About 60 percent of the eggs sold in the United States come from farmers 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.

Egg farmers who participate must spray-wash their eggs with warm water and use a sanitizing rinse and air-drying techniques specified by USDA’s Agricultural Marketing Service (AMS).

“We require very specific washing and rinsing procedures for participation, but as a practical matter, every producer in the United States washes their eggs. It’s an industry standard,” says Mark Perigen, national supervisor for AMS’s shell egg quality assessment division. After washing, all eggs must be stored at refrigerated temperatures that meet USDA Food Safety and Inspection Service requirements.

According to researchers, egg quality stays high with refrigeration and degrades rapidly without it. And only the highest quality eggs will make the grade.