The following is an excerpt from Storey’s Guide to Raising Chickens by Gail Damerow (Storey Publishing, 2010). This new edition of a best-selling classic will tell you everything you need to know about raising your own backyard flock.
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 (see the “Anatomy of an Egg” diagram in the Image Gallery). Between these two membranes is the air space that develops at the large end as the egg ages.
The spongy, or calcareous , layer is made up of tiny calcite crystals consisting of 94 percent calcium carbonate with small amounts of other minerals. Viewed through a microscope, these crystals look like thousands of thin pencils standing on end. The spaces between them form pores connecting the surfaces of the inner shell and outer shell so moisture and carbon dioxide can get out of the egg and air can get in to create the air space.
The bloom, or cuticle , is a light coating that seals the pores to preserve the egg’s freshness by reducing evaporation and preventing bacteria from entering through the shell. Sometimes you’ll find a freshly laid egg before the bloom has dried. Most of the pigment that gives the shell its color is in this layer.
When you wash an egg, the bloom dissolves, making the egg feel temporarily slippery. To replace natural bloom, commercial producers spray shells with a thin film of mineral oil, which is why store-bought eggs sometimes look shiny. If you wash an egg, rubbing the dried egg with clean vegetable oil somewhat replaces the bloom.
An eggshell’s strength is naturally influenced by the vitamins and minerals in a hen’s diet, especially vitamin D, calcium, phosphorus and manganese. Shell strength is also influenced by a hen’s age — older hens lay larger eggs with thinner, weaker shells.
A shell gets strength from its shape as well as from its composition. The curved surface is designed to distribute pressure evenly, provided the pressure is applied at the ends of the egg, not at the middle. The middle of a shell must be weak enough to allow an emerging chick to peck all around and break out of an incubated egg. Some chefs take advantage of this characteristic to make a big show of breaking an egg with one hand — what you don’t see is the thumb they press against the middle of the shell. By contrast the ends of an egg must be quite strong so a newly laid egg won’t crack when it plops into a nest, blunt end down.
One way to test the strength of an egg is to press the ends between the palms of your hands. For a more precise measurement, use an ordinary bathroom scale. Stack some boards or books on the floor to equal the height of the egg when it’s standing on end. With paper towels, fashion a ring around the bottom of the egg to stand it on end, next to the books. Rest one edge of the scale on top of the books and the other edge on the top of the egg. Press on the part of the scale that’s just above the egg. A well-formed shell should support up to 9 pounds (4 kg) before it breaks.
Except for preserving the freshness of eggs, shells have no culinary use (although I was once served a blended health-food drink containing a raw egg, shell and all, and I must admit it tasted pretty good). Shells have plenty of other uses. They may be:
1. Dried, crushed and fed back to hens as a calcium supplement
2. Added to compost to sweeten the soil
3. Placed in tomato planting holes to prevent blossom-end rot
4. Decorated for a variety of arts and crafts
Interior Egg Quality
Interior quality refers to the appearance and consistency of an egg’s contents, which may be determined easily by breaking the egg into a dish for examination. In doing so, you will discover the egg has more than one kind of egg white, or albumen . Two clearly visible kinds are the firm white around the yolk and the thinner white closer to the shell. A less obvious second, or inner, thin layer lies between the outer thick white and the yolk.
The outer thin egg white repels bacteria by virtue of its alkalinity and its lack of the nutrients needed by bacteria for growth. The firm or thick egg white surrounding the yolk cushions the yolk, and its composition includes defenses against bacteria. The older an egg gets, the more thin white and the less thick white it has.
Another kind of white is the chalaziferous , sometimes called the inner thick (in contrast to the other thick egg white, which is called the outer thick), layer made up of dense albumen surrounding the yolk. During its formation, as the egg travels through the oviduct and rotates, the ends of this layer become twisted together to form a cord of sorts, or chalaza (pronounced kah-lay’-za), on each side of the yolk. These two cords anchor the chalaziferous layer and protect the yolk by centering it within the white.
When you break an egg into a dish, the chalazae snap away from the shell membrane and recoil against the yolk. Misinformed cooks sometimes mistake the resulting two white blobs at opposite sides of the yolk for the beginnings of a developing chick.
A chick develops instead from a round, whitish spot on top of the yolk called the germinal disc or blastodisc . When an egg is infertile, the blastodisc has an irregular shape. If the egg has been fertilized, the blastodisc becomes the blastoderm and organizes into a set of tiny rings, one inside the other.
Egg yolks get their color from xanthophyll , a natural yellow-orange pigment in green plants and yellow corn and the same pigment that colors the skin and shanks of yellow-skin hens. The exact color of a yolk depends on the source of the xanthophyll. Alfalfa, for example, produces a yellowish yolk, while corn gives yolks a reddish-orange color.
Excessive amounts of certain pigmented feeds can affect yolk color. Alfalfa meal, clover, kale, rape, rye pasture, and certain weeds including mustard, pennycress and shepherd’s purse make yolks darker. Too much cottonseed meal can really throw off yolk color, causing it to be salmon, dark green or nearly black.
The yolk is not of uniform color throughout. Look closely, and you will see that it consists of concentric rings. At the center is a ball of white yolk, around which are alternating layers of thick dark yolk and thinner white yolk. Although you might never see it — except maybe in a hard-cooked egg — a neck of white yolk extends from the center to the edge of the yolk, flaring out and ending just beneath the blastodisc.
As an egg ages, both its white and yolk deteriorate. Their quality may not have been all that great to start with, depending on the hen’s age and health, the use of medications, the weather and hereditary factors. The better an egg’s starting quality, the better it keeps.