Making Alcohols (Fermentation)

Fermentation Process

Fermentation is a biological process where yeast converts glucose into ethanol and carbon dioxide without oxygen. This is an example of anaerobic respiration.

The overall word equation for fermentation is:

Glucose → Ethanol + Carbon dioxide

In chemical terms, the equation is:

$$\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2 \text{C}_2\text{H}_5\text{OH} + 2 \text{CO}_2$$

Yeast contains enzymes that catalyse the conversion of glucose molecules into ethanol and carbon dioxide.

The process works best at a temperature between 306C and 406C. If the temperature is too low, the reaction is slow; if too high, the yeast enzymes denature and stop working.

For example, if you start with 180 g of glucose, fermentation produces 92 g of ethanol and 88 g of carbon dioxide (based on molar masses and stoichiometry).

Conditions for Fermentation

Fermentation requires specific conditions to work efficiently:

• Anaerobic environment: No oxygen must be present because yeast respires anaerobically to produce ethanol.
• Optimum temperature: Around 30-406C to keep enzymes active without denaturing.
• Sugar source: Glucose or other sugars (like sucrose or fructose) are needed as the raw material.
• Maximum ethanol concentration: The reaction stops when ethanol concentration reaches about 15% because ethanol becomes toxic to yeast.

If oxygen is present, yeast respires aerobically, producing carbon dioxide and water instead of ethanol.

For instance, if fermentation is carried out at 256C instead of 356C, the rate of ethanol production will be slower because enzyme activity decreases at lower temperatures.

Uses of Fermentation

Fermentation has important practical uses:

• Alcoholic drinks: Brewing beer, making wine, and producing spirits all use fermentation to produce ethanol.
• Biofuel production: Ethanol made by fermentation can be used as a renewable fuel, either pure or blended with petrol.
• Renewable resource: Fermentation uses sugars from crops like sugar cane or maize, which can be regrown, making it more sustainable than fossil fuels.

Advantages and Limitations of Fermentation

Fermentation has several advantages:

• Renewable raw materials: Uses sugars from plants, which can be replanted each year.
• Low energy process: Occurs at moderate temperatures and pressures, so less energy is needed compared to chemical synthesis.

However, there are limitations:

• Slow reaction rate: Fermentation takes hours or days, slower than industrial chemical methods.
• Limited ethanol concentration: Maximum ethanol concentration is about 15% before yeast dies, so further purification (distillation) is needed for higher concentrations.

Example: Calculating Ethanol Produced from Glucose

If 180 g of glucose undergoes fermentation, how much ethanol is produced?

Molar masses: glucose (C₆H₁₂O₆) = 180 g/mol, ethanol (C₂H₅OH) = 46 g/mol.

From the equation:

$$\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2 \text{C}_2\text{H}_5\text{OH} + 2 \text{CO}_2$$

1 mole of glucose produces 2 moles of ethanol.

Number of moles of glucose = $\frac{180}{180} = 1$ mole.

Therefore, moles of ethanol produced = $2 \times 1 = 2$ moles.

Mass of ethanol = $2 \times 46 = 92 \text{ g}$.

So, 180 g of glucose produces 92 g of ethanol.