What Do Yeast Eat...and How Can You Tell?
Talk It Over
Those brown granules in a packet or jar of baking yeast may look dead, but they actually contain millions of microscopic, living yeast cells that eat, grow, and make more yeast cells when conditions are right. You can't see them eating, but you can see evidence that they do.
- 2 empty plastic lemons or limes with dropper inserts and screw caps)
- Small fishing weights
- Measuring cup
- Active dry yeast
- Measuring Spoons
- Deep pot
- Warm water
- Stopwatch (optional, for "Go Far")
- Remove the caps and dropper inserts from the plastic fruits. Wash and rinse them well inside and out.
- Count out enough fishing weights to fill each fruit about ¼ full. Put the same number and size of weights in each fruit.
- Fill the measuring cup with warm (not hot) water to the 1-cup mark. Add a teaspoon of active dry yeast. Stir with a spoon until all the yeast grains dissolve.
- Carefully pour the yeast-and-water mixture into 1 of the fruits. Fill it all the way to the top. Put on the dropper insert and screw cap.
- Add 1 teaspoon of molasses to the yeast-water mixture that is still in the measuring cup. Stir well.
- Fill the other plastic fruit with the yeast-water-molasses mixture. Put on the dropper insert and cap. With the marker, write a big "M" on the fruit, so you know it has molasses in it.
- Set both fruits in the pot. Add enough warm (not hot) water to cover the fruits completely.
- Working underwater, remove the caps (but not the dropper inserts) from both fruits.
- Within 10–20 minutes you should see something start to happen. Describe the differences you see and try to explain them.
- Determine what the yeast cells are using for food. How do you know?
Avoid tap water that is too hot. You'll burn your fingers and kill your yeast.
Mix 1 teaspoon of dry yeast into each of 2 cups of warm water. Add 1 teaspoon of molasses to 1 cup but not to the other. Leave both for 30 minutes in a warm place. What differences do you see?
Set up the experiment as in "Go," but use a stopwatch and count the number of bubbles that rise from each fruit each minute. Keep counting and recording minute-by-minute until the bubbles stop. What does the change in the number of bubbles per minute tell you about how the yeast population is growing? (Hint: The more yeast cells there are, the more food they consume.)
Use the bubble-counting method to measure and compare yeast growth rates when you test
- Equal amounts of different yeast foods such as sugar, honey, corn syrup, or jelly
- Different amounts of the same food (such as ¼, ½, and 1 teaspoon of molasses added to equal amounts of water)
- The effects of temperature by submerging your filled fruits in hot, warm, and cold water
- How other substances added to the mixture (such as vinegar, lemon juice, salt, or baking soda) affect yeast growth
Show Your Results
For "Go Easy," report your observations like this: In the cup without molasses, I saw _____. In the cup with molasses, I saw _____. I think this means that _____.
For "Go," draw diagrams of your setups and observations. Write a few sentences telling what you saw and explaining what you think caused it.Put fresh setups in your project display so others can see for themselves what happens.
For "Go Far," make a line graph showing the number of bubbles you counted each minute under the different conditions you tested.
Trips and Tricks
If you use too few fishing weights, your plastic fruits may start to float after a while. If that happens, repeat the experiment, using more weights.
Warning is hereby given that not all Project Ideas are appropriate for all individuals or in all circumstances. Implementation of any Science Project Idea should be undertaken only in appropriate settings and with appropriate parental or other supervision. Reading and following the safety precautions of all materials used in a project is the sole responsibility of each individual. For further information, consult your state’s handbook of Science Safety.