Making ethanol by fermentation

Ethanol is the alcohol found in beer, wine and spirits. It is also used as a fuel for vehicles, either on its own or mixed with petrol. Ethanol is also used as a solvent. Ethanol is produced by fermentation and concentrated by fractional distillation.

Fermentation is an anaerobic process:

glucose → ethanol + carbon dioxide

Yeast provides the enzymes required for fermentation. The temperature has to be warm but not too warm.

The typical conditions required for fermentation are:

  • sugars dissolved in water, mixed with yeast
  • anaerobic conditions (no air can get in)
  • 25°C – 35°C temperature

Fermentation is a slow reaction. It may take days or weeks to finish. It ends when the ethanol concentration reaches about 15%, killing the yeast. If oxygen gets in, the ethanol will oxidise into ethanoic acid and the drink will taste like vinegar.

Two test tubes connected by tube. Left tube contains yeast in sugar solution, vegetable oil on top.  Right tube contains limewater. Bubbles are coming from bottom of connecting tube within limewater.Investigating fermentation in the lab

Describe what happens to the limewater during the experiment shown above.

The limewater changes from colourless to milky. This is because fermentation produces carbon dioxide.

Reactions of alcohols

Methanol is used as a chemical feedstock. It’s toxic, so it is added to industrial ethanol (methylated spirits) to stop people from drinking it.

Ethanol is the alcohol in beer, wine and spirits. It is also a fuel and a solvent.

Propan-1-ol and propan-2-ol are fuels and solvents.