Eggs are categorized into three grades, AA, A, and B based on quality considerations. To determine these grade ratings, eggs are sorted into categories by size, weight and “quality” factors concerning the shell, white, egg yolk, and the internal air cell. Also, abnormalities are looked for, such as the presence of spots of blood, which indicates a fertilized egg, unfit for consumption. Historically, a candling device was used and a similar method is used even today, except with a flashlight and a dark sleeve rather than a specific device. I certainly will not attempt to explain the whole procedure for egg grading, but the inspection consists of using a device, any device, that can “candle” or shine a light on the egg in darkness so that the internal characteristics of the egg can be determined. A few eggs, now and again, are ‘broken out’ to compare the results with the candling results and determine the “Haugh Unit value.”
In case you’re wondering, the color of an egg has nothing to do with its quality.
In case you are really interested or plan to start your own egg plant (egg plant, not eggplant, joker) you can get the lowdown in the USDA Egg Grading Manual and the specifications in the Federal Register of the USDA, Code of Federal Regulations 7, part 56, 2008.
Now, the thing about eggs, like any fresh food, is that the quality is not static. It deteriorates as the egg ages. So the eggs must be stored and handled carefully to minimize this deterioration. Some of the characteristics that determine egg quality are:
External Factors: Shell Shape and Texture, Cleanliness
All chicken eggs are oval. You’re not going to find a round one. So the different shapes are variations on “rounded with large end and smaller end.” There is considerable variation allowed. The image below shows ideal, practically normal, practically normal with slight ridges and rough areas, and abnormal eggs.
Eggs are checked for cleanliness after washing. Eggs considered clean can have some very small stains, cage marks, or specks but are generally free from foreign material, stains, and discolorations. An egg considered dirty has dirt of foreign material clinging to its surface, or has prominent or moderate stains. If localized, the “dirt” cannot cover more than 1/32 of the egg’s surface. If scattered, it cannot cover more the 1/16 of the egg’s surface.
Internal Quality: Air Cell, Yolk, and White
Hen eggs contain an air cell at the large end which forms as the egg cools. When the egg is first laid, there is no air cell. As the egg cools, the liquids inside the shell contract more than the shell itself, which forms an air filled space, separating the inner membrane from the outer shell. The air space may grow further as water evaporates from the egg. Although it is thought that the air cell is unimportant compared to the broken out appearance of an egg, the U.S. standards still consider it a factor in grading, although only the depth is considered important. It does not matter whether the air cell moves or whether it is bubbly (small separate air cells beneath the main one). An air-cell gauge is used to easily determine the depth of the cell, although experienced handlers can judge it by eye. The depth allowances for AA, A, an B are:
|AA||l/8 inch (3.2 mm)|
|A||3/l6 inch (4.8 mm)|
The appearance of the yolk, upon candling, is considered one of the best indicators of quality. The appearance of the yolk and the condition of the white are interrelated. To judge egg yolk quality the distinctness of the yolk shadow outline, size and shape of the yolk, and defects and germ development are considered.
the less defined you yolk outline is indicates a higher quality egg. An AA egg should have a yolk outline that is only slightly defined and that pretty much blends into the surrounding white. An A grade egg should have the yolk outline fairly well defined, meaning it is discernible but not clearly outlined. A B grade egg has a yolk outline that is clearly defined and is visible as dark shadow when the egg is twirled in front of the candler. The image below shows the yolk outline grades of candled eggs.
In a freshly laid egg, the yolk is round and firm. As the egg ages, the yolk’s membrane weakens and water is absorbed into the yolk, causing it to grow in size and weight. These changes only become apparent in the lowest quality B grade eggs. These yolks will appear enlarged and flattened like a “balloon slightly filled with water.”
The main cause of yolk defects is germ development. Other various defects can be observed but their exact causes are unknown and subject to speculation. The three terms used are:
- Practically free from defects — AA and A grade. There is no germ development but there may be other very slight defects on the surface of the yolk.
- Serious defects — B grade. There is no germ development but there are well developed spots or areas and other serious defects such as olive yolks. These defects do not render the egg inedible.
- Clearly visible germ development — B quality. There is a clearly developed and visilbe germ spot on the yolk but there is no evident blood.
- Blood due to germ development — Inedible. There is clearly visible blood which can be seen as lines or a blood ring due to the germ in a fertile egg that has developed further.
Egg white or “albumen” usually has four layers, the chalaziferous, inner thin, thick, and outer thin. The relative proportions of these layers before the candling light is the main thing that determines its appearance. As stated above, the condition of the yolk and the white are interrelated. The intensity of the yolk shadow outline and the movement of the yolk as the egg is twirled in front of the light is determined by the viscosity of the albumen. The thicker the albumen, the less movement of the yolk and the more indistinct its outline. As can be seen by the yolk outline descriptions above, then, higher quality (fresher) eggs have thicker whites. When you cook eggs and the white spreads out across the pan, being very watery, you have an older egg. The following factors are considered:
- Clear — AA or A grade. There are no discolorations or foreign bodies in the albumen.
- Firm — AA grade. Albumen that is thick or or viscous enough to prevent the yolk outline from being more than slightly defined or only indistinctly visible when the is twirled. If such an egg is broken-out, the albumen has a Haugh unit value of 72 or higher when measured at a temperature between 45 and 60 °F (7.2 and 15.6 °C) See Egg Grading manual link above for Haugh unit description.
- Reasonably firm — A grade. Albumen that that is a bit lees thick or viscous than firm albumen and permits permits the yolk to approach the shell more closely, resulting in a fairly well defined yolk outline when the egg is twirled. when broken out, there is a Haugh unit value of 60 up to, but not including 72 when measured at a temperature between 45 and 60 °F (7.2 and 15.6 °C)
- Weak and watery — B grade. Albumen that is weak, thin, and generally lacking in viscosity, permitting the yolk to approach the shell closely, thus causing the yolk outline to appear plainly visible and dark when the egg is twirled. When broken out, the weak and watery albumen has a Haugh unit value lower than 60 when measured at a temperature between 45 and 60 °F (7.2 and 15.6 °C)
- Blood spots or meat spots — B grade. Small blood spots of not more than one-eight inch aggregate diameter are present. Or, small meat spots of not more than one-eighth inch aggregate diameter. Larger blood spots, or blood that has diffused into the albumen surrounding the spot are classified as loss. These blood spots should not be due to germ development but they may be on the yolk or in the albumen. Meat spots may be blood spots which have lost their characteristic red color, or tissue from the reproductive organs.
- Bloody white — An egg which has blood diffused through the albumen. Such a condition may be present in freshly laid eggs. Eggs with bloody albumen are classed as loss.
Eggs Defined as “Loss”
Various other factors can help determine if an egg is defined as a loss. An egg that is a loss is an egg that is inedible, cooked, frozen, contaminated, sour, musty, or moldy, or an egg that contains a large blood spot, large meat spot, bloody white, green white, rot, stuck yolk, blood ring, embryo chick (at or beyond the blood ring state), free yolk in the white, or other foreign material.
Summary of Specifications for AA, A, and B Grade Eggs
The following table is a reproduction of the Summary of U.S. Standards for Quality of Individual Shell Eggs table given in the USDA Egg Grading Manual.
|Quality Factor||AA Quality||A Quality||B Quality|
|Shell||Clean||Clean||Clean to slightly stained.*|
|Unbroken. Practically Normal||Unbroken. Practically Normal.||Unbroken. Abnormal.|
|Air Cell||1/8 inch or less in depth.||3/16 inch or less in depth.||Over 3/16 inch in depth.|
|Unlimited movement and free or bubbly.||Unlimited movement and free or bubbly.||Unlimited movement and free or bubbly.|
|White||Clear.||Clear. Reasonably Firm||Weak and Watery. Small blood and meat spots.**|
|Yolk||Outline slightly defined.||Outline fairly well defined.||Outline plainly visible|
|Practically free from defects.||Practically free from defects.||Enlarged and flattened. Clearly visible germ development but no blood or Other serious defects.|
* Moderately stained areas permitted (1/32 of surface if localized,
or 1/16 if scattered.).
** If they are small (aggregating not more than 1/8 inch in diameter).
For eggs with dirty or broken shells, the standards of quality provide two additional qualities.
- Dirty – Unbroken. Adhering dirt or foreign material, prominent stains, moderate stained areas in excess of B quality.
- Check – Broken or cracked shell, but membranes intact, no leadking. A “leaker” has broken or cracked shell membranes, and contents leaking or free to leak.
Weight Classes: Jumbo to Peewee
Eggs are also classified according to weight. They are separated into jumbo, extra large, large, medium, small, and peewee sizes. This table breaks down the sizes by minimum weight per dozen eggs, 30 dozen eggs, and individual eggs.
|Size or Wt. Class||Min. net wt. per dz. (oz)||Min. net wt. 30 per dz. (lbs)||Min. net wt. individual eggs (oz)|
How Do You Know if An Egg is Good?
Regardless of the eggs quality grade when you buy it, the egg’s freshness determines its true quality for cooking. A grade A or AA egg, if you store it too long, will become a grade B egg. The cook does not need to be concerned with high, average, or low AA, A, or B grade eggs but their appearance will help determine their best use in cooking. Eggs that are AA quality, very fresh, will have high and firm yolks and thick whites which do not spread. A grades will have slightly thinner whites. B grades will be watery and thin.
All of these eggs, even the B grades, are still useful, just for different purposes. Any egg will work well in a recipe where the egg is incorporated. However, certain properties are present in fresher or older eggs.
When you are serving eggs fried, poached, or cooked in the shell, ideally you want the freshest eggs possible, AA grade being the best. Break your eggs out into a separate container to judge the thickness of the white and the firmness of the yolk. Remember, B grade eggs will have yolks that are enlarged and slightly flattened. The smaller and firmer the yolk, the fresher the egg. These eggs will not spread out like water. For the most part, you simply want to avoid B grade eggs for this use, but when poaching, which is difficult to master, go for the freshest egg possible. For frying or boiling, AA or A should be fine for most, unless you are a really obsessive foodie.
When you are going to incorporate your eggs into recipes with other ingredients, they can be older. For whipping, older egg whites will actually whip up to a greater volume!
Although we are often warned never to use an egg that is cracked, if the egg has been refrigerated and if it is going to be cooked very thoroughly, it should be fine, as long as it does not smell sulfurous. Cracked eggs should be used in recipes, therefore. If you are dealing with older “B grade” eggs and you are unsure if they are too old, a sniff will tell you quickly. Good eggs have either no smell or a slightly sweet smell. On the other hand, the smell of bad eggs is hard to mistake. Any type of phosphorous or musty odor indicates an inedible egg.
If you find blood spots or germ spots in your eggs, as in the image above, these are harmless to you and it is up to you whether you can handle the idea that a bit of blood or the beginnings of germination are present. Obviously, you wouldn’t want to serve these fried or poached to guests. You may be able to remove the spot with the tip of a spoon.
Green whites could mean contamination by Pseudomonas bacteria. They may or may not have a sour odor. Discard any such eggs.
Sometimes when you crack an egg you may break the yolk. However, the white of a fresh egg is thick and the yolk is not watery, so the yolk will not mix with the egg white very freely. On the other hand, if you break open and egg and the yolk is all mixed up with the white, causing the whole thing to be a murky mess, that’s a bad egg. This is called ‘mixed rot’ or an ‘addled egg.’ This may be caused by the yolk migrating and having become stuck to the shell, then dislodged upon handling, causing the membrane to burst. At first the yolk just seeps a little and over time it becomes mixed with the white. In order for the yolk to have become stuck to the shell to the degree that it cannot be dislodged without breaking the membrane, it had to have been stored for a long period of time in a fixed position. That’s a bad egg.