Tuesday, October 10, 2006

All about eggs

I’ve been rediscovering eggs lately. They are such a perfect source of quick, inexpensive protein. I’ve become a bit of an organic / farm fresh zealot. The fresh eggs available at the farmer’s market taste so much better than the store bought variety that it’s worth waiting to stock up. This all got me thinking about eggs – what makes eggs different colors? How can you tell a fresh egg from an old-ish egg? What is that slimy membrane-y thing you see in eggs? What’s that red dot you sometimes find? I thought I’d compile all my learnings in a post. I’ve used four main sources for this: Wikipedia, The American Egg Board, Martha Stewart (from which most of the anatomy and the preparation / cooking information is drawn) and Delicious Organics (an online retailer of organic produce). In most cases I’ve taken the information from these sources verbatim, meshing together some facts from different sites and adding a few of my own. I found all of the information across sources to be consistent. If you have any additional thoughts you’d like to add or any corrections to point out, please do post a comment!


  • Egg cartons from USDA-inspected plants must display a Julian date--the date the eggs were packed (this date starts with January 1 as number 1 and ends with December 31 as 365). Although not required, they may also carry an expiration date beyond which the eggs should not be sold. In USDA-inspected plants, this date cannot exceed 30 days after the pack date. It may be less through choice of the packer or quantity purchaser such as your local supermarket chain. Plants not under USDA inspection are governed by laws of their states.
  • As an egg ages, the porous shell will allow more and more air to enter into the egg. A fresh egg will sink to the bottom of a glass of water: an older egg will become increasingly buoyant.
  • The freshest eggs you can find are the best for frying and poaching, as they are the firmest and will hold their shape well. Eggs that are ten days old and older are actually better for certain purposes, such as whipping and beating for recipes, and for hard-boiling, since a slightly older egg is easier to peel once it is cooked.

(photo above from The University of Nebraska Cooperative Extension)


  • Yolk: The yolk makes up approximately one third of the weight of an egg and contains all of the fat and just under half the protein. It is extremely vitamin-rich, containing every vitamin except C; it’s also one of the few natural food sources of vitamin D. The yolk also contains more phosphorus, manganese, iron, iodine, copper, and calcium than the white, and it contains all of the zinc. The yolk of a Large egg contains about 59 calories. The yolk acts as an emulsifier in recipes, allowing combinations of ingredients—such as oil and water—that normally don’t mix. A fresh yolk will be firm and will stand tall when the egg is broken. The color of the yolk is largely determined by what the hen is fed.
  • Albumen (egg white): The albumen makes up the remaining two-thirds of an egg’s liquid weight, and is itself approximately 87 percent water. It contains more than half of the protein of the egg, is fat-free, and is rich is niacin, riboflavin, chlorine, magnesium, potassium, sodium, and sulfur. The albumen directly surrounding the yolk is called the “thick white,” and is firmer and thicker than the outer layer. This is especially true in a very fresh egg; as an egg ages, the entire white thins and becomes more watery. A cloudy egg white often seen in very fresh eggs is caused by the harmless presence of carbon dioxide, which escapes as the egg ages.
  • Chalaza: These small, white, stringy pieces act as an anchor between the yolk and the thick white, and are most apparent in fresh eggs. They are harmless and needn’t be removed (they are neither imperfections nor beginning embryos, contrary to popular belief), though they can be strained away for aesthetic purposes if desired. The more prominent the chalazae, the fresher the egg. Chalazae do not interfere with the cooking or beating of the white and need not be removed, although some cooks like to strain them from stirred custard.
  • Air cell: A pocket of air that forms between the wide end of the eggshell and the albumen and increases in size as an egg ages. A very fresh egg will have little or no air cell.

Grading of Eggs

  • The United States Department of Agriculture (USDA) has established a grading system for eggs sold in the shell. It is not mandatory that eggs be graded by this system to be sold, but only those that meet the standards will carry the USDA grade shield on the package. In order of quality, USDA grades are AA, A, and B. Grades are not determined by size or nutritional value, but rather by age, shape and appearance of the shell, yolk, and albumen, and size and location of the air cell.
  • An egg graded AA will have a firm yolk and a thick albumen, a small air cell, and will generally not be more than 10 days old. Grade A eggs can be ten days to several weeks old, and will have a fairly firm, upstanding yolk and a good proportion of thick white to thin. Both AA and A must have oval-shaped shells with one larger end; a misshapen egg will automatically be grade B. A grade B egg is more watery than the higher grades, and will have a larger air cell. B eggs are rarely sold in grocery stores and generally go straight to manufacturers of egg products.

Blood Spots

  • Blood spots are occasionally found on an egg yolk. Contrary to popular opinion, these tiny spots do not indicate a fertilized egg. Rather, they are caused by the rupture of a blood vessel on the yolk surface during formation of the egg or by a similar accident in the wall of the oviduct. Less than 1% of all eggs produced have blood spots.
  • Mass candling methods reveal most eggs with blood spots and those eggs are removed but, even with electronic spotters, it is impossible to catch all of them. As an egg ages, the yolk takes up water from the albumen to dilute the blood spot so, in actuality, a blood spot indicates that the egg is fresh. Both chemically and nutritionally, these eggs are fit to eat. The spot can be removed with the tip of a knife, if you wish.
  • Eggs with blood spots are not kosher. This is because blood spots can also occur due to fertilization. Of course, most hens nowadays are not allowed anywhere near a rooster so fertilization is an impossibility in today's eggs, but religion dictates that we toss the egg with the blood spot. The laws of Kashrut do not dictate white eggs over brown or other color eggs, just that it not be a fertilized egg. Some people believe that there are more blood spots in brown eggs than white eggs because they mistakenly think that a naturally occurring brown coloration in the brown egg is a blood spot and it is not. The laws of Kashrut are clear that it is the red blood spot that is not allowed for fear of fertilization and that brown spots can be ignored.


  • If a boiled egg is overcooked, a greenish ring sometimes appears around egg yolk. This is a manifestation of the iron and sulfur compounds in the egg. It can also occur when there is an abundance of iron in the cooking water. The green ring does not affect the egg's taste; overcooking, however, harms the quality of the protein.
  • Egg shell color is caused by pigment deposition during egg formation in the oviduct and can vary according to breed, from the more common white or brown to pink or speckled blue-green. Aracona chickens, for example, lay a lovely green shaded egg. Breeds with white feathers and ear lobes lay white eggs; breeds with red feathers and ear lobes lay brown eggs. White eggs are most in demand among American buyers. In some parts of the country, however, particularly in New England, brown shells are preferred. The Rhode Island Red, New Hampshire and Plymouth Rock are breeds that lay brown eggs. Since brown-egg layers are slightly larger birds and require more food, brown eggs are usually more expensive than white. Egg color is simply an aesthetic preference.
  • Yolk color is dependent on the diet of the hen; if the diet contains yellow/orange plant pigments known as xanthophylls, then they are deposited in the yolk, coloring it. A colorless diet can produce an almost colorless yolk. Farmers may enhance yolk color with artificial pigments, but in most locations, this activity is forbidden
  • Size is actually determined by the age of a hen. The older the hen, the larger the egg.
  • Double yolks come from the same hens making the XL eggs. A large egg should weigh no less than two ounces, whereas a jumbo will weigh about two and three-quarter ounces, and a small about one and one-half ounces.

Preparing and cooking

  • Separating eggs: Even a drop of fat will decrease volume, so be careful not to let any yolk fall into the bowl. Eggs are easiest to separate when they are cold, but bring the separated egg whites to room temperature before beating them.
  • Whipping egg whites: Use a stainless-steel and copper bowls to yield the best results. Avoid plastic, which can retain traces of fat even when washed.
  • Boiling eggs: Despite the name, boiled eggs should not be boiled throughout the cooking process—a method that yields a rubbery result—but instead should be brought to a boil and then removed from the heat.

Instructions for the Perfect boiled egg:

  • Place eggs in a single layer in a saucepan, and cover with 1 inch of cold water. Bring to a boil, cover, and immediately remove from heat. Let stand approximately 1 1/2 to 2 minutes for soft-boiled, 2 to 2 1/2 minutes for medium-boiled, and 12 to 13 minutes for hard-boiled. Remove eggs from water.
  • Soft- and medium-boiled eggs should be served immediately in egg cups—perfect for cracking and scooping the egg right from the shell.
  • Hard-boiled eggs should be removed from the pot and plunged into a bowl of ice water. This prevents the yolk from discoloring due to overcooking and facilitates peeling. Let stand for 2 minutes, then crack by gently pressing the egg against a hard surface. Peel under cold running water. Serve.


  • For best results, store eggs in the container they are sold in, on a refrigerator shelf rather than in an egg compartment on the door, which is subject to frequent temperature changes as the door opens and closes.
  • Inappropriate temperature and humidity will age an egg. (A week-old egg can actually be fresher than a day-old egg.) In the U.S. eggs are refrigerated but in other parts of the world they are not. They will keep fresh out of the refrigerator if never placed in cold. Once they are refrigerated, they must always be refrigerated to maintain freshness.
  • Refrigerate unbeaten whites, tightly covered for up to four days, or freeze (in ice-cube trays, then transfer to a freezer bag), up to six months (thaw overnight in the refrigerator).


  • Recently, chicken eggs that are especially high in Omega 3 fatty acids have come on the market. These eggs are made by feeding laying hens a diet containing polyunsaturated fats and kelp meal.
  • If a raw egg is spun, abruptly stopped and then quickly released, it will start to spin again as the liquid inside continues to rotate. This technique can be used to reliably determine whether an egg is raw or already boiled — a solid egg will remain stationary once halted. Additionally, if a cooked egg and a raw egg are spun with the same force, the cooked egg will spin much faster. Another way of determining if an egg is raw or already boiled is to spin it quickly. An already boiled egg will spin into an upright position after a few seconds, but the raw egg will continue to spin on its wide side.


    Anonymous said...

    We eat a lot of eggs and your information was excellent. Thank you for all the facts...and by the way, we love the puppy! WendyPfromCharlotte,NC

    Anonymous said...

    A good egg is hard to find ... and I found mine.

    Anonymous said...
    This comment has been removed by a blog administrator.
    Anonymous said...

    Seems it's time to reveal the "Eggs Shaken" recipe: crack two eggs into a small bowl. Heat butter and (if desired) tapatillo or other hot sauce in small frying pan at medium heat. Add eggs. Pepper liberally, salt if desired. Sprinkle with wheat germ. Cook to desired firmness and serve.

    Anonymous said...

    WOW! What great info. I learned a lot. Thanks for all your hard work.

    Anonymous said...

    Dear "S"
    The office has decided to take "R"s lead and commit to a healthy, unrefined-carb-only diet for two weeks. We are creating our own version of the BIGGEST LOSER. I think eggs for breakfast is the answer rather than my usual toast (Guy Wilson will be disappointed). Your research is quite handy!

    Anonymous said...

    Someone please tell me if you can taste the wheat germ in the egg recipe offered in the "eggs shaken" recipe by a commenter here. I did not know anything as simple as an egg would disguise the taste of wheat germ. I would like to know 1st hand before I try it.

    Sabra said...

    Re: eggs shakin' -- it's a recipe we both have shared for a long time, and no - you would never know that it's wheat germ. The one thing "A" forgot to add to the recipe is to sprinkle with freshly grated parmesan cheese (to be true to the original recipe) -- it's delicious. I might post a photo at some point. I'm usually really be generous with the pepper (I usually use coarser vs. finer pepper) and pretty generous with the wheat germ too. Try to make it in a frying pan that's about the same size as the two eggs and make individual portions (ie cook two pans at the same time -- I use little 8 inch omelette pans -- cast iron would be even better). At the end, just before you take it off the stove, turn the temperature up so the eggs crackle at the table. You can gently slide them off onto a plate or serve in the pan at the table.

    Anonymous said...

    Thanks for deciphering the information on the egg carton relating to freshness. Regarding fresh eggs you purchase from a farm that is not USDA inspected: how long does such an egg remain fresh assuming it has been consistently refrigerated?

    Sabra said...

    Your eggs should stay fresh 4-5 weeks after their pack date. It always amazes me how long the keep!

    Anonymous said...

    I am flabbergasted to find an acceptable way to pile up wheat germ on something in which I won't even taste it. I'll try it, but I must say, I remain skeptical. I am putting my trust in you for this one.

    tigerfish said...

    That's good egg info. I once kept some eggs in the fridge for several month.Hope it's ok consuming them after so long. I know about expiry dates and I did not wanted to throw them away.So wasteful.

    Anonymous said...

    Thanks for the great info. on egg whites . my husband actually threw out the perfect egg because the white was cloudy as compared to the other one. I will be sure to inform him to not throw out the next one. This is the best site ever. Thanks Meg

    Anonymous said...

    What does it mean if the yolk inside a scotch egg is red?

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