March, 2003, Biology Experiment:

Yielding Yeast

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

PASPORT Turbidity Sensor
(PS-2122)
PASPORT Turbidity Sensor

 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 turbidity of the suspension.
 

Background Information:

Yeast are simple unicellular fungi, the most familiar forms of which are brewer's yeast and baker's yeast. Both 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.

Turbidity refers to the relative clarity or cloudiness of a solution: the more turbid, the more suspended particles are present in the solution. Turbidity is measured in units called NTU's (nephelometric turbidity units), based on how light is scattered by the particles suspended in the solution. Measuring the turbidity of an activated yeast suspension will give some indication of the concentration of activated yeast in the sample.

Hypothesize: How will the activated yeast suspension change the turbidity of the solution? Will the solution become more or less turbid as the yeast continue to respire? What other observations can you expect to make as the yeast becomes activated?

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Equipment & Supplies:

For each lab group:
  • Turbidity Sensor: PASPORT (PS-2122)
       (Note: This sensor is not available in the ScienceWorkshop line)

  • Computer Interface:
       PASPORT: Xplorer GLX (PS-2002)  |  Xplorer (PS-2000)  |  USB Link (PS-2100A)  |  PowerLink (PS-2001)
  • Cuvettes for samples (come with the Turbidity Sensor)
  • Dry yeast -- i.e. Fleishmann's® Active Dry Yeast or similar product
  • Sugar, 1 teaspoon per package of yeast
  • Warm distilled water (approximately 40° C, or 100° F - 110° F), 200 mL per package of yeast plus
    100 mL to be used for dilution
  • Room-temperature distilled water, 6 mL for control
  • Lab glassware: flasks or beakers for mixing suspension and for dilution, pipettes and graduated cylinder (100 mL) for measuring samples

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Experimental Procedure:

Software & Probeware Setup:
  1. Ensure that your USB Link, Xplorer or PowerLink is connected to the computer.

  2. Connect the Turbidity Sensor to the USB Link, Xplorer or PowerLink. Select "Launch DataStudio" from the PASPORTAL window that appears. A Graph Display of Turbidity (NTU) vs. Time will be automatically displayed.

  3. Fill a control cuvette to the lid with approximately 6 mL of distilled water. After securely tightening the cap, use a tissue or other soft cloth to wipe fingerprints or other residue off the glass surface of the cuvette.
    Note: label cuvettes only on the screw cap (i.e. with small stickers) -- do not mark or label the glass.

  4. Calibrate the Turbidity Sensor using the cuvette filled with distilled water (0 NTU) and the 100 NTU standard solution that comes with the sensor. Refer to the sensor's Quick-Start Card for specific calibration instructions.
Equipment Setup:
  1. Prepare activated yeast solution: first dissolve 1 teaspoon sugar (sucrose) in 200 mL of warm water. Add 1 package dry yeast and stir quickly until yeast is dissolved.

  2. Immediately dilute 1.0 mL of the activated yeast suspension with 100 mL of warm distilled water. Fill a second cuvette to the lid with approximately 6 mL of this diluted sample solution. Securely tighten the cap and wipe the outside of the cuvette clean.
Data Collection & Recording:
  1. Insert the cuvette containing the diluted yeast solution into the sensor and close the lid. (Note: closing the lid keeps stray light from entering the sensor, which minimizes error. Refer to the sensor's Quick-Start Card for more tips about sample preparation.)

  2. Click the Start button ( ) to begin collecting data. Monitor the turbidity reading for 5-10 minutes, or until the reading is stable. Click the Stop button ( ) to end data collection. Remove the cuvette from the sensor.

  3. As a control, insert the cuvette containing distilled water into the sensor, click the Start button, and monitor the turbidity reading for 10-30 seconds or until the reading is stable. Click the Stop button ( ) to end data collection.

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Data Analysis:

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

  2. Examine the turbidity graph for the activated yeast suspension. Observe and discuss any changes that occurred over the course of the experiment.

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Conclusions and Extensions:

  1. Did the activated yeast suspension change the turbidity of the solution? Did the solution become more or less turbid over time as the yeast continued to respire?

  2. What additional observations provide evidence that the yeast are respiring? Contrast the appearance of the dry yeast granules with the yeast as it appears in solution.


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