Fermentation

Summary: The anaerobic conversion of glucose into ethanol and carbon dioxide by yeast enzymes. Tags: igcse chemistry definition organic-chem Created: 2026-07-14 Last Updated: 2026-07-14

Fermentation is the anaerobic biochemical process by which microorganisms such as yeast convert glucose into ethanol and carbon dioxide, catalysed by enzymes (zymase). The overall word equation is glucose → ethanol + carbon dioxide, with the balanced symbol equation C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂, and the reaction is classified as exothermic, releasing energy that the yeast cells use for growth. The key conditions for successful fermentation are: an absence of oxygen (strictly anaerobic), a temperature maintained between 30 and 35 degrees Celsius (the optimum for the zymase enzyme in yeast), and a neutral pH; if the temperature exceeds about 40 degrees Celsius, the enzymes denature and the process halts, while oxygen exposure leads to aerobic respiration instead, producing CO₂ and water rather than ethanol. Fermentation stops naturally when the ethanol concentration reaches approximately 10 to 15 percent, because ethanol is toxic to the yeast at higher concentrations, which is why stronger alcoholic drinks must be produced by fractional distillation of the fermented mixture. This process is industrially significant at IGCSE level because it is the only route to ethanol that uses renewable raw materials (sugars from plants), in contrast to the non-renewable catalytic hydration of ethene; exam questions often require comparison of these two methods, including advantages and disadvantages of each.

Key term — IGCSE Chemistry (0620/0971) glossary.