Energy from Yeast Cells
All living things require energy from food to carry out their life functions. This energy is
released by the breakdown of energy rich compounds such as sugar. The breakdown of sugar
produces waste products. The presence of these waste products provides evidence of these
waste products to determine if fermentation has taken place.
We will be using two different chemical indicators: Benedicts solution and Bromothymol Blue.
Benedicts solution is used to indicate glucose, fructose, or galactose. Bromothymol Blue is an
indicator of acidity and carbon dioxide. Both solutions will change color when in the presence of
what they indicate.
Objectives:
 Detect the waste products of fermentation with an indicator.
 Observe the relationship between energy consumption and waste production within the
process of fermentation.
Materials:
 Yeast, sucrose solution, bromothymol blue solution, benedict’s solution, graduated
cylinder (10-100mL), test tubes, Erlenmeyer flasks (125 or 250 mL), rubber stoppers
(one hole and two hole), rubber tubing, stirring rod, marking pen, test tube holder,
safety goggles, lab apron
Procedure:
1. Using a rinsed graduated cylinder, measure 15ml of warm water. Pour the water into a
rinsed 125(250) mL Erlenmeyer flask.
2. Add ½ a packet of yeast to the water and mix them together with a stirring rod.
3. Using a rinsed grad. Cylinder, measure 50 mL of sucrose solution. Add the solution to
the flask containing the yeast mixture and mix well using a stirring rod. The sucrose will
serve as food for the yeast.
4. (SAFETY) Put on safety goggles, a lab apron, and tie back long hair.
5. Set up a hot water bath. (DONE BY THE TEACHER)
6. Using a grad. cylinder, measure 5 mL of sucrose solution. Transfer this solution to a test
tube. Rinse the grad. cylinder, then use it to measure 5 mL of Benedict’s solution. Add
this solution to the same test tube. With a marking pen, label this test tube “A”.
Caution: Benedict’s solution contains copper sulfate and is poisonous. Do not get any
in your mouth and do not swallow any.
7. Rinse the grad. cylinder. Use it to measure 5 mL of the yeast-sucrose mixture prepared
in step 3. Transfer this solution to another test tube. Rinse the grad. cylinder, then use
it to measure 5 mL of Benedicts solution. Add this solution to the same test tube. With
a marking pen, label this test tube “B”.
8. Using a test tube holder, place the two test tubes, one at a time, into the hot water
bath. After five minutes, remove the test tubes from the hot water bath using the test
tube holder and place them in a test tube rack. Note any changes in the contents of the
tubes. Answer Investigation questions 1 through 3 on the Lab report.
9. Clean up your area prior to going on the next step.
10. Measure 75 mL of water and 2 mL of Bromothymol Blue solution and pour into an
Erlenmeyer flask.
11. Obtain a rubber stopper/rubber hose apparatus from your teacher. Insert the one hole
stopper into the flask containing the yeast mixture. Insert the two hole stopper into the
flask containing the bromothymol blue solution. The longer end of the tubing should be
below the surface of the bromothymol blue solution. (Look at drawing on board).
12. Place the set up in a warm place. Clean up all other materials. Before leaving the
laboratory, wash your hands thoroughly with soap and water.
Energy from Yeast Cells
Prelab questions:
1. Why are you directed to add warm water, not hot water, to the yeast culture?
2. Why is a two hole stopper used in the flask containing the indicator?
3. How does Benedict’s solution indicate the presence of glucose?
4. What color change did you observe in the bromothymol blue solution?
5. For what substance is bromothymol blue an indicator?
6. What two waste products are contained in your breath?
7. Explain why the presence of carbon dioxide can be used to indicate if fermentation
has taken place.
8. What will the results of the test tell you?
Investigation:
Observations and Data:
1. After heating, what is the color of the contents of test tube A?
2. After heating, what is the color of the contents of test tube B?
3. What does the color of the contents of test tube B after heating indicate? Explain.
4. How did the bromothymol blue solution react on the first day of testing?
5. What does this result indicate?
6. How did the bromothymol blue solution react on the second day of testing?
7. What did this color change indicate?
Analysis and Conclusions:
1. On the basis of your observations, summarize the chemical changes that occur
during fermentation in yeast.
2. Alcoholic fermentation is the process that causes bread dough to rise. Explain how
this happens.