Appliance Science: The biology of the chicken egg

I've always been a firm believer in knowing what is in the things I eat. I check food labels, watch out for the salt content and avoid saturated fats. But I realized recently that I don't know much about one of the staples of my diet: chicken eggs. What is in an egg, and how does that affect the way we cook them? Let's take a look at the chemistry of eggs.

An egg is rather like a spaceship: it contains everything that a baby chicken needs to go on a strange journey from fertilization to emerging as a chick, to travel from being a tiny bundle of cells to a fully formed chick, ready to start pecking at the world. That's a journey that takes about 21 days, so it's no surprise that eggs are incredibly nutritious.

Chickens are curious creatures. First domesticated thousands of years ago, the biology of these birds has been reshaped by the needs of man over this time. Specifically, the way they produce eggs has been harnessed. With a male chicken around, a hen (a female chicken) will lay 10 to 12 eggs, one a day, then sit on them and wait for them to hatch, a process called brooding. If there are no males around, the hen will still lay the eggs. So, humans took this behavior and adapted it by separating the males and females, and removing the eggs on a daily basis. If you do this, the hen keeps laying an egg a day, often for months at a time. This unfertilized egg is, in most ways, identical to a fertilized one, except that it won't grow: there will be no chick growing inside.

A voyage into the egg

On the outside of the egg is the shell. Made mostly of a mineral called calcite (which is composed of calcium carbonate), this protects the inside of the egg, keeping out bacteria and other icky things. It lets some things through, though, letting oxygen in and carbon dioxide out so the growing chick can breathe. In fresh eggs, the shell is coated by a waxy substance called the cuticle that helps to seal it, but this is washed from the eggs that you buy in the store. Although the shell is less than half a millimeter (under 0.001 inch) thick, it is surprisingly tough: it can withstand drops from several inches without cracking. It does have one weakness, though: calcite reacts with acids. This means that even a weak acid like vinegar can dissolve an egg shell, producing the rather creepy so-called naked egg: an egg without a shell that still stays together.

This naked egg is possible because of the next layer of the egg, two tough membranes that surround the contents. One of these sticks to the egg shell, while the other sticks to the albumen inside the egg. These membranes are mainly composed of keratin, a fibrous protein. Chemically, these membranes aren't that different from your own hair and nails. At one end of the egg, these two membranes separate to leave a gap, called the air chamber.

Inside these membranes is the albumen, or the egg white. Mainly composed of water (about 90 percent) and proteins (most of the remaining 10 percent), the albumen supports the yolk and provides water and proteins for the growing chick. The albumen also contains the chalaza, two strands of fibrous proteins that connect the yolk to the inner membrane, helping to keep it in place.

Although the albumen is mostly water, it is one of the most versatile parts of the egg for cooking. With a little prompting, the proteins in it will break apart (chemists call this denaturing) and fit together in new ways. If the prompting is physical (such as being whipped with a whisk), the albumen becomes a foam that can support its own weight. If the prompting is heat, the albumen solidifies and becomes opaque, forming the classic cooked egg white, a high protein dish that is a favorite of body builders.

Chemists used to think this process was irreversible, that there was no way to return the proteins to their raw state, but a team from the University of California in Irvine recently demonstrated a way to reverse the process, to pick apart the disrupted proteins and allow them to return to their raw state. In effect, they managed to uncook the egg white. Although this process isn't really practical for cooks, it could have huge implications for the manufacture of complex drugs. That's because it provides a new way to separate the chemicals produced in living cells, such as genetically engineered bacteria that produce hormones and other drugs.

The yolk is a yellow sphere that contains most of the nutrients in the egg: a heady mixture of fat (about 26 percent, less than a third of which is saturated fat), protein (16 percent), carbohydrates (4 percent) and cholesterol (about 1 percent). Biologists like to argue over the semantics of this, but it is generally accepted that the yolk of an egg is one single, massive cell, thousands of times bigger than typical cells. The yellow yolk is most of the mass of the cell, but the important parts of the cell (such as the nucleus that contains the DNA) are in a small spot on the surface of the yolk called the germinal disc. This is often invisible in the eggs you buy, because they are unfertilized, so the cell has not split and grown. It is sometimes visible as a small spot on the surface of the yolk, though, called a blood spot. If the egg was fertilized, the germinal disc is the spot where the chick would begin, as it splits into multiple cells and grows, fed by the nutrients in the yolk.

The cholesterol in egg yolks gets a bad rap: although it is an essential nutrient that plays an integral part in making cells, it was thought to have a large role in conditions like heart disease, by helping to clog the arteries. More recent research indicates that this concern is overstated, though: this research suggests that having too much of the "bad" LDL cholesterol in the blood may not be related to eating food that contains cholesterol. Instead, it seems that other factors (such as genetics and general diet) may have a more significant impact than the number of eggs consumed.

So, consider this the next time you eat an egg: the humble egg is a complex chemical and biological phenomenon, a fascinating mix of nutritious chemicals that is the result of millions of years of evolution and thousands of years of human manipulation. And they taste good, too, especially on a fried-egg sandwich...