April, 2003, Biology Experiment:

Yielding Yeast, Part 2

- Purpose
- Background Information
- Equipment & Supplies
- Experimental Procedure
- Data Analysis
- Conclusions and Extensions

Ohaus ScoutTM II Balance
(model #SR2020)

Ohaus Balance
Yeast

(Photo published in Microbiol. Rev. 54: 381-431, 1990)
Saccharomyces cerevisiae, baker's or brewer's yeast. Bud scars of six daughter cells are shown in blue.

Purpose:

Students will investigate the activation of baker's yeast through observation and by monitoring the temperature and mass of the suspension.

*Note: This experiment requires DataStudio version 1.8.5, available approximately May 1, 2003, as a free download from store.pasco.com/forms/DSdownload.cfm.

Background Information:

Yeast are simple unicellular fungi, the most familiar forms of which are brewer's yeast and baker's yeast. Both of these types of yeast are strains of the species Saccaromyces cerevisiae. Yeast cannot carry out photosynthesis but still have simple nutritional needs -- a carbon source, a nitrogen source, salts and trace elements, and the vitamin biotin. Baker's yeast is an essential ingredient for breadmaking since it causes the dough to rise: as the yeast metabolize sugar, carbon dioxide gas is produced. These gas bubbles cause the dough to rise and contribute to bread's light texture. When purchased in its dry form, yeast must first be activated by dissolving it in warm water and introducing sugar to the solution. The foam or froth that is produced and the "yeasty" aroma are signs that the yeast is actively respiring.

Hypothesize: Considering the metabolic processes of the yeast in the activated suspension, predict what will happen to the net mass of the system as activation proceeds. What other observations can you expect to make as the yeast becomes activated?

Back to top

Equipment and Supplies:

For each lab group:

Activated yeast suspension: Dissolve 1 teaspoon table sugar (sucrose) in 200 mL warm water (approximately 40°C, or 100°F - 110°F). Add one envelope (approximately 7 grams) dry yeast (Fleishmann's® Active Dry Yeast or similar product) and mix well. This 200-mL suspension will be enough for four lab groups.

*Note: Do not prepare this ahead of time! Prepare the suspension just before students are ready to collect data so that activation is just beginning.

Back to top


Experimental Procedure:

Software & Probeware Setup

 ScienceWorkshop users: This experiment can be conducted using the Ohaus Balance only, without the Temperature Sensor. See "Teacher's Hints" for setup instructions.
  1. Plug the Ohaus electronic balance into an electrical outlet and then connect it to the computer using the 9-pin serial connector (RS232 cord) supplied with the balance.
    * Note: The cable is unidirectional; be sure to plug the labeled "scale" end into the balance. If the cable is plugged in the opposite way, the balance will blink continuously once the power is turned on and you will have to reverse the cable.

  2. Turn the power on to the balance.

  3. Ensure that your USB Link, Xplorer or PowerLink is connected to the computer and then connect the Temperature Sensor to the interface.

  4. The PASPORTAL window will automatically appear. Choose "Launch DataStudio."

  5. Click the Setup button ( ) in the main toolbar to open the Experiment Setup window.

  6. Click the Add Ohaus Mass button. The Ohaus balance icon will appear in the Summary window and a graph of Mass vs. Time will be automatically added to the Graph window.

  7. Close the Setup window. Click the Align matching X Scales button ( ) and resize the display as needed.

Data Collection & Recording

  1. Measure 50 mL of activated yeast solution and pour into the plastic beaker.

  2. Place the beaker containing the yeast suspension atop the balance. (We recommend using a plastic beaker in order to stay within the measuring limits of the balance.) Place the Temperature Sensor’s probe into the suspension.

  3. Click the Start button ( ) to begin recording temperature and mass data.

  4. As the computer continues to collect data, record additional observations that indicate the yeast are becoming activated. Continue for approximately 20 minutes, and then click the Stop button ( ) to end data collection.
    *Note: Observe the suspension carefully so that it does not become too foamy; avoid spillover onto the balance. If necessary, stir briefly and as gently as possible with the Temperature Sensor to reduce the foam level.

Data Analysis:

  1. Scale the axes to fit the data using the Scale to Fit button ( ) in the Graph toolbar.

  2. Examine the temperature and mass graphs for the activated yeast suspension. Observe and discuss any changes that occurred over the course of the experiment.

Conclusions and Extensions:

  1. How does the appearance of the activated yeast suspension change as activation occurs? What evidence of physical changes and chemical changes do you observe?

  2. How quickly did the temperature of the suspension change as activation proceeded? Did the rate of temperature change correspond to any changes in mass?

  3. How does the graph of Mass vs. Time change as activation proceeds? Propose an explanation.

  4. Predict how the results of this experiment would be different if warmer or cooler water, or more or less sugar, was used. Repeat the experiment and test your predictions.