| Orientation:
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| This activity is designed to show
students how fermentation occurs. |
| Supplies: |
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1. Brewer's yeast
2. Closed container with a sealed balloon on top
(like a coke bottle)
|
Background:
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.
Experiment:
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.
Sources:
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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