Grading of Eggs
Once eggs pass inspection, a producer can pay the USDA to have them graded for quality. The best quality eggs are graded USDA Grade AA, followed by USDA Grade A. USDA Grade B, the lowest grade, is available to food service establishments and not sold directly to consumers. Grade AA and A eggs are the grades sold at supermarkets. Their firm, high albumens and yolks make them suitable for frying, coddling, poaching, and hard cooking. Grade B eggs have thinner whites and somewhat flattened yolks, making them better for scrambling or baking, or as ingredients in other food products.
Grades are determined by graders who incorporate three methods to judge the quality of eggs: candling, measuring Haugh (pronounced “how”) units, and evaluating appearance, specifically that of the shell, white, yolk, and air cell. Commercial egg grades are assigned based on both interior and exterior quality. Interior quality is primarily determined by candling. The exterior quality is determined by the cleanliness of the shell, the shape of the egg, and the presence of shell irregularities such as pimples (calcium deposits) and weak shells.
Candling. As the name implies, the original method of candling involved holding an egg up to the light of a candle to view its contents. Although it is still a good way to determine an egg’s inner quality and to detect defects, eggs are now mechanically rotated over lights, many at a time, by rollers.
Candling is based on the principle that eggs start to deteriorate the minute they are laid, and these changes can be seen vaguely through an egg held against a light. The whites become more thin and transparent as carbon dioxide departs through the shell; and the yolk membrane stretches as the yolk absorbs water from the white, which eventually will cause the yolk to break easily when the shell is cracked.
The yolk in fresh, high-quality egg is suspended tightly by the chalazae, seen in candling only as a slight shadow. The yolks in older eggs, on the other hand, are surrounded by thinning clearer egg whites and deteriorating chalazae. These older yolks lie closer to the shell because they are no longer suspended well by the chalazae; they are looser in consistency, and cast a darker shadow. The egg’s air cell, too, becomes wider or moves as the egg ages. Grade AA eggs must have an air cell depth smaller than 1/8 inch; Grade A eggs are limited to 3/16 inch; and Grade B eggs have no limit to the size of their air cell.
Haugh Units. The freshness of an egg can be detected by cracking it open onto a flat surface and looking at the height of its thick albumen. Fresh egg whites sit up tall and firm, while older ones tend to spread out. Professional graders cannot evaluate every egg for freshness, so an egg is randomly selected and measured using a special instrument called a micrometer. The Haugh unit, a numerical value reflecting an egg’s freshness, is obtained by mathematically combining the thick albumen height with the egg’s weight, and then using a formula or table to convert this number into a Haugh unit. As Haugh units decrease, so does egg quality, and this is reflected in grading: Grade AA is given to eggs with a Haugh unit of 72 or higher, Grade A for a measure of 60 to 71, and Grade B for a measure of 31 to 59.
Appearance. Grading can also be based on the appearance of eggs broken on a flat surface. Graders evaluate the quality of an egg by observing the thickness of the albumen, the prominence of the chalazae, the roundness and firmness of the yolk, and the shape, cleanliness, and texture of the shell. The vast majority of eggs are graded on interior quality by candling. Grading of broken-out eggs is reserved for research purposes or random spot checks of candled eggs.
