Yeast fermentation gives the following during reaction:
C6H12O6-------> 2CH3CH2OH + 2CO2 + energy
Enzymes
Glucose ---------------> carbon dioxide + ethanol + energy (ATP)
(a gas)
Material:
- 5 – fermentation tubes, labeled 1 through five.
- 5 large test-tubs
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constant-temperature water bath (37oC)
- pipettes
- mm ruler
- 20% solution of each: dextrose, fructose, sucrose, maltose
- distilled water
- 10% baker’s yeast suspension
Procedure:
- Five fermentation tubes were labeled upside down near the bottom.
- To each test tube 2 ml of water is added and 2ml of each of the sugar solution in different test tubes, to each tube 2ml of yeast suspension and then the tubes were brought up with water.
- Each fermentation tube was inserted in the larger tube as far it could go and then it was inverted.
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After all the fermentation tubes were set up, the height of the liquid was recorded and the tubes were placed in a constant water bath of (37oC).
- Tubes were left in this condition for an hour, after they were removed from the water bath and re-measured.
- The difference in height between the initial and final reading was recorded in the observation.
Observation:
Test tube 1 – no initial reaction between the yeast suspension and water. After an hour in the water bath no CO2 was evolved.
Test tube 2 – one to two bubbles seen in test tube after yeast was added to the dextrose. After an hour 3 mm of CO2 was evolved.
Test tube 3 – a few more bubbles could be seen in the yeast and fructose reaction than the reaction with dextrose. After an hour the fermentation test tube was completely buoyant and CO2 completely filled the tube, therefore 94mm of CO2 was evolved during this reaction.
Test tube 4 - reaction with sucrose and yeast similar to that of test tube 2. After an hour 8mm of CO2 was evolved.
Test tube 5 – reaction with maltose was similar to that of test tube 3. After an hour the test tube was completely buoyant and the test tube was filled with the gas. Therefore 94mm of CO2 was evolved in this reaction.
Discussion:
The yeast, Saccharomyces cerevisiae, performs alcohol fermentation when oxygen in unavailable. One of the products alcohol fermentation is CO2. The rate of CO2 evolution can be used as an indication of the relative rate of fermentation of the yeast organisms. Several factors can affect fermentation rate: concentration of yeast, concentration of the fuel molecules, type of fuel molecules, and temperature. Not all cell respiration is aerobic. Fuel molecules can be oxidized without oxygen to yield smaller amounts of ATP. The fermentations involve the partial breakdown of glucose without using oxygen. Fermentations include glycolysis, but not the oxidative reactions that occur in the mitochondria. Many prokaryotes have a variety of fermentation pathways, using a variety of different fuel molecules. Most eukaryotes have a fermentation pathway, used when oxygen is lacking. However, most eukaryotes are obligate aerobes. They cannot survive without aerobic respiration.
In test tube one there was no fuel present for the yeast to act upon so there was no production of carbon dioxide. In test tube two, dextrose reacted with the yeast to yield some carbon dioxide. While this was occurring, simultaneously the enzymes present in the yeast broke down the dextrose (glucose) to produce ethyl alcohol.
In test tube three, the entire test tube was filled with gas by the buoyancy that it exhibited. Fructose like glucose is a six-sugar carbon of a different shape.
Also fructose is sweeter than glucose and is preferred by the commercial industry when making goods that need sweetening. Because of this the amount of energy provided by the fructose was higher than that of the dextrose even though their chemical makeup is the same. In test tube four the reaction with sucrose yielded a little more carbon dioxide than that of test tube two. The yeast enzymes needs to first hydrolyze sucrose, a disaccharide and a reducing sugar before fermentation can occur. When hydrolyzation occurs two 6-carbon sugars are produced, one glucose and one fructose.
There is a 50-50 mix of the sugars so it is called an invert sugar.
In test tube five, maltose is also a disaccharide and reducing sugar but when it is hydrolyzed it produces two glucose molecules, there is no mixed sugars.
Because of this the entire of test tube five was filled with carbon dioxide indicated by its buoyancy and the liquid being pushed out into the larger test tube. There may be a difference in the amount produced by sucrose even though sucrose when hydrolyzed produced fructose, which by itself produced more carbon dioxide than glucose; because it is a mixed sugar the amount of carbon dioxide evolved was less. On the other hand maltose’s hydrolyzation produced two glucose molecules, which is twice as much fuel for fermentation and therefore evolved a test tube full of carbon dioxide gas.
Conclusion:
The enzyme alcohol dehydrogenase of yeast is stereo specific to monosaccharide to produce an ethyl alcohol in its presence. In this experiment the temperature was kept at 37oC in order to maintain the rate of the reaction.
A graph of fermentation rate shows that between 20-40oC the rate of an enzyme yeast reaction rises and then tapers off at around 40oC because yeast cells work faster at this temperature because they have more energy. At 40oC the enzyme changes shape because of its stereo specificity at a particular temperature and the enzyme stops working, and no more product is produced.