Under What Kinds of Conditions Does Yeast Ferment Sugar Most Quickly?

Under What Kinds of Conditions Does Yeast Ferment Sugar Most Quickly?

Controlling the Rate of Fermentation by a Fungus


The action of yeast is important to the manufacture of bread, beer, and wine. Baker’s yeast, Saccharomyces cerevisiae, participates in alcoholic fermentation under anaerobic conditions. What substances and conditions promote fermentation by yeast? In this investigation, you will examine the effects of two substances on the rate at which yeast cells perform fermentation.


Under what kinds of conditions does yeast ferment sugar most quickly?

Pre-Lab Discussion

Read the entire investigation. Then, work with a partner to answer the following questions.

1. What process causes production of the bubbles in bread and in some alcoholic beverages? What gas forms these bubbles?

2. Identify the manipulated variables and responding variables in this experiment.

3. In which of the five flasks do you expect to observe the evidence of fermentation? Explain your answer.

4. What results do you expect from bottle D (contents include water, salt, and yeast)? Explain your answer.

Materials (per group)

5-250 mL flask


5 round balloons



Warm (40-45⁰C) water

Dry, powdered yeast

100-mL graduated cylinder

1 teaspoon measuring spoon (1 teaspoon equals 5 ml)


1. Obtain five 1-inch strips of tape. On the different pieces of tape, write the letters A, B, C, D, and E.

2. Tape one piece of tape on each of the five different flasks.

3. Using the measuring spoon,measure 5 mL of sugar(1 teaspoon = 5 mL). Place the sugar into flask A.

4. Add 30 mL of sugar to flask B, C, and E.

4. Add 5 mL of salt to flask C, D, and E.

5. Add 2 mL of dry, powdered Yeast to flask A, B, C, and D.

6. Fill flask A about 150-mL of warm water and quickly place a balloon over the opening of the flask. Make sure that the balloon fits tightly around the neck of the flask. Mix the dry ingredients and the water by swirling the bottle gently.

7. Repeat step 6 for bottles B, C, D, and E. Be sure to add equal volumes of water to all five flask.

8. Place the flask in a warm spot away from drafts. Wash your hands thoroughly after carrying out this investigation. Carefully observe the bottles each day for several days and record your observations in the Data Table.

Data Table

Sugar / Salt / Yeast / Day 1 / Day 2 / Day 3 / Day 4
A / 5 mL / none / 2 mL
B / 30 mL / None / 2 mL
C / 30 mL / 5 mL / 2 mL
D / None / 5 mL / 2 mL
E / 30 mL / 5 mL / none

Diagram each flask on the final day of experiment:

A / B / C / D / E

Analysis and Conclusions

1. Describe the overall changes observed in every bottle over the course of the experiment.

2. How could you explain the changes that occurred?

3. In what way did the contents of the bottles with the inflated balloonsdiffer from those whose balloons did not inflate?

4. How is sugar related to fermentation?

5. Was there a difference in the rate of inflation of the balloons, and if so, what variable does it appear to be related to? Formulate a hypothesis about the relationship between the manipulated variables in the experiment and the rate of fermentation.

Going Further

Bread and alcoholic beverages are not the only food products produced by yeasts. Using library or internet resources, research the use of fungi in the production of cheese and of soy products. Write a short report (3 paragraphs) explaining what type of fungi are used to make cheese and soy, the process by which fungi are used to produce the food products, and specific names of food items made by this process.