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Anatomy of a Fresh Egg


We all know that an egg is comprised of a shell, and then inside there's the yolk and the white. But an egg is actually oh-so-much-more than that! The anatomy of a fresh egg is pretty fascinating.

Do you know that an egg has every nutrient needed to for life except  Vitamin C?

Do you know that the white takes up about two-thirds of the volume of the contents of the eggshell and the yolk takes up the remaining one-third?

And the yolk contains about half of the protein in an egg, the majority of nutrients, but also most of the fat.

Do you know what the thin twisted strands are that you sometimes find in an egg? 

Or why the "white" is  sometimes clear and sometimes cloudy? 

Or how about the blood spot you sometimes find on the yolk?

Anatomy of a Fresh Egg

Albumen (or Egg White)

The albumen or egg white of a freshly laid egg will be cloudy, not completely clear. This is because it contains carbon dioxide that slowly seeps out as the egg ages, turning the white clear. 

The fresher the egg, the thicker and cloudier the albumen. 

The albumen actually consists of four parts: the inner thick albumen, the inner thin albumen, and the outer thick albumen and the outer thin albumen. These layers are designed to protect and cushion a chick embryo as it develops.

These layers get more watery as the egg ages and the proteins change, so an older egg will spread out in the pan more than a fresh egg.

An egg white does contain protein, as well as niacin, riboflavin, magnesium, potassium and sodium, but not nearly as much nutrition as the yolk. But on the flip side, an egg white only contains about 17 calories and no fat or cholesterol.

Blood Spot

Occasionally you'll find an egg with a tiny red speck on the yolk called a blood spot. This is not an indication of a fertile egg, it's merely a blood vessel on the surface of the yolk that broke during the formation of the egg. 

Although it's perfectly edible, most people prefer to remove a blood spot with the tine of a fork or tip of a knife before preparing the egg to eat.

Bloom (or Cuticle)

The bloom or cuticle is a nearly invisible natural coating applied to the eggshell that helps to seal the pores in the shell and keep out air and bacteria. 

Washing an egg removes that bloom, allowing the egg to age more quickly and lose its freshness, as well as increase the chance of bacteria entering into the egg. 

As the egg ages, the bloom dries and starts to flake off, allowing limited air to get inside to form the air sac that provides air to a developing embryo to breathe.

Chalazae

When you crack a very fresh egg, you might see thin, white ropes trailing from the yolk. These are twisted strands of protein that anchor the yolk in place in the center of the egg white. 

They are normally only visible in very fresh eggs. 

Like blood spots, they are perfectly edible, but you might want to remove them before eating the egg.

Germinal Disc

A small, almost unnoticeable white dot on the yolk of an egg is called the germinal disc. It's the entrance to a tiny path that leads to the center of the yolk. 

And it's this path that the sperm in a fertilized egg will travel to create the embryo and begin to form a baby chick.

Related to the germinal disc is the "bulls eye".  

Once the egg is fertilized, several concentric rings form around the dot of the germinal disc, creating a bulls eye, which is the one visible indication of a fertile egg.

Membrane

There are actually two membranes inside an egg that separate the shell from the egg white - an inner membrane and an outer membrane. 

Once the egg is laid and begins to cool to room temperature, the membranes begin to separate and air starts to seep in between them, as well as create an air sac at the blunt end of the egg.

Fresh eggs don't hard boil well because until a couple of weeks go by, there's not enough air in between the membranes to allow them to easily separate and for you to peel the shell off.

Pigment

All eggshells start out white. Then, depending on the breed of chicken, pigment is applied to the outside of the shell. That pigment is either brown or blue. 

The blue pigment is applied early in the laying process and that color seeps through to the inside of the shell. The brown pigment is applied much later, so the inside of brown eggshells is still white.

Some breeds have both brown and blue pigment, and their eggs turn out green, with blue insides!

Shell

The eggshell covers the contents of the egg and is the first line of defense against air and bacteria.  It makes up about 10% of the total weight of the egg. 

There are between 7,000-17,000 tiny pores in an eggshell. These pores regulate the movement of air and bacteria into the egg - and carbon dioxide and moisture out of the egg. 

Laying hens require adequate levels of calcium, in the form of crushed eggshells or oyster shell, to create strong shells for the eggs they lay. Without enough calcium, they will leach it from their bones, or lay eggs with thin, breakable or soft shells.

Vitelline Membrane

The vitelline membrane is a thin covering that helps keep the yolk contained. It's strongest when the egg is fresh, so fresh egg yolks stand up nice and tall and don't spread out when you crack the egg. 

The yolk of a fresh egg is less likely to break because of the vitelline membrane. As the egg ages, the vitelline membrane thins and becomes weaker, making the yolk more likely to break when you crack it into your skillet.

Yolk

The yolk of the egg contains all of the fat and cholesterol, and most of the calories in an egg, but also the majority of the vitamins and nutrients, including vitamins A, B, D, E and K, as well as calcium, folic acid, iron, manganese, phosphorus, protein, selenium, thiamine and zinc.

Usually an egg is laid with only one yolk, but occasionally, two yolks are released into the hen's oviduct too close together and they both are encased in the same shell resulting in a double-yolked egg. 

Perfect for scrambling or making an omelet, I would skip using a double-yolked egg for baking because you might throw off your ratios!

Now you know! 

The next time you crack open a fresh egg, you'll be able  to spot all the various parts that make up the egg.

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Further Reading/References:
https://www.exploratorium.edu/cooking/eggs/eggcomposition.html
https://www.fresheggsdaily.blog/2013/07/egg-anatomy-whats-inside-that-eggshell.html
https://www.incredibleegg.org/eggcyclopedia/a/albumen/

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