Everything Worth Knowing About Chicken Eggs

Everything Worth Knowing About Chicken Eggs

Reading Time: 11 minutes

Important chicken egg facts, including egg sizes, odd eggs, and egg quality.

“It was a brave man who first ate an oyster,” Jonathon Swift said, but the first person to eat an egg must have been even braver. Or very, very hungry. Imagine breaking open an egg, not knowing what you’ll find inside! Animals of all kinds, including early humans, have eaten eggs in this manner for eons: chicken eggs, pigeon eggs and peahen eggs, pelican and ostrich eggs, turtle and alligator eggs. Like the wild animals that even today relish an egg at any stage of development (raccoons and bears enjoy them already hatched), our ancient ancestors probably didn’t care: it was food.

Today, Quality is Important

Today we have higher standards. We prefer chicken eggs or duck eggs from known sources, and the quality of the eggs we eat is very important from several standpoints. The backyard poultry keeper has more reasons than most to pay close attention to quality: nobody else is going to do it for you. (And finding a nest a free-range chicken has hidden puts you in the position of that primitive egg-eater.)

However, “quality” can have different meanings for different people.

Some factors of quality, such as egg freshness and safety, are important, while others, such as chicken eggshell color, are aesthetic, and a matter of personal or cultural preference or customer appeal. Increasingly, many people are including such considerations as the humane treatment of hens and organic chicken feed in their definition of quality.

The USDA Grading System

The basic elements of chicken egg quality are covered in the USDA grading system. Most people are somewhat familiar with this, even though—unless they were in a 4-H poultry project —they seldom know what the grades actually mean.

Chicken eggs are graded for both interior and exterior quality. The lowest grade for either determines the grade of the chicken egg. Exterior evaluation considers cleanliness, shape, texture, and soundness. AA and A are the only grades sold in stores. Consumers never see grade B eggs, which go to bakers and other food processors. “Dirty” chicken eggs cannot be sold for human consumption.

How Chicken Eggs are Graded

Chicken eggs are clean when they’re laid except perhaps for an occasional streak of blood incurred in the process, but a warm moist egg can easily become contaminated by manure or other foreign material. The problem is largely avoided with rollout nests, which do not use straw or other nesting material. Nesting boxes and bedding material should be kept clean.

If not too badly soiled, chicken eggs can be washed. Here’s how to wash a fresh egg, but keep in mind that washing removes the “bloom,” the moist outer membrane that coats and protects a freshly laid chicken egg. Clean, unwashed chicken eggs are preferable and will stay fresher longer. Sometimes slightly soiled eggs can be dry cleaned with an abrasive such as fine sandpaper, but if washing is necessary, use clean water with a temperature of 110 – 115°F with an approved detergent sanitizer (not dish soap).

Stains can remain even after washing, downgrading the egg to B or “dirty” depending on the extent of the stain.

Elliptical eggs are so common we often talk about “egg-shaped” objects, but eggs that are round, very long, or otherwise different from the norm aren’t all that unusual. They result from perfectly natural causes and there is nothing wrong with them from the standpoint of the cook or the eater. Nevertheless, they are automatically downgraded to B because of their appearance—and because they don’t fit well into chicken egg cartons and are much more likely to be broken.

Similarly, eggs with extremely rough or uneven shells are B grade. In addition to the aesthetics, rough-shelled eggs fracture more easily than those with smooth shells. “Pimpled” shells, the result of calcium deposits, fall into this category, as do mottled shells, which involve pale translucent spots that develop half an hour or more after laying. Both of these involve heredity, although other factors can be involved.

“Body checks” have been cracked inside the hen during shell calcification, then covered with another layer of calcium, sometimes making them hard to discern without candling. They can also appear as ridges or bulges on the shell. Body checks are said to increase if the hens become agitated just as the chicken egg shell is starting to form in the oviduct. Depending on the extent and severity, these may be downgraded to B.

Interior Quality

One of the most important measures of chicken egg quality is the condition of the albumen. When we break an chicken egg into a frying pan we want to see a round yolk in the center, surrounded by thick albumen. If the yolk is flattened, probably well off-center, with a large area of albumen producing a wide pool of liquid, we automatically know that chicken egg isn’t fresh.

Some sources claim that grade AA chicken eggs, properly stored in a carton in a home refrigerator, will deteriorate to grade A in about a week, and to grade B in another five weeks. An chicken egg at room temperature will lose quality much more quickly.

It should come as no surprise that disease can cause loss of chicken egg quality. The most economically important is probably infectious bronchitis, which affects shell as well as interior quality, with watery whites being common.

In addition to the age of the chicken egg and disease, albumen quality is affected by the age of the hen: older hens lay lower quality eggs. Nutrition is not considered a major factor, nor is environment, including heat stress, although high amounts of vanadium have been shown to cause watery egg whites. (Chickens require tiny amounts of vanadium for growth and reproduction.)

Yolk quality depends on appearance, texture, firmness and smell.

The yolk of a fresh chicken egg is round and firm. As it ages, it absorbs water from the albumen, and enlarges. This weakens the vitelline membrane, causing a flattened top and a generally out-of-round shape. The weakened yolks are prone to rupture in the frying pan.

Rubbery yolks can be traced to freezing or severe chilling of fresh chicken eggs, but also to crude cottonseed oil or velvetleaf seed in the feed. The yolks of eggs of free-range hens consuming velvetleaf seeds appear normal at first, but become rubbery, viscous and pasty after even a brief period of cold storage. The culprit has been traced to cyclopropenoid compounds, which tend to increase the saturated fat in eggs, tissue and milk. (Velvet weed, or velvetleaf, Abutilon theophrasti, introduced from Asia in the mid 1700s as a potential fiber crop, has become a major and widespread invasive weed, particularly in corn and soybean fields. Be aware that the seeds are often found in corn screenings.)

Double yolk eggs occur when two yolks move through the oviduct together, either from simultaneous ovulations or a delay in the yolk’s passage through the oviduct. Such chicken eggs are usually large and don’t reach the market, but there’s nothing wrong with them.

Yolk color is another example of aesthetic quality that has nothing to do with health, nutrition or safety: it depends on the hen’s diet. Yellow-orange plant pigments known as xanthophyllis affect yolk color: hens eating yellow corn and alfalfa meal lay eggs with darker yolks than those eating white corn, milo, wheat or barley. Yolk color can be enhanced by adding marigold petals to feed. Interestingly, yolk color is like shell color in that different cultures have different preferences.

One yolk “problem” we often hear about is a greenish ring around hard-cooked yolks. This happens when “boiled” eggs are overcooked (they should simmer, not boil), or when there is a lot of iron in the water, resulting in sulfur and iron compounds in the egg reacting at the yolk’s surface. This is strictly cosmetic: nutrition and flavor aren’t affected.

In the Dr. Suess story, Sam-I-Am learned to like green eggs and ham, probably scrambled. This is more common when cooking large batches at too high a temperature, or if the eggs are held too long after cooking. Avoid holding scrambled eggs over direct heat before serving: if they won’t be served immediately, keep them warm with a container of hot water between the eggs and the heat source.

Off-odors are relatively uncommon, and can usually be traced to chemicals used for treating parasites, household detergents used for washing eggs, moldy egg cases or something the hen ate. Don’t store eggs in a cooler with fruits, flowers, or vegetables, especially onions. A United Nations bulletin for less-developed countries warns not to store eggs near diesel fuel.

A Look Inside

External appearance doesn’t tell us much about the qualities of the edible interior. For that, we turn to candling, which every poultry raiser is, or should be, familiar with: most basic poultry books cover it.

The depth of the air cell, determined by candling, is considered in grading an egg. The depth is the distance from the cell’s top to bottom when the egg is held with the air cell up (normally the large end of the egg). A fresh egg will have a cell less than 1/8-inch deep. With time, water in the egg evaporates through pores, and is replaced by air. This is why fresh eggs sink and old eggs float in water. As the air cell grows larger the egg is downgraded.

The most common goal of candlers is detecting blood in chicken eggs, or meat spots. Small spots are easily removed after the egg is cracked open. Such eggs aren’t marketed — the reason for candling—but there’s no reason a poultry owner couldn’t use them.

Occasionally the chalazae is mistaken for a meat spot, and some people have been known to mistake it for an embryo. It is, of course, not only natural, but necessary for centering the yolk in the shell. It consists simply of twisted strands of protein, or egg white.

Technically speaking, eggs are sold by weight, which has nothing to do with quality or grade. Eggs of any size can be grade AA, A or B. While most consumers prefer large and extra large eggs (24 and 27 ounces per dozen), small and medium eggs (18 and 21 ounces per dozen) provide the same food value, and in many cases might be the better buy. According to a 1966 Consumers Report, small and mediums are likely to be more plentiful, and cheaper per ounce, than larger eggs in late summer and fall. The USDA calculated that if extra large eggs were 54¢ a dozen, large eggs were a better buy at less than 47¢ a dozen and mediums were a better buy at less than 41¢.

Why are eggs sold by the dozen? Nobody seems to know. At one time there was a move to sell them in lots of 10, to match the decimal system. But today cartons of six and 18 are common, so it probably doesn’t matter.


Several recent recalls, one alone involving more than half a billion eggs, focused attention on the bacterium salmonella. Even though cooking destroys salmonella, more than 2,000 people were sickened. Raw eggs, as used in hollandaise sauce and other recipes, are implicated, but the majority of cases simply involve undercooking. Salmonella enteritidis can infect the ovaries of healthy-looking hens and contaminate the eggs before the shells are formed. It’s less likely to grow in eggs stored at 45°F or less.

The Center for Disease Control says an average of 130,000 people get sick from salmonella in eggs every year — but eggs are responsible for less than one percent of all food-borne illnesses.

Food process engineers at Purdue University say that on average, one of every 20,000 chicken eggs contains a small amount of salmonella (of which there are 2,300 different kinds). The birds pick up the bacteria from their environment, which is contaminated by rodents, wild birds and flies. The bacteria thrive inside the chicken, with a temperature of about 102°, but the hen shows no signs of illness, making it impossible to know which birds are infected.

The few eggs that become infected usually contain a very low level of bacteria, 2-5 milligrams; it takes at least 100 to make a person sick. But the bacteria multiply rapidly if the eggs aren’t properly cooled, doubling every 20 minutes under ideal conditions. Two could become 32 within an hour.

In addition, a lapse in cleaning or an undetected outbreak among the chickens can greatly increase the percentage of tainted eggs. Every egg has about 9,000 pores through which bacteria can pass, originating from a conveyor belt, or a vat of egg-cleaning fluid that’s not kept at the proper temperature and pH.

Maintaining Perspective

In spite of the concerns, whether safety or aesthetic, it helps to maintain a rational perspective. Three helpful examples: methods of long-term egg storage in early America; balut; and the well-known Chinese thousand-year-old egg.

In American Frugal Housewife, (1833), author “Mrs. Child, Boston,” advises buying eggs by the case, in spring and fall, rather than by the dozen as you need them:

“Eggs will keep almost any length of time in lime-water properly prepared. One pint of coarse salt, and one pint of unslacked lime, to a pail full of water. If there be too much lime, it will eat the shells from the eggs; and if there be a single egg cracked, it will spoil the whole. They should be covered with lime-water, and kept in a cold place. The yolk becomes slightly red; but I have seen eggs, thus kept, perfectly sweet and fresh at the end of three years. The cheapest time to lay down eggs, is early in spring, and the middle and last of September. It is bad economy to buy eggs by the dozen, as you want them.”

This was not unusual advice at that time and other ways of preserving eggs without refrigeration were used well into the 20th century.

Balut is a fertilized duck egg embryo, boiled and eaten in the shell, often considered an aphrodisiac as well as a tasty treat sold by street vendors in the Philippines and Southeast Asia. They are often served with beer. The broth around the embryo is sipped from the egg, then the shell is peeled off, and the yolk and the embryonic chick are eaten. Various condiments are used in diverse places: chili, garlic, coconut vinegar, lemon juice, Vietnamese mint leaves—it varies.

Sound yucky? In some places, this is now considered haute cuisine.

And in stark contrast to fresh, day-old eggs, what can we say about hundred- or thousand-year-old eggs? They aren’t really that old, of course: maybe a couple of weeks, or months, but still…

These traditional Chinese eggs (duck, chicken or quail) are preserved in a mixture of clay, salt, lime, and rice hulls. The yolk becomes a dark green to grey color, with a creamy consistency (and an odor of sulfur and ammonia) while the white becomes a dark brown, translucent jelly with little flavor. The chemists’ explanation is that the alkaline material raises the pH of the egg, which breaks down some of the complex, flavorless proteins and fats, producing a variety of small flavorful compounds. Sounds like limburger cheese.

We have progressed a great deal since our ancient ancestors robbed bird’s nests to enjoy the gustatory pleasures and nutritional benefits of eggs. Our cooking arts have been honed and our science has been sharpened, enabling us to get the most satisfaction and value from eggs. We’re concerned about food safety, but it’s interesting to know that there’s no accounting for taste, either for individuals or entire cultures, and quality can be subjective. As a producer, you set your own standards.

Protein Quality of an Egg

The protein quality of an egg can be scientifically measured, based on the height of the albumen (egg white). The Haugh unit, introduced by Raymond Haugh in 1937, is widely used as an industry measure of egg quality.An egg is weighed, broken onto a flat surface, and the height of the albumen immediately surrounding the yolk is measured with a micrometer. The height, correlated with the weight, produces the Haugh unit (HU). The higher the number, the better the quality of the egg; fresher eggs have thicker whites.

If you want to try this at home the formula is: HU = 100 * log (h-1.7w0.37 +7.6) Where HU = Haugh unit; h = observed height of the albumen in millimeters; w = weight of the egg in grams

Egg “Quality” Has Various Meanings

In the egg-breaking industry — businesses that produce over 1,660 million pounds of liquid egg products each year for food service industries — the strength of the vitelline (yolk) membrane is an important quality issue because even a tiny amount of yolk contaminating the white (albumen) can reduce the foaming properties of the albumen, a requirement in baking and in preparing other confections. High-speed egg-breaking equipment increases the possibility of ruptured yolks.

So, to a food scientist, vitelline membrane strength (VMS) is an important factor in egg quality. To the backyard poultry-keeper, not so much. Of far greater interest are such questions as freshness, appearance, safety, cleanliness, and perhaps size. Is the shell smooth and free of blemishes? When the egg is cracked into the frying pan, does the white run all over? Does the yolk stand up? What color is it?

Egg Grading System

The USDA egg grading system greatly simplifies the one described in The Grocer’s Encyclopedia, an interesting book about food production and preparation written by Artemus Ward in 1911: “to the man who handles [eggs] between the farm and the breakfast table there are Fancy Fresh, Fresh Gathered, Storage Packed, Storage, Limed, Known Marks, Extras, Firsts, Seconds, Dirties, Checks, etc.” The four USDA grades first surfaced in 1943.

Jd Belanger founded the original Backyard Poultry in 1979 and is the author of many homesteading books.

Originally published in Backyard Poultry October / November 2012 and regularly vetted for accuracy.

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