STUDENT HANDOUT:

• Yeast Fermentation Student Handout

INTRODUCTION:

When most people think of microbes, they think of "germs,"  which they view as causing infections. Actually, most microbes do not cause disease and many are beneficial to the environment and to human life. For as long as the human race has been on Earth, people have been using the activities of microbes to develop beneficial products. In the mid-1800's, Louis Pasteur finally explained the science which we now know as "microbiology." This science deals with the many types of microorganisms, both beneficial and disease causing. Microbiology is concerned with the way these microorganisms behave and how they interact with one another and with other living things.

One such microbe is common bakers’ yeast, or Saccharomyces cerevisiae.  This is one of the most readily available microorganisms and probably one that is most familiar to you.  It is used in the baking of bread.  The typical ingredients in bread include yeast, sugar, flour and warm water.

The warm water and sugar help activate the yeast to produce carbon dioxide in a reaction known as cellular respiration.  This gas is trapped by the dough, making the dough expand and rise.  The chemical reaction for cellular respiration is:  

Glucose + Oxygen   →   Carbon Dioxide + Water + Energy (ATP)

ATP = Adenosine Triphosphate 

Since this reaction has oxygen as a reactant, this process is known as aerobic cellular respiration.  In eukaryotic cells, this reaction takes place in the cytoplasm and mitochondria of the cell.  In prokaryotic cells, this reaction takes place in the cytoplasm and cell membrane of the cell.  

Aerobic cellular respiration takes place in four steps (Figure 1).  These steps are glycolysis, pyruvate oxidation, the Krebs cycle and the electron transport chain.  Glycolysis, pyruvate oxidation and the Krebs cycle result in electron carriers delivering electrons to the electron transport chain.  The oxygen acts as the last electron acceptor in the electron transport chain.  

Figure 1.  Overview of aerobic cellular respiration.

In the absence of oxygen, this reaction is known as anaerobic cellular respiration.  A special type of anaerobic respiration is known as fermentation.  In the process of fermentation, only the step of glycolysis occurs (Figure 2).  The electron carriers give their electrons to pyruvate, which can be further oxidized.  As a result of this, different organisms produce different products of fermentation (Table 1). 

Figure 2.  During fermentation, only the process of glycolysis occurs.

Table 1.  The products of fermentation are made by various microorganisms. 

OBJECTIVES:

1. Demonstrate the beneficial activities of yeast.

2. Observe whether yeast can produce carbon dioxide from sugar.

3. Visualize yeast cells using the microscope.

MATERIALS (Work in pairs or small groups):

1. 3 clean, empty plastic water bottles w/ caps

2. 3 packets of dry-active yeast (7g, 2¼ teaspoons per packet)

3. 3 cups of warm water (700 milliliters)

4. 3 latex balloons – small to fit on top of bottle

5. funnel

6. measuring cup or graduated cylinder

7. measuring spoon (1 teaspoon)

8. balance (if measuring each material)

9. scissors (to cut yeast packet)

10. string or tape measure

11. ruler

12. marker

PROCEDURE:

1. Line up the plastic bottles.

2. Label bottles 1, 2, 3 with marker.

3. Remove caps and set aside.

4. Using a funnel, add 7 grams of yeast to a bottle (1 packet).

5. Repeat step 4 for the other 2 bottles.

6. Using a funnel, add 4.5 grams of sugar (sucrose) to bottle 2 (1 teaspoon).

7. Using a funnel, add 13.5 grams of sugar to bottle 3 (3 teaspoons).

8. Using a funnel, add 235 milliliters of warm water to each bottle (1 cup).

9. Close caps and shake bottles gently for 10 seconds.

10. Remove caps and place a balloon on top of each bottle.

11. Let bottles sit undisturbed for 20 -30 minutes.

12. Measure the height and circumference at the widest point of the balloon using the string.  Lay flat against the ruler to record the measurement. Alternatively, you could use a tape measure. 

COROLLARY ACTIVITY - BUDDING OF YEAST:

While the yeast culture is incubating, prepare wet mount slides of yeast cells. 

MATERIALS (Work in pairs or small groups):

1. packet of dry-active yeast

2. warm water

3. glass microscope slide

4. coverslip

5. Pasteur pipet

6. microscope 

PROCEDURE:

1. Add a pinch of dry-active yeast to some warm water.

2. Wait until the mixture begins to become cloudy and bubbly.  This may take anywhere from 30 minutes to an hour.

3. Using a Pasteur pipet, place a drop on a microscope slide and add a coverslip.  With the iris diaphragm on your microscope nearly closed, a magnification of approximately 400x should be adequate to view the yeast cells.  You should be able to see the cells reproducing asexually.  This process is called budding. 

OBSERVATIONS/CONCLUSIONS:

1. After 20 - 30 minutes, observe the balloons.  Measure the height and circumference at the widest point of the balloon using the string.  Lay flat against the ruler to record the measurement.  Alternatively, you could use a tape measure. Record your data in Table 1 of the Laboratory Report Form. 

2. What is the difference in the height of the balloons between the bottles with 1 teaspoon and three teaspoons of sugar added to the yeast?  

3. Draw your microscopy observations of yeast cells in Figure 1 of the Laboratory Report Form.  Were you able to see any budding?

QUESTIONS:

1. Imagine we performed the same experiment, but this time, rather than sugar, we added…

(a) honey.  What do you think would happen?  Why?

(b) olive oil.  What do you think would happen?  Why?

2. What other foods or beverages are made with yeast?

KEY TERMS:

• aerobic cellular respiration

• anaerobic cellular respiration

• budding

• cellular respiration

• eukaryotic cells

• fermentation

• iris diaphragm

• microbes

• prokaryotic cells

• yeast

SUGGESTED READINGS:

Gillen, A.L. and Williams, R.P. January 1991. Kitchen microbiology and microbe phobia. The American Biology Teacher, 53.1: 10-11.