Green Eggs??

Have you ever noticed the green film that sometimes forms around the yolk in a hard-boiled egg? It doesn't look very yummy - what is that stuff? Eggs don't start out with anything green in them! The green film is iron sulfide (chemical formula, FeS), and it forms during cooking when iron in the yolk combines with some of the egg's proteins. You can't taste it, and it's not dangerous to eat.

How can you avoid forming this green film? You need to cool the eggs quickly after they are done cooking. As soon as the eggs are finished, pour off the hot water and cover them with cold water. The reaction happens at high temperature, and by cooling the eggs, you stop it from occurring!

Whip It Up!

Do you like lemon meringue pie? Did you know that the white meringue topping is made from egg whites? Making a meringue is an amazing trick. You start with a small amount of a clear, yellow liquid, and end up with huge mounds of fluffy white foam. This happens partly because of denaturation. The protein knots we talked about before can't withstand the beating, and they fall apart. But the whipping process also mixes a lot of air into the egg whites.

If you've never seen the changes egg whites undergo, it's a cool experiment to try. Take two eggs from the refrigerator, and separate the whites from the yolks while the eggs are cold (ask an adult for help). Make sure that there is absolutely no dark-yellow yolk mixed in with the whites. Then let the egg whites stand on the counter for about 30 minutes to warm up a little. Pour them into a large bowl made of glass, ceramic, copper, or stainless steel that has been carefully cleaned. Using either a hand whisk or a mixer (which is much easier), start beating the egg whites. Take a look at them every few minutes to observe what's happening.

First you'll notice that the egg whites form bubbles, although they're still yellow at this point. Then you'll notice that the liquid turns white, but is still thin and bubbly. Pretty soon, however, you'll notice that the egg whites become stiffer. If you lift the beaters out of the beaten egg whites, they form little peaks that bend over (this is called the soft-peak stage). But keep going! Eventually, you'll get stiff, shiny peaks that look like little snow-covered mountains (not surprisingly, this is known as the stiff-peak stage). At this point, you can even turn the bowl over, and the beaten egg whites won't come out. It's a pretty amazing transformation!

If you like to cook, you can sweeten the beaten egg whites with sugar, and bake them to make a kind of cookie called a meringue. You can even flavor the egg whites with things like chocolate or coconut. Do a Google search for a recipe!

The White Stuff

Now that we've talked about eggshells and the thin membrane that lines them, let's move on to the egg white. The egg white is the clear, light yellow liquid that surrounds the darker yellow yolk (more on that later!). Wait a minute - why is it called the egg white if it's yellow? Anyone who has seen a hard-boiled egg knows the answer to that. The yellow liquid becomes a white solid when it is heated. But what's in the egg white, and why does it change in this way? And can we change the color in any other way?

The egg white is also known as albumen (pronounced al-BYU-min). Egg whites are mostly (85%) water, but some protein molecules are dissolved in it. These proteins are long, stringy molecules which normally have particular shapes when dissolved in water. When they are heated, the proteins denature, or become a big tangled mess. Imagine a bunch of long strings that are all tied into the same kind of very fancy knot - that's sort of like the original proteins. Then picture all of those knots untied, and all the strings become tangled up into a big ball - that's what denaturation does. The change in color from light yellow to white is caused by this change in shape.

We can denature albumen proteins in ways other than heating. For example, if you pour some rubbing (isopropyl) alcohol into egg whites, you'll see white streaks forming. The proteins can't keep their knotted shapes in alcohol, so they become denatured. Another way of denaturing proteins is by beating them. That will be the topic for the next post.

Now Make a Shriveled Egg!

Once you have made a "rubber" egg, you can do another experiment with it. Place the egg into a jar full of corn syrup and let it sit for a few days. You will notice that all of the liquid gets drawn out of the egg, and you end up with a shriveled-up, pruney-looking egg. Why does that happen?

You're actually watching a process called osmosis (pronounced oz-MO-sis). The thin protein skin is actually a membrane, a thin film which lets some molecules pass through it, but not all molecules. When you put the egg in vinegar, the shell dissolved, but something else happened, too. The molecules in the vinegar were able to pass through the protein skin into the egg. That's why it got bigger. Now, when you place the "rubber" egg into corn syrup, the liquid inside the egg gets drawn out into the syrup, and the egg "deflates".

Osmosis is important to a lot of biology. The cells of your body are surrounded by membranes, and water and other molecules (like nutrients or waste products) can move in and out.

Make A "Rubber" Egg!

What happens when you drop an egg? It breaks, right? Well, not if you remove the shell first! Here's how to do this...

What you'll need:
1 egg
Vinegar
1 jar with lid
Spoon

This is very simple - put the egg into the jar, then pour vinegar over it to cover. Place the lid on the jar loosely, just to trap the vinegar smell. Look at the egg - what's happening? Soon after adding the vinegar, you'll see small bubbles forming on the eggshell. These are bubbles of carbon dioxide (CO₂) gas. They come from the chemical reaction of the vinegar with the calcium carbonate in the shell. Remember that the formula for calcium carbonate is CaCO3. When you remove CO₂ from CaCO₃, you're left with CaO, or calcium oxide.

But wait - this gets better! Let the egg stay in the vinegar for several days. Eventually, the whole shell will dissolve! The egg will get larger because it absorbs some vinegar, and it will be held together by the proteins that were originally in the shell. The vinegar makes these proteins tougher, and the egg will now feel like a water balloon! Using a spoon, remove it from the jar and hold it in your hand. Hold the egg about an inch above a counter and drop it - you will actually see it bounce! But be very careful when doing this. This "rubber" egg is very delicate, and if it breaks, you will have a small "eggsplosion"!!

Shell Games

Let's start talking about the various parts of an egg from the outside in. The hard outer covering of the common chicken egg is, of course, called the shell. Eggshells contain 95% of the mineral calcium carbonate and 5% of protein. Calcium carbonate (CaCO₃) can also be found in other places - sea shells, pearls, and rocks such as limestone and marble.

In the supermarket, you will see eggs with both white and brown shells. There isn't any difference in the nutrition you get from either color. It's just that some breeds of chickens (such as the Leghorn) lay white eggs, and some (like the Delaware) lay brown eggs. There is actually a kind of South American chicken (the Araucana) that lays blue-green eggs!

Hard-Boiled or Raw?

Have you ever made hard-boiled eggs, then mixed them up with raw eggs, so that you couldn't tell them apart? Even if you haven't done this, is there a way to distinguish them? They certainly look the same on the outside. Can we tell what the inside is like without breaking the egg?

Yes, we can! Ask yourself - what is inside a raw egg? There's a yolk and some runny egg whites - but everything is a liquid. Now what about a hard-boiled egg? Everything has turned to solid because it was cooked (more on that later!). We can make observations about the behavior of these eggs that tell us what is inside.

Take one hard-boiled and one raw egg. Clear an area on a countertop, lay the hard-boiled egg on its side, and give it a spin. It will wobble a little, but will keep turning. Now place your index finger lightly on the egg just long enough to make it stop. Remove your finger and watch what happens. Big deal - the egg remains stopped.

But now, try the same experiment with the raw egg (make sure it doesn't spin off the countertop). What happens when you remove your finger? The egg starts to turn again! Why?
Remember that the inside of this egg is filled with a liquid. When you put your finger on the shell, you stop the shell, but the liquid keeps moving. When you take your finger away, the moving liquid gets the shells going again. In the hard-boiled egg, everything stops when you stop the spinning, and the egg doesn't start turning again.

This effect actually has a fancy science name - inertia (pronounced in-er-shah), which is the tendency of moving things to stay in motion. Think about riding in a car when the driver suddenly puts on the brakes, you are thrown forward because of your inertia. At first, you and the car are both moving at the same speed. But when the brakes applied, the car stops but you don't. Can you think of any other examples of times when you feel inertia?

Denature An Egg!

Meat isn't the only protein-containing food that people commonly eat. Eggs are another good source of protein. In fact, the effect of denaturation are more obvious when you cook an egg than when you cook meat.

What's inside a raw egg? There's a dark yellow, liquid yolk floating in a light yellow liquid called the egg white. Now, what about a hard-boiled egg? The yolk turns light yellow, and the white becomes really white. Those changes occur because the heat from boiling denatures the proteins in the egg - exactly the same process that happens in meat.

Here's a pretty amazing way to denature the proteins in egg white without cooking them! Crack open an egg and separate the white from the yolk (you may need an adult to help with this). Put the white into a small bowl, and add some rubbing (isopropyl) alcohol to it. Pretty soon, you'll see white strings, which are denatured proteins. So, heating is only one way to mess up the protein knot - you can also do it simply by placing the proteins in an environment that they don't like.

Warning here: DO NOT EAT THE DENATURED EGG WHITE. YOU CAN POUR THE EGG/ALCOHOL MIXTURE DOWN THE DRAIN.