About 60 percent of the eggs sold in the United States come from processors who participate in USDA’s grading service, voluntarily paying to have their eggs graded so the eggs can display a “USDA Grade A” or “AA” shield on their cartons. The grade is based on qualities that can be observed in the shell, yolk, and egg white when the egg is inspected with lights and other specialized equipment. Specifics on egg-grading criteria can be found here.
Egg processors who participate are required to spray-wash their eggs with warm water and use a sanitizing rinse and air-drying techniques specified by USDA’s Agricultural Marketing Service (AMS).