Submitted by Karen S. Gottsch
Cambridge High School, NE
Page 1/3
BIOLOGY
YEAST FERMENTATION
Objective: Yeast requires several factors for optimum activity. These include a source of
carbohydrates, water, pH level, and temperature. This activity will illustrate to the
student that each factor has an optimum level and that small changes in any one factor
can affect the amount of growth in yeast and therefore the production of carbon dioxide.
DAY 1 (30-40 min)
A) A basic yeast fermentation demonstration is set up by the instructor.
.
80 g. water (room temperature)
5 g. sucrose
4 g. yeast
The materials are placed in a 250 ml flask or small plastic or glass container (the 10ozsize club soda or water bottles work well) and stirred gently. A balloon (round if
possible) is carefully placed over the opening of the container. The container is placed in
a location that is easily viewed by the students for the next 20-30 minutes.
B) As the yeast demonstration is progressing, background information and growth
requirement variations are discussed.
Background information
-Yeasts are single-celled fungi.
-Requirements for growth are a carbohydrate source and water as well as proper
temperature and pH
-The production of enzymes by the yeast and why.
-Beneficial uses and spoilage problems
-The process of fermentation and the production of alcohol and carbon dioxide.
Growth requirements discussion:
1) Carbohydrates: simple sugars vs. starches
Sources of sugars
2) pH: yeast grows best at a pH around 7
-methods to alter pH
3) Water: H+ and OH-, binding of water and sugar molecules
4) Types of yeast: Baker’s vs. brewer’s
Submitted by Karen S. Gottsch
Cambridge High School, NE
Page 2/3
C) After approximately 20 minutes, observations of the yeast demonstration are made by
the students. Review the use of a string to measure circumference and determine volume
if necessary.
D) Assignment:
Students are to design an experiment to compare the growth of yeast under
various conditions. The experiment should include one dependent variable with four
levels (eg. 4 different temperatures). The amount of water, sugar, and yeast used in the
teacher demonstration will act as the control. Each experimental design should include a
hypothesis.
Equipment available:
Test tubes, flasks, and various sized containers.
Honey (fructose), table sugar, glucose (dextrose), corm starch, lactose, gelatin, etc
Water, ice, hot plate
Vinegar, ammonia, fruit juices
pH meter, graduated cylinders, scale
Balloons (round)
Other materials can be used.
The instructor must approve the design of the experiment and all equipment gathered
prior to the next class period.
DAY 2 (50 min)
A) Each student (group) sets up their experiment.
During the time between the set-up and the initial observations, each student or
group will describe their experimental design to the class as well as their
hypothesis.
B) Initial observations and measurements can be made and recorded after 20 minutes and
then two more times at 10 minute intervals. Measure the amount of gas by using a string
to measure the circumference of the balloon and convert to cm3.
C) Make a chart that includes a description of the conditions in each container,
observations for each time period, and amount of gas produced.
D) Use the information in your chart to produce a bar graph.
Submitted by Karen S. Gottsch
Cambridge High School, NE
Page 3/3
QUESTIONS:
1. What were the contents of the control container?
2. What is your dependent variable?
3. What are your independent variable(s)?
4. What was the most favorable condition for growth?
5. What was the least favorable condition for growth?
6. How could you determine that the gas in the balloon was carbon dioxide?
7) Describe one or two changes you would make in you experimental design if you were
to redo it and explain why you would make the changes
Extensions:
1) What other method could be used to determine the amount of gas produces by the
fermentation of yeast?
Describe the benefits and disadvantages of this method compared to the method
used in this experiment.