Tiny Bubbles
Yeasts are single-celled eukaryotic organisms that use sugar as a food supply. In the following investigation, students
will observe the substances produced by yeast cells from the breakdown of food molecules.
Purpose:
The purpose of this investigation is to determine which kind of sugar produces the most rapid
fermentation in yeast cells.
Materials:
1 plastic shoe box, 4 Large test tubes (2.2 cm id x 15 cm long), 4 small test tubes (1.1 cm id x 10 cm
long), 4 disposable plastic pipettes, 4 metal nuts (weights), thermometer, 5 different Yeast solutions:
(1) sucrose, (2) apple juice, (3) dextrose, (4) molasses, (5) distilled water (negative control in this
experiment), tap water (to fill plastic shoe box and large test tubes. Timer with second hand, test tube
rack, test tube template (determines which test tube contains specific yeast solutions). Metric ruler.
Procedures:
1. Place the test tube template where it can be viewed during setup.
2. Fill the plastic shoe box 75% full of warm 45°C tap water.
3. Completely fill the four pipettes with the sugar-yeast solutions. To fill the pipette, pull up as much
liquid as possible into the stem by squeezing the bulb with thumb and index finger and then slowly
releasing it. Turn the pipette upside down and tap the pipette to move the liquid into the bulb. If this is
not efficiently successful, then try the “Cartoon Air Brakes Method.” This method is achieved simply
by moving the pipette downward quickly while suddenly stopping the pipette in mid motion.
Centrifugal force also works well. Fill the remaining portion of the pipette until it is completely filled
with yeast-sugar solution.
4. Slide a metal nut over the stem where it rests above the bulb of each pipette.
5. Gently place the pipette with nut applied into one of the large test tubes. Submerge the test tube in
the shoe box. Allow the test tube to completely fill with water. Submerge a small test tube allowing it
to completely will with water. While submerged, slide the small test tube over the stem of the pipette
until it rests against the nut. Remove this setup from the water bath and place it in the test tube rack
in accordance to the test tube template. Repeat this process for all sugar-yeast solutions being tested
as quickly as possible. Place the entire test tube rack into the plastic shoe box water bath.
6. Allow a minimum of 10 minutes or maximum of 20 minutes to pass (This will be determined by the
instructor). Use a metric ruler to measure the distance from the rounded end of the test tube to the
3
gas-liquid line. This measurement is in cm. We can convert this value into cm or mL easily since the
small test tube is 1 cm in diameter.
6. Collect data from the class and calculate the average number of bubbles produced by each
solution. Construct a graph of your data and the class average by plotting the number of bubbles
counted for each test tube tested for your group. Choose a colored pencil to connect these plots. Use
a second colored pencil to connect the plots of the Class Average number of bubbles formed for each
test tube.
Observations:
1. What differences are noted in the number of bubbles observed leaving the tips of the pipettes?
2. How did the small group data compare with the class average?
Analysis and
Conclusions:
1. What was the purpose of the yeast-and-water solution?
2. What is the name of the gas that is inside the bubbles released from the solutions?
3. Explain how the counting of the gas bubbles is a way of measuring fermentation.
4. Which sugar was the best food source for yeast? Give evidence to support your answer.
5. How does the molasses differ from the other sugars that you used?
JOHNS/Fall 2008
DATA TABLE
Name:
Class Period:
CLASS DATA
Number of Bubbles T1-T12
Sugar/Yeast Solution
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
Class Average
number of
bubbles or
volume of gas
Sucrose
Dextrose (Glucose)
Apple Juice (Fructose)
Molasses
Water
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Place answers to Analysis and
Conclusions questions here.
1._________________________
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2._________________________
3._________________________
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4._________________________
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5._________________________
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Water
Sucrose
Dextrose
Apple Juice
Molasses
JOHNS/Fall 2008
JOHNS/Fall 2008
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JOHNS/Fall 2008
Material Preparation and Instructor Notes:
•
Figure 1 shows Test tubes (25mm x 200mm), disposable pipets (SAMCO ® Fine Tip Std Bulb TRANSFER
PIPETS 232), and 20mm X 9mm X 10mm hex nuts (Figure 2). Any test tube rack with wide enough holes will
suffice.
Figure 1
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•
•
•
Figure 2
Figure 3
Combine 72 ml black strap molasses and 408 ml distilled water in a beaker or any container. Repeat this process,
replacing sugars of your choice. I used sucrose, dextrose, & apple juice. This makes a 15% solution.
Cover these containers with plastic wrap if planning to perform the activity at a later date.
Figure 3 illustrates two different amounts of yeast to be used. For one class of twenty-four students (12 stations),
add 1.5 grams of yeast and 80 ml of each sugar-water solution. This will allow each station to acquire
approximately 5.8 ml of yeast-sugar solution to completely fill one disposable pipet. For six periods, use a 7 gram
packet of yeast with 480 ml of each sugar-water solution.
Figure 4 shows the volume obtained by squeezing the bulb just once. The pipet should be filled completely. The
initial intake of yeast-sugar solution should be manually forced into the bulb of the pipet. This is achieved through
centrifugal force. The left pipet in Figure 5 shows the result of this procedure.
Figure 5
Figure 7
Figure 4
Figure 6
JOHNS/Fall 2008