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This activity is designed to show students how fermentation occurs.

1. Brewer's yeast
2.  Closed container with a sealed balloon on top (like a coke bottle)


Fermentation is a "respiration" which does not use oxygen and which for this reason is called anaerobic ("without air"). In these experiments we will point out the production of carbon dioxide during fermentation.

The oxidization of glucose in living organisms occurs in two principal phases:  glycolysis and the oxygen-burning respiration.

Glycolysis happens in the cytoplasm of cells, respiration inside the mitochondria. Yeast cells can grow both with and without oxygen, but in the absence of oxygen, the yeast cells limit themselves to use glycolysis. In this case, they demolish the molecule of glucose into molecules of pyruvic acid which then they convert into acetaldehyde and lastly into ethyl alcohol or ethanol. During these reactions, the yeast cells obtain energy for their own needs.

Yeast organisms are a kind of mold that "feeds on" food rich in carbohydrates, such as corn, rice, or cereal grains. Yeasts secrete enzymes that break down these complex carbohydrates into simple sugar, which can get inside the yeast organisms because their membrane has transporter proteins that move glucose into the organism.

During fermentation, the yeast cells yield from 12 to 17% alcohol. Besides the energy, the other important product of this fermentative process is carbon dioxide. In fact wine and beer are often rich in this gas.


1 - Introduce some brewer's yeast (Saccharomyces cerevisiae) mixed with water and sugar in a flask. Close the container with a rubber balloon. As time passes, you will see the balloon inflate, showing the production of carbon dioxide.

2 - It is possible toget quantitative measures, which will allow you to evaluate the effect of parameters such as the temperature, the substrate composition, etc. Close the bottle with a rubber plug and pass a pipette with a capillary end through it. Place a drop of colored water inside the capillary. Now, knowing the internal diameter of the capillary, the shifting of the drop along the tube in a given period of time will give you the amount of CO2 produced.
3 - Make other tests replacing the sugar with cabbage cut into thin pieces and boiled, potatoes, apples, crushed grapes, vegetables, etc. Often, these products are rich in starch, a compound similar to sugar and which is used the same as sugar by the yeast. Evaluate the different productivity in carbon dioxide by the different substrates. It is indicative of how much sugar and starch are in these substances.


http://gened.emc.maricopa.edu/bio/bio181/BIOBK/BioBookGlyc.html   Cellular Metabolism and Fermentation

http://www.accessexcellence.com/LC/SS/ferm_index.html   Microbial Fermentations: Changing The Course Of Human History (Information)

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