The fermentation equation
Word equation: glucose → ethanol + carbon dioxide (+ small amount of energy released)
Chemical equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Like human anaerobic respiration, fermentation in yeast releases much less energy per glucose molecule than aerobic respiration because the glucose is only partially broken down. The remaining energy stays locked in the ethanol molecule.
Applications of yeast fermentation
Bread making: yeast is mixed into dough. The carbon dioxide produced during fermentation forms bubbles that expand in the warm dough, causing it to rise. The ethanol evaporates during baking. The dough is kept warm to increase the rate of fermentation (up to the enzyme optimum temperature).
Brewing and wine making: yeast ferments sugars in grain (beer) or grape juice (wine) to produce ethanol. The carbon dioxide may be retained (carbonated drinks) or allowed to escape. The process is carried out in sealed containers to maintain anaerobic conditions.
Investigating fermentation rate
A common practical involves placing yeast and glucose solution in a flask connected to a gas collection system (for example, an inverted measuring cylinder over water). The volume of carbon dioxide collected per unit time measures the fermentation rate.
To investigate the effect of temperature, change only the temperature of the water bath and keep glucose concentration, yeast concentration and volume constant. Expected result: rate increases with temperature up to about 40 °C, then decreases as enzymes in the yeast become denatured.