6.08 Fungus Lab: Growing Yeast

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This experiment will take two days to complete.

Background Information:

Yeast is a very important group of fungi. The common yeast used in baking bread grows very fast. The basic idea in this lab is that the yeast will use an energy source and in doing so, will produce carbon dioxide gas. You will measure the amount of carbon dioxide (CO₂) released during the growth of yeast. The growth of the yeast stops when the energy source has been used up by the yeast and CO₂ production slows down or stops. Your variable then is? ________________

Note: You will need to complete the sections with the "Red" headings.

Hypothesis: (6 points)

1. Read through the experiment procedure.
2. Based on that, what do you think the outcome of the experiment will be?

   _____________________________________________

Materials:

• A teaspoon measure
• A permanent marker
• Active dry yeast (used in baking bread—do not use quick-rising varieties.)
• 2 empty & clean (14-20 oz.) soda bottles
• 1 (14-20 oz.) cola (Coke, Pepsi, or a generic equivalent) — open and allow to go flat overnight
• 1 (14-20 oz.) fruit juice—uncarbonated (make sure all your bottles are the same oz.)
• Water
• Sugar
• 4 identical round, thin latex balloons—"water balloons" are slow to expand. Non-Mylar® "helium-quality" balloons give good results. But, water balloons will work.
• String or yarn
• Ruler

Procedure: (30 points)

Step 1: Label each bottle with a number to keep track of what each one contains.

1. Control (one of the empty bottles)
2. Coke
3. Fruit Juice
4. Sugar (one of the empty bottles)

Step 2: Fill both of the empty bottles (#1 and #4) with water. Fill them to the same height as the soda and juice.

Step 3: Put a teaspoon of sugar into bottle #4.

Step 4: Put a teaspoon of dried yeast in each bottle. Seal the bottles tightly and shake the bottles.

Step 5: Remove the lids very slowly and stretch a balloon over the mouth of each bottle. The balloon should fit very tightly so that the carbon dioxide (CO₂) does not leak into the air.

Step 6: Make a hypothesis: Which bottle do you think will have the most yeast growth? (2 points) (Growth of the yeast will be measured by the amount of CO₂ that is released.)

Step 7: Make observations at 0, 24, and 48 hours. Record the circumference of the balloons and the circumference of each bottle.

*One good method of measurement is to wrap a string around each bottle or balloon at its widest point, and then measure the length of the wrapped string against a ruler.

Record any other things you see happen. Did the color change? Did one balloon have a hole in it?

Step 8: Place the containers in a warm area out of direct sunlight (top of refrigerator or clothes dryer) where they will not be disturbed.

Conclusion Questions: (3 points each)

1. Restate your hypothesis here. Based on the results of your experimental data was your hypothesis correct? Explain.
2. Based on the results of your experimental data, which energy source had the highest amount of sugar? On what do you base this? Explain.
3. In addition to the amount of sugar, what other factors may have had an impact on the growth of the yeast? (Think about substances in your test liquids, environmental conditions, and so forth.)

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