Commercial chicken eggs are sorted — according to exterior and interior quality — into three grades established by the United States Department of Agriculture (USDA): AA, A and B. For all grades, the shell must be intact. Nutritionally, all grades are the same.
Grades AA and A eggs are nearly identical, the main difference being that Grade A eggs are slightly older than Grade AA eggs. Grade AA eggs therefore have firmer, thicker whites that hold the yolks up high and round, whereas the white of a Grade A egg is “reasonably firm,” meaning it spreads a little farther when you break the egg into a frying pan. Grade A are the eggs you are most likely to see at a grocery store. Both grades are suitable for frying, poaching and other dishes in which appearance is important.
Grade B eggs have stained or abnormal shells, minor blood or meat spots and other trivial defects. They are used in the food industry to make liquid, frozen and powdered egg products, so you are unlikely to find them at a grocery store. Homegrown Grade B eggs are best used for scrambling, baking and similar recipes in which the eggs are stirred.
Any egg that does not fit into one of these three categories is unfit for human use and consumption. Although you needn’t worry about grading your homegrown eggs, the USDA grading system offers a guideline for assessing the quality of the eggs your hens produce.
Exterior Egg Quality
Exterior quality refers to a shell’s appearance, cleanliness and strength. Appearance is important because the shell is the first thing you notice about an egg. Cleanliness is important because the shell is the egg’s first defense against bacterial contamination; the cleaner the shell, the easier it can do its job. Strength influences the egg’s ability to remain intact until you’re ready to use it. The shell accounts for about 12 percent of the weight of a large egg. It is made up of three layers:
The inner, or mammillary, layer encloses the inner and outer membranes surrounding the egg . Between these two membranes is the air space that develops at the large end as the egg ages.