light. An egg’s interior is judged by the size of the air cell (the empty space between the egg white and shell at the large end of the egg — smaller air cells are preferred); the albumen (egg white) proportion and quality; and the yolk’s firmness. Grade AA eggs have high, round yolks and thick whites. Grade A eggs have whites that are “reasonably” firm, while Grade B eggs have thinner whites and flatter yolks; these eggs are often processed into liquid, frozen or dried products. According to the USDA, Grade A eggs are the most popular sold in stores.
Egg size is independent of grade. For example, a store may sell grades AA and A large eggs, or grades A and B jumbo eggs. Computer-controlled scales weigh eggs and sort them according to minimum weight per dozen:
Jumbo: 30 oz. per dozen
Extra large: 27 oz. per dozen
Large: 24 oz. per dozen
Medium: 21 oz. per dozen
Small: 18 oz. per dozen
Peewee: 15 oz. per dozen
(Most recipe measurements are calculated on the assumption that the cook is using “large” eggs.)