Fermentation of Carbohydrates

Specification Point 9.33C:
  • Describe the production of ethanol by fermentation of carbohydrates in aqueous solution, using yeast to provide enzymes
  • Ethanol (C2H5OH) is one of the most important alcohols.
  • It is the type of alcohol found in alcoholic drinks such as wine and beer and is also used as a solvent and as a fuel in some cars.
  • It can be produced either by fermentation or by hydration of ethene by steam.
  • Most ethanol is made by fermentation which uses plants as the raw material, which are renewable making it a sustainable process.
  • The plants used are sugar beet or sugar cane, both of which are high in sugar content.

 

  • The sugar is dissolved in water and yeast is added.
  • The mixture is then fermented between 15 and 35°C in the absence of oxygen for a few days.
  • If the temperature is too low the reaction rate will be too slow and if it is too high the enzymes will become denatured.
  • Yeast contains enzymes that break down starch or sugar to glucose.
  • The yeast respire anaerobically using the glucose to form ethanol and carbon dioxide:

C6H12O6 + Enzymes → 2CO2 + 2C2H5OH

  • The yeast are 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.

Separation of Ethanol from Fermentation Mixture

Specification Point 9.34C:
  • Explain how to obtain a concentrated solution of ethanol by fractional distillation of the fermentation mixture
  • 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 78ºC which is the boiling point of ethanol but below that of water (100ºC).
  • 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. 

Fractional Distillation of Ethanol, Edexcel GCSE Chemistry

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

Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.