Egg quality grading is a standardized inspection process that evaluates both the inside and outside of each egg, then assigns it a letter grade: AA, A, or B. In the United States, grading is overseen by the USDA, though it’s actually voluntary. Companies pay for the service, and eggs that pass inspection under a licensed USDA grader earn the familiar shield on the carton. Eggs without the shield may still carry a grade assigned by the producer or a state agency, but they weren’t federally verified.
Candling: Seeing Inside the Shell
The core technique behind egg grading is called candling. The egg is held up to a bright light source (originally a candle, now an electric lamp or LED array), which illuminates the interior through the shell. A trained grader or automated sensor can see three things: the air cell at the wide end of the egg, the yolk, and the surrounding white (albumen). The lowest-quality factor among those three determines the egg’s final grade. One excellent yolk won’t save an egg with a poor air cell.
A fresh, high-quality egg has a small air cell no deeper than 1/8 of an inch. As an egg ages, moisture evaporates through the shell and that air pocket grows. An air cell deeper than 3/8 of an inch is a significant defect. This is why air cell depth works as a reliable freshness indicator built right into the grading process.
The yolk of a top-grade egg barely moves when the egg is rotated in front of the light. It stays centered because it’s surrounded by thick, dense albumen holding it in place. As quality drops, the white thins out, the yolk drifts more freely toward the shell, and its outline becomes much more visible during candling. Graders watch for this movement as a key sign of interior quality.
Blood spots and meat spots also show up during candling. Spots smaller than 1/8 inch in diameter drop an egg to Grade B. Spots larger than that make the egg inedible by USDA standards. Graders learn to distinguish blood spots from the chalaza, a natural rope-like strand of albumen that anchors the yolk in place. The chalaza produces a distinctive bright ring of refracted light around its shadow, while a blood spot does not.
What Each Grade Means
Grade AA eggs have the firmest, thickest whites and the smallest air cells. When you crack one open, the white stands up tall around the yolk rather than spreading thin across the pan. The yolk is round, centered, and well-defined. The shell is clean, unbroken, and normally shaped. These are the best eggs for poaching or frying, where appearance matters.
Grade A eggs are very close to AA. The white is reasonably firm, the air cell is slightly larger, and the yolk may be a bit more visible during candling. In practice, most eggs sold in grocery stores are Grade A. The differences between AA and A are subtle enough that most consumers wouldn’t notice them in everyday cooking.
Grade B eggs have noticeably thinner whites, larger air cells, and yolks that may be flattened or off-center. The shells can have slight staining or abnormal shapes. These eggs are perfectly safe to eat but don’t look as appealing when cooked whole. They’re commonly used in liquid, frozen, or dried egg products rather than sold in the shell.
Shell Inspection
Before the light even turns on, graders evaluate the outside of the egg. The shell should be intact, clean, and free of cracks, stains, or unusual texture. Even minor cracks are a problem because they compromise the natural barrier against bacteria. Shells with rough patches, thin spots, or odd shapes get downgraded. For the top grades, the shell needs to be smooth, normally shaped, and practically spotless.
Sizing by Weight
Size and grade are two separate evaluations, though both appear on the carton. Size classification is based strictly on the minimum weight per dozen eggs, not per individual egg. The six size classes are:
- Jumbo: 30 ounces per dozen
- Extra Large: 27 ounces per dozen
- Large: 24 ounces per dozen
- Medium: 21 ounces per dozen
- Small: 18 ounces per dozen
- Peewee: 15 ounces per dozen
Large eggs, at 24 ounces per dozen (about 2 ounces each), are the standard size used in most recipes. Size has no bearing on quality grade, and grade has no bearing on size. A Medium egg can be Grade AA, and a Jumbo can be Grade B.
Washing Before Grading
In the United States, commercial eggs are washed before grading. This removes dirt and bacteria from the shell surface but also strips the egg’s natural protective coating, called the bloom or cuticle. To compensate, processors apply a thin layer of food-grade mineral oil to reseal the shell and slow moisture loss. This step matters for grading because it helps preserve the small air cell and firm albumen that earn higher grades. The standard commercial wash uses warm water at an alkaline pH to reduce Salmonella contamination, though research has shown that acidic washes at room temperature can achieve similar pathogen reduction while potentially extending shelf life.
How Modern Machines Handle It
Large-scale egg operations process tens of thousands of eggs per hour, far too many for human candlers to inspect one by one. Automated grading systems use high-intensity lights, cameras, and computer vision to evaluate each egg as it moves along a conveyor. These machines can detect dirty shells, blood spots, and cracks in real time, sorting eggs into grade categories without human intervention.
Recent systems use neural networks trained on thousands of egg images to identify defects. Blood spots on white-shelled eggs are relatively easy for these models to catch because the color contrast is so stark. Cracks, however, remain a challenge. Microcracks and fractures on the bottom or sides of the egg can escape detection if the camera only captures one angle. Some facilities are exploring 360-degree camera setups to close this gap. Despite these limitations, automated systems achieve high sorting accuracy for the most common defects and have made consistent, large-scale grading economically feasible.
Voluntary but Widespread
Federal egg grading is not required by law. Producers choose to participate and pay the USDA for the grading service. The USDA shield on a carton confirms that a licensed federal grader supervised the quality and weight checks. Cartons without the shield may still display a grade, but that grade was assigned by the company itself or verified by a state agency. Some states actively monitor egg quality at the retail level, providing an additional layer of oversight even for eggs that weren’t federally graded.
For consumers, the practical takeaway is straightforward: the grade tells you about the egg’s interior quality and freshness at the time it was packed, while the size tells you its weight. Both are checked during the same processing run, but they measure completely different things.