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Unit 19: Biology Part 2 Page 116 of 137

Materials:

 4 cups

 Water
 Sour milk, expired sour cream or fresh yogurt
 Methylene blue
 Mineral oil
 Large clear bowl

1. Label four cups as “control,” “milk,” “yogurt,” and “sour cream”

2. Fill the control cup with water.

3. Fill each of the other cups half full with sour milk, expired sour cream, or

fresh yogurt.

4. Add 20 drops of methylene blue to each cup.

5. Add 2 drops of mineral oil to each cup.

6. Place the cups in a large clear bowl filled with warm water.

7. Record the time it takes for the color to fade.

What’s Happening: Methylene blue is a dye used to detect bacteria. The
bacteria use up oxygen, and the dye turns colorless in the absence of
oxygen. So, the faster the color fades when the dye is added to a substance,
the less oxygen there is, and the more bacteria the substance has. (There
are some bacteria that don’t need oxygen, so this test wouldn’t work for
them.) You may have noticed that there are bacteria present in the yogurt
although it’s fresh. These bacteria are helpful to you. They improve your
ability to digest food.

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