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Finding Fat in Foods

We are going in search of fats in our foods. This test does not require you to taste the food, but that doesn't mean you can't have a snack along the way.

To find some fats, you will need:

  • a brown paper bag (This will work with typing paper, but it does not show up as well.)
  • a pen or pencil
  • a variety of foods
  • water
  • cooking oil

Start by drawing a grid of squares on the paper bag, making each square about 2 inches across. They don't have to be exact. Once you have at least ten or fifteen squares, its time to head for the kitchen.

Before we start foraging....I mean rummaging through the refrigerator, lets get an idea of what we are doing. Place two of your paper squares on the kitchen counter. In the center of one square, put a drop of water. Use your finger to smear the water around the paper. Label that square "water."

In the center of the next square, put a drop of cooking oil, and smear it around too. Label that "cooking oil."

Notice that both liquids make dark spots. Hold the paper up between your eyes and a light source, and you will see that the wet spots look brighter than the rest of the paper. The liquid transmits the light through the paper. Because the light goes through the paper, from above it looks like a dark spot. (See the Why Wet Things Look Dark video.)

Now, place the paper on the kitchen counter, and let it sit for about thirty minutes. That should give you plenty of time to have a nice bowl of ice cream, especially if you use part of the time slicing bananas and nuts to go with the chocolate sauce, and the ....well, you get the idea.

After thirty minutes, check the piece of paper. The water has probably evaporated, so that dark spot is gone. On the other hand, fats evaporate MUCH slower, so you should still see a dark spot there.

That is how we will test for fats. For each square, we will rub some of the food into a spot, label the paper, and then let it sit long enough for the water to evaporate away. If that food contains fat or oil, you will still have a dark spot. Notice that it is not just a colored stain. Instead, it still looks wet.

So, now you are ready to start testing foods, but first a couple of words of warning. Wash your hands well before you begin, use small food samples, and don't put the food sample back into the refrigerator. No one will want to eat the bit of food that you have been experimenting with. Do not experiment on those nice steaks, or any other expensive foods, unless you ask for permission first.

OK, so which foods do you think will contain fats and oils? Some are obvious. Meats contain fat, so that one is easy. Fats and oils contain lots of stored, chemical energy. When an animal eats more energy (calories) than they need, their bodies store some of that energy as fat.

Butter contains a LOT of fat. And where does butter come from? Milk, right? (See the Making Butter video: ) That fat makes milk a very good food for young mammals. Look at other things that are made from milk, such as cheese and yogurt, as they should contain fats too. You might want to compare regular yogurt with fat-free yogurt.

Now, move beyond the animal world, and look at some of the plant materials in your kitchen. A good place to look for plant fats and oils is in seeds, where the oils provide energy for the young sprout. But do you have any seeds in your kitchen? Probably so. Nuts, including peanuts are seeds. If you happen to have some peanuts, rub one of them briskly on one of your paper squares, and you should get a nice oil spot. If you don't have any whole peanuts, try a tiny bit of peanut butter. Be aware that some people are very allergic to peanuts.

What about other things made from seeds? If you have any mustard in your refrigerator, test it. Mustard is made from mustard seeds. It may also have oils added to it, to help dissolve some of the flavor rich chemicals. While you are testing condiments, mayonnaise also contains oils, as do most salad dressings.

As part of your fat finding, keep in mind where the fat came from, and remember that fats and oils are a way for living things to store energy. That stored energy explains why so many of the foods in your kitchen contain those fats and oils, including that bowl of ice cream, the chocolate sauce, and even the banana peel, which makes that yummy snack part of a great science experiment.

Have a wonder-filled week.