Fermentation – IGCSE Chemistry Definition and Key Facts

Fermentation is part of the organic chemistry section of the Cambridge 0620 syllabus. It is tested as a method for producing ethanol and is compared with the industrial hydration of ethene — a comparison that appears as a classic 4-mark question on Paper 4.

The 0620 definition

Fermentation is the anaerobic conversion of glucose (or other sugars) to ethanol and carbon dioxide, using yeast. Yeast provides enzymes (zymase) that act as biological catalysts.

The equation

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

glucose → ethanol + carbon dioxide

This equation must be balanced — the 2 in front of both products is essential for the mark.

Conditions required

Condition	Detail	Why it matters
Yeast	Contains the enzyme zymase	Catalyses the reaction at low temperature
Temperature	25–37 °C (optimum ~35 °C)	Below this, reaction is too slow; above ~40 °C enzymes are denatured
Anaerobic (no oxygen)	Air must be excluded	If oxygen is present, ethanol is oxidised to ethanoic acid (vinegar)
Aqueous glucose solution	Dissolved sugar substrate	Provides the reactant for the enzyme to act on

The anaerobic condition is a frequently tested point: candidates must state that air/oxygen is excluded and explain why.

The reaction stops when the ethanol concentration reaches about 15%, because at this concentration the ethanol kills the yeast.

Apparatus for fermentation

In a school laboratory, glucose solution and yeast are mixed in a flask. The flask is sealed with a delivery tube leading into limewater. The limewater turns milky, confirming CO₂ production. No air can enter through the limewater, maintaining anaerobic conditions.

After fermentation, the mixture is filtered to remove yeast and then fractionally distilled to separate ethanol (boiling point 78 °C) from water (boiling point 100 °C). Fractional distillation can achieve about 96% ethanol.

Fermentation vs hydration of ethene

The 0620 syllabus requires comparison of these two methods of making ethanol.

Feature	Fermentation	Hydration of ethene
Raw material	Glucose (from crops — renewable)	Ethene (from crude oil — non-renewable)
Catalyst	Yeast/enzymes (zymase)	Phosphoric acid (H₃PO₄)
Temperature	~35 °C (low)	~300 °C (high)
Pressure	Atmospheric	~60–70 atm (high)
Rate	Slow (takes days)	Fast (continuous process)
Purity of ethanol	Low (~15% solution, needs distillation)	High (nearly pure)
Type of process	Batch	Continuous
Sustainability	Renewable and carbon-neutral	Non-renewable; releases net CO₂

Paper 4 commonly asks candidates to discuss advantages and disadvantages of each method.

Ethanol as a biofuel

Ethanol produced by fermentation of sugarcane or corn is used as a biofuel. In Brazil and parts of Southeast Asia, bioethanol is blended with petrol. Advantages include: renewable feedstock, carbon-neutral combustion in principle (CO₂ released equals CO₂ absorbed during plant growth), and cleaner burning than petrol. Disadvantages: uses farmland that could grow food, requires energy for distillation, and has lower energy density than petrol.

Worked exam question

Ethanol can be made by fermentation. (a) Write the equation for fermentation. [1] (b) State two conditions needed. [2] (c) State one advantage of fermentation over the hydration of ethene. [1]

(a) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ [1]

(b) Yeast / enzymes [1]; warm temperature / 25–35 °C / anaerobic conditions [1]

(c) Uses renewable raw material / lower energy costs / lower temperature needed [1]

Common exam mistakes

1. Writing an unbalanced equation — forgetting the 2 in front of C₂H₅OH and CO₂.
2. Not stating that conditions are anaerobic — this is a frequently required mark.
3. Saying “yeast is a catalyst” instead of “yeast contains enzymes that act as catalysts.” Yeast is an organism, not a catalyst itself.
4. Saying “room temperature” instead of “warm temperature (25–35 °C)” — the optimum is above room temperature.