To Freeze, Or Not To Freeze?

In the last posting, you learned that some foods shouldn't be frozen, if you want to eat them later. They don't go bad, or become unsafe to eat, but they turn icky and disgusting. In this activity, you can experiment with different foods, to see which kinds of foods you won't want to eat after they've been frozen and thawed.

What You'll Need:
Small paper cups, one for each food you want to freeze
Metal baking pan
Marker
Freezer
Several foods to try freezing, such as lettuce, apples, carrots, milk, tomatoes, bread, cottage cheese, mayonnaise, oranges, pasta (cooked), meat, yogurt, eggs, water, soda, or potatoes

First, clear out some space in the freezer so that the baking pan will lay flat. Place a little bit of each food into each cup and label. Place all the cups onto the baking pan, then put everything into the freezer. After 2-3 hours, take the pan out and put it on a table or counter to thaw. After two hours, inspect the foods and see how they look. Which ones would you still eat? Which ones look really yucky? Even though all of these foods are still safe to eat, it's probably a good idea to throw away anything that doesn't still look right.

What you'll find is that foods with lots of water tend to turn mushy after they're frozen, and that foods that are mixtures of many different things usually separate into different layers.

Lots of Leftovers

Now that Thanksgiving is over, you probably have lots and lots of leftovers. You'll probably freeze much of what you can't eat. Freezing is a great way to save both cooked and uncooked food, as long as your freezer is colder than 0^oF. The U. S. Department of Agriculture has a fact sheet that answers lots of questions about freezing food safely. Here are some of the highlights:

• Almost anything can be frozen, except for eggs in their shells and anything in a can.
• Some foods turn yucky when they're frozen, but they're still safe to eat (there will be an experiment on this)
• Food never "goes bad" in the freezer. It never loses its nutritional value, although it may get dried out and not taste very good.
• Bacteria "sleep" when they're frozen, but become active again when the food is thawed. Remember that freezing does not kill bacteria or other nasty beasties.
• Fresh vegetables should be cooked briefly in boiling water or a microwave oven before freezing. This is called blanching. Vegetables sold in frozen form have already been blanched.
• It's OK to freeze meat in the supermarket packaging, but only if you're going to use it in a month or two. Otherwise, the meat will dry out.
• Freezer burn can occur if food is exposed to air in the freezer. It causes dried-out, grayish spots. They're pretty disgusting, but you can just cut these spots off and use the food as usual. To prevent freezer burn, be sure to wrap food tightly.
• Freezing can cause some foods, particularly meats, to change color, but it doesnt't make them unsafe or less nutritious.
• You should only thaw foods in the refrigerator, in the microwave, or in cold water. Never thaw foods on the kitchen counter, outdoors, or in the dishwasher!
• Once food has been defrosted, it is safe to freeze it again, even if it hasn't been cooked.

Cran-tastic Sauce!

Do you like to eat cranberry sauce on Thanksgiving? Here's an easy recipe that also teaches you something about a molecule called pectin. Be sure to do this with an adult. But best of all, it's an experiment you can eat when you're done!

Pectin is a polysaccharide, a long-chain molecule that is made up of sugar units - a sugar chain. Pectin dissolves in water, but it thickens the liquid. It's what's commonly used to make jam and jelly thick and spreadable. Pectin is found in many fruits, such as cranberries.

What you'll need:
4 cups (1 pound) of fresh cranberries
2 cups sugar
1 1/2 cups water
Deep pan for cooking
Spoon with a long handle

Wash the cranberries, picking out any that aren't red, or any leaves or other bits of plants. Mix the water and sugar and boil. Keep heating over medium until the solution turns clear. Add the cranberries and turn up the heat to high. Make sure that your pan is deep enough that the cranberry sauce won't spatter out. Watch the berries as they cook, but don't get too close to the hot mixture! Keep stirring, and notice how the cranberry sauce feels. After about 5-10 minutes, the sauce will start to thicken up, and the pectin molecules dissolves out of the berries. Cook just a few minutes more, then let the sauce cool. Store in the refrigerator, then eat within a few days. Enjoy!

Sob Story

Have you ever cut an onion? Did your eyes water? Onions are famous for their ability to make you cry when you cut them. Why?

Actually, it's related to the sulfur-containing molecules discussed in the previous post. The particular versions that are found in onions actually dissolve in your tears and form a little bit of sulfuric acid! Acids burn, so your eyes fill with water to try to wash the acid away.

What's That Smell??

You've undoubtedly noticed that some vegetables are smellier than others. Corn, carrots, lettuce, green beans - they don't have much odor to them. But what about onions, garlic, and cabbage? Whew! These vegetables can make your eyes water, especially when they're cooking. Why does that happen?

All of these vegetables contain the element sulfur. In its pure form, sulfur is a light yellow powder that doesn't have much of an odor. But if you combine it with some carbon, hydrogen, and maybe a little oxygen and/or nitrogen, you can make some of the stinkiest molecules on earth! These molecules evaporate into the air easily, which is why you can smell them. The good part is that, when you cook these vegetables, the smelly sulfur-containing molecules leave. That means that they're pretty stinky while they're cooking, but that they taste much different (and often better) when they're done.

Try cooking some cabbage to check this out. Take a head of raw cabbage and cut it in half (have an adult help). Smell and taste both halves - they should be the same. Now cut one half into small pieces and place the leaves into a saucepan with 1 cup of water. Boil uncovered for 20 minutes. How does the cooking cabbage make the room smell? When the cabbage is done, remove a piece and let it cool. Compare its taste and smell to that of the uncooked half. What do you notice?

The kitchen got pretty smelly while the cabbage was cooking. That's because the sulfur-containing molecules left the leaves and went into the air. But the cooked cabbage tasted and smelled quite different from the uncooked cabbage because all or most of the stinky molecules had evaporated!

How to Cook Your Veggies

You know that you have to eat your vegetables, even though you may not like to. Did you know that how you cook vegetables affects how they look? Here's a simple experiment that you can do!

What You'll Need:
3 small, micowave-safe bowls
Measuring cup
Measuring spoons
Vinegar
Baking soda
Water
Microwave oven
Oven mitt
Spoon
Frozen green vegetable, such as peas

Pour 1/4 cup of water into each bowl. Add 1 tablespoon of vinegar to one bowl, and add 1 teaspoon of baking soda to another one. The third bowl should only have water in it. Add about 1/8 cup frozen vegetable pieces to each bowl (about 20 peas, if that's what you're using). Cook all three bowls in the microwave using high power for about 5 minutes. It doesn't matter whether you do them one at a time or all at once. When they're done, carefully remove the bowls from the microwave with the oven mitt. They will be very hot!!! Compare the vegetables - look at their color and mash them with a spoon to check their texture. What do you see?

You'll immediately notice that the vegetables cooked in vinegar (an acid) look grayish and very yucky. Who would want to eat them? The veggies cooked with baking soda (a base) look nice and green, but they are disgustingly mushy when you mash them with a spoon. Why do they turn out so different?

The green color is due to a molecule called chlorophyll, which has a magnesium (Mg) atom in the middle. Cooking in acid removes the magnesium and puts two hydrogens (H) in its place. The H-containing version of chlorophyll is a dull, ugly green color, and that's what you see. Cooking in a base doesn't remove the Mg, but it does break down the walls of the plant cells that keep the vegetable nice and firm. Without those strong walls, you can easily smoosh the cells together using a spoon. Vegetables cooked in water only keep a pretty good green color, and don't get mushy. Which one would you like to eat??

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??

Mighty Muscles

Let's talk some more about how muscles work, whether they're in a turkey leg or in your arm.

Make a muscle with your upper arm like the yellow guy at right is doing. Place your other hand on top of the bulging muscle (called the bicep) and straighten your arm. Can you feel something moving? What's really going on in there?

Muscles are made up of bundles of muscle fibers, which are long and skinny. Inside these fibers are smaller units, called fibrils, which contain stringy molecules of two kinds of protein: myosin (orange) and actin (blue). When your brain tells the muscle to get shorter, or contract (when you "make a muscle"), the myosin and actin molecules slide along each other. When you stretch, they slide apart and get longer:

Now place your hand on the muscle (tricep) underneath your arm, and straighten and bend your arm again. Can you feel the tricep move? Notice that when the bicep is contracted, the tricep is stretched, and when the bicep is stretched, the tricep is contracted. Many of the muscles in your body work in opposite ways like these two. This helps to keep you balanced.

So, now do a little dance around the room, and imagine all of your muscle fibrils sliding in and out, in and out, in and out... And the next time you eat a piece of meat, look for the muscle fibers!

Talking Turkey

The centerpiece of most people's Thanksgiving dinner is the turkey. Turkey is, of course, a kind of meat, by which we usually mean the muscle tissue of an animal - beef from cows, pork from pigs, veal from calves, turkey from turkey, etc. Meat is, in general, a very nutritious food because it contains a lot of the proteins we need to stay strong and healthy (although it is possible to get those proteins without eating meat).

You've probably noticed that turkey (and chicken) meat comes in two colors, which we call white and dark. But we never see "white" beef, or "dark" fish. Why not? The color difference is due to a chemical called myoglobin which stores oxygen. Darker meats contain more myoglobin than whiter meats, and muscles that get more exercise have more myoglobin. Cows walk around a lot finding grass to eat, so they get plenty of exercise, and all of their meat is dark. Turkeys, on the other hand, stand around in pens just waiting to get fed, and they rarely, if ever, fly. Their legs and thighs work hard, but their breast muscles, which control their wings, don't do very much. So, their leg and thigh meat is dark, but the breast is white!