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.

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!

Chewy, Gooey Caramels

Here's another recipe for one of Dr. B's favorite candies - caramels!

What you need:
1 cup heavy cream
12 tablespoons butter (1 1/2 sticks), cut into small pieces
2 2/3 cups light brown sugar
1/2 teaspoon vanilla
Water
Large saucepan with cover
Stove
Candy thermometer
9" x 9" baking pan
Nonstick cooking spray
Spoon
Spatula
Knife
Waxed paper or plastic wrap

Coat the baking pan with nonstick cooking spray. Put the cream, butter, brown sugar, vanilla, and water into the saucepan. Heat on low for a few minutes while stirring until the butter melts and the sugar dissolves. Turn the heat up until the mixture boils, then cover and let cook for three minutes. Uncover the pan, turn the heat down, and continue to boil; don't stir the mixture during this time. When the candy thermometer reads 250^oF, remove the pan from the heat and pour the caramel into the pan. This mixture will heat up quickly - make sure it doesn't burn! Let the caramels cools until they are solid but still slightly warm. To get them out, turn the pan upside down onto a piece of waxed paper or plastic wrap and push on the bottom of the pan. Cut into small pieces and wrap individually. Enjoy them!

So, what's so scientific about making caramels? While the mixture of cream, butter, and brown sugar is yummy enough on its own, it tastes even better after it cooks a while. This is an example of the Maillard reaction, which takes place between a protein (in the cream) and a sugar. This is the first type of candy you've made that contains any proteins. When the two molecules react, the mixture turns brown. It's the same reaction that occurs when you make toast (the flour used to make bread has both protein and sugar) and when onions are cooked until they turn brown. Of course, caramels, toast, and onions (while all delicious) don't taste at all alike. Each of these foods contains lots of other molecules that give them their particular flavor. But the fact that they all turn brown is due to the same reaction!

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.

De-Nature of Proteins

We've talked about what meat is made of - muscle (apologies to any vegetarians reading this!). But what happens when you cook meat? Why does it look and taste so different?

Meat is mostly made up of water, fat, and proteins, which are very long molecules composed of building blocks called amino acids. You can think of a protein like a necklace, with the amino acids being the individual beads. Normally, these protein necklaces are coiled up in a very specific shape. But when they are heated, this shape falls apart, and they just become shapeless blobs - this process is called denaturation. Imagine a rope tied in a very complicated knot that somehow becomes untied and end up just as a pile of rope. When you cook meat, you denature the proteins, which has several effects:

• Denatured proteins are easier for our stomachs to digest
• Denatured proteins make meat firmer and not so slimy
• Denaturation disrupts the stringy muscle fibers, making the meat easier to chew
• Denaturation changes the color of meat from translucent to opaque

Cooking has other benefits, as well, since it kills bacteria and improves flavor. However, those aren't related to the denaturation process. So, how do you like your meat cooked - rare, or well-denatured??