Fermentation (Edexcel GCSE Chemistry)

Fermentation

• Ethanol (C₂H₅OH) is one of the most important alcohols

• It is used as fuel (for vehicles in some countries) and as a solvent

• It is the type of alcohol found in alcoholic drinks such as wine and beer

• It can be produced by fermentation where sugar or starch is dissolved in water and yeast is added

• The mixture is then fermented between 15 and 35 °C with the absence of oxygen for a few days

• Yeast contains enzymes that break down sugar to alcohol

• If the temperature is too low the reaction rate will be too slow and if it is too high the enzymes will become denatured

• The yeast respires anaerobically using the glucose to form ethanol and carbon dioxide:

C₆H₁₂O₆ + enzymes → 2CO₂ + 2C₂H₅OH

• The yeast is killed off once the concentration of alcohol reaches around 15%, hence the reaction vessel is emptied and the process is started again

• This is the reason that ethanol production by fermentation is a batch process

Purifying the Product

• Fermentation produces a dilute solution of ethanol which needs to be separated from the reaction mixture.

• This is done using fractional distillation.

• The mixture is heated to 78 ºC which is the boiling point of ethanol but below that of water (100 ºC).

• The ethanol evaporates and its vapours pass through a condenser, where they cool and condense, forming liquid ethanol.

• The water and any other impurities remain behind in the reaction flask.

• When the temperature starts to increase to 100 ºC heating should be stopped. The water and ethanol have now been separated.

Diagram showing how fractional distillation is used to separate ethanol from water in the laboratory