Students create 2 liters of a medium to which 100 grams of glucose (or other carbon source) is added. The medium contains KH2 PO4, (NH4)2 SO4, MgSO4-7H2O, yeast extract, and sodium bisulfite, and was diluted to the total volume (2 liters) by adding distilled water. We chose to create two feed solutions: one containing glucose, a simple sugar, as its carbon source; and the other containing orange-flavored Gatorade brand drink powder as the carbon source. Gatorade powder contains dextrose, a form of glucose, and sucrose, a disaccharide (more complex than glucose or dextrose). We wanted to compare the two solutions to see which would produce more ethanol per gram of carbon provided.
The mixture was pumped from a feed flask to a reactor flask, where all of the yeast resided. The pump rate was adjusted to a setpoint. Each minute, the amount of carbon dioxide expelled from the reaction flask was measured by a gas flow meter. Based on the amount of carbon dioxide produced, a computer adjusted the flow rate of nutrient feed to the reactor flask. This is an example of indrect control.
A more desirable, but also more difficult technique, would involve measuring the amount of the carbon source itself (direct control ). By measuring the amount of glucose or drink powder remaining in the reactor flask, one would know exactly how much more feed to supply in order to have just enough to keep production at a maximum without inhibiting cell growth. However, the complexity of such a measurement, and the limitations of the lab equipment make such a technique unavailable for this experiment.
The experiment was set up to run over a period of approximately 24 hours. At the end of the fermentation, students can measure the solution in the reaction flask to determine the alcohol content by percent, and therefore how much ethanol, by volume, was produced.
Click here to return to table of contents.