Eggs sold at supermarkets are typically cleaned as part of the processing process before reaching consumers, so they do not need to be washed again, according to Jennifer Pallian, a Canadian food scientist and dietitian.

Eggs laid by hens have a natural protective coating on the surface of the shell. This thin layer acts as a seal that helps prevent bacteria from entering the egg.

“Eggshells are porous, with thousands of tiny holes that let air pass through,” Pallian told Simply Recipes.

After eggs are cleaned, producers typically coat the shells with a thin layer of mineral oil that acts as a protective barrier. Washing the eggs can remove this layer and increase the risk of bacterial contamination.

If washed with cold or tepid tap water, bacteria can be carried through the pores and into the egg.

For eggs from farms or farmers’ markets, consumers should ask the seller whether the eggs have been washed. If they have not and the natural protective coating remains intact, the eggs should not be washed before being stored. Consumers can use a dry cloth, paper towel or brush to gently remove any dirt.

If eggs must be washed, Pallian recommends using water that is warmer than the eggs rather than cold water. After washing, the eggs should be dried immediately and used soon afterward.

Scientists at the University of Arizona in the U.S. also recommend refrigerating eggs from backyard chickens even if they have not been washed.

The safe way to store eggs

Eggs should be kept in their original carton and left in the coldest part of the refrigerator rather than in the door.

The U.S. Food and Drug Administration recommends keeping the refrigerator temperature at 4 C (40 F) or below. Consumers should also wash their hands and any utensils that come into contact with raw eggs after handling them.

Once they have been refrigerated, the eggs should not be left at room temperature for more than two hours, as bacteria can grow more quickly in warmer conditions.