Fermenting Power of Bread Yeasts

Introduction:

Bread dough is usually leavened by baker's yeast (actively gas-producing strains of Saccharomyces cerevesiae). Yeasts ferment the sugar in the dough, producing ethanol and carbon dioxide. CO₂ is the leavening agent and the alcohol evaporates off during baking. Sometimes other gas-producing microorganisms are involved in bread leavening--these usually are heterofermenting lactic acid bacteria (sourdough bread or salt-rising bread).

Commercial yeast is prepared and sold in two forms: yeast cakes and active dry yeasts. Yeast cakes contain, in addition to yeast cells, small amounts of starch, vegetable oils, and some lactic acid bacteria. Active dry yeast is made by drying the yeast cells to less that 80% moisture. These yeast cells are dried carefully at low temperatures so the cells will survive. When these yeast cells are stored at room temperature, they will retain their "dough-raising" ability for many months.

You will be studying the difference between the active dry yeast and the yeast cakes.

Materials:

50 or 100 mL graduated cylinder (to measure 30 mL distilled water)

100 mL graduated cylinder, greased with vaseline

flour

two brands, A and B, of active dry yeast (either A or B should be a yeast cake)

Saccharomyces cerevesiae, young streak culture (solid medium)

square sheets of brown wrapping paper

buffered methylene blue stain:

mix 1 part of 1:5000 methylene blue and 1 part of a phosphate buffer solution (99.25 mL of 0.2 M KH₂PO₄ to 0.25 mL of 0.2 M Na₂HPO₄) to give pH or 4.6

microscope

microscope slides and coverslips

Procedure:

A. Examine yeast cells

• prepare wet mounts of yeasts
• stain slides using methylene blue stain
• microscopically examine yeast cells
• record the percent of living cells (will appear white)
• record the percent of dead cells (will appear blue)

B. Fermenting ("dough-raising") power of yeasts

• combine 50 g flour, 1 g dry yeast A, and 30 mL water on brown wrapping paper--knead vigorously for 5 minutes
• repeat step above using yeast B
• place into separate greased cylinders and record volume
• incubate at room temperature and record volume at various time intervals

References:

• Boyd, Robert F. 1988. General Microbiology 2nd ed. Times Mirror/Mosby College Publishing, St. Louis
• Frazier, W.C., and D.C. Westhoff. 1978. Food Microbiology, 3rd ed. McGraw-Hill, Inc., New York
• Grula, Dr. Mary M., Oklahoma State University, Dept. of Botany/Microbiology, Stillwater, OK 74.78-0289

Back to Laboratory Exercises

Back to Biotechnology Homepage

Up to Top of Current Page