Yielding Yeast

Investigate the activation of baker's yeast through observation and monitor the turbidity of the suspension.

Lab Summary

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

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.

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: 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?

Published: March 2003

Downloads

• Yielding Yeast (10 KB, .zip)

  Includes experiment setup and procedures

Here's What You Need

U.S. Educator prices shown.

Probeware (PASPORT Systems)

Xplorer GLX (PS-2002) - $349

The Xplorer GLX is a data collection, graphing, and analysis tool designed for science students and educators.

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PASPORT Turbidity Sensor (PS-2122) - $129

Measures the turbidity level (cloudiness) of water in nephelometric turbidity units (NTU). For use with PASPORT Interfaces.

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Other Materials

• 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