Alcohol Production by Yeasts

Many microorganisms produce materials of food value and/or are important in the food industry. For many years, the only source of ethanol for industrial use was from the fermentation of sugars by yeast. All alcohol produced for human consumption is still prepared this way. The basic reaction involves the breakdown of glucose to carbon dioxide and ethanol. The glucose is converted to pyruvic acid and the pyruvic acid may be converted into many different end products such as ethanol, lactic acid, etc.

Alcohol is a food item produced by yeast when sugars are fermented. Fermentation is mainly the breakdown of glucose into smaller molecules. The chemical formula for fermentation can be diagramed as:

Living cells can tolerate only certain concentrations of alcohol (or lactic acid). In media in which yeast are fermenting sugars, once the concentration builds beyond 12% the cells die and fermentation ceases. This is why most wines have an alcohol concentration of 12%.

The major types of fermentation are:

Lab Materials:

SDA culture of Saccharomyces cerevisiae
malt extract broth
sterile water
hot plate and beaker with water

Lab Exercise:

Each group will:

  1. Inoculate a malt extract broth with Saccharomyces cerevisiae. Label this tube A. Do not boil.
  2. Add three raisons to each of two tubes of sterile water. Label these tubes B and C. Tube B will not be boiled or inoculated.
  3. Boil tube C for 10 minutes. Let the tube cool to room temperature, and inoculate with Saccharomyces cerevisiae.
  4. Incubate all three tubes at room temperature.


Questions for Review:

The University of Southern Mississippi
Delia Anderson and John C. Jones
Last updated November 10, 1999