Organic Chemistry – Fermentation of Glucose

Fermentation of glucose

The fermentation process is carried out by enzymes in yeast which converts glucose into ethanol.

C₆H₁₂O₆ (aq) → 2C₂H₅OH (aq) + 2CO₂ (g)

Wine and beer are the products of fermentation. Wine is produced by fermenting sugars in grapes. Beer is produced by fermenting malt. Fermentation is a slow process.

A simple fermentation set up is shown in the image.

Procedure

Mix the glucose solution with yeast catalyst in a conical flask.

Maintain the temperature at 37°C.

Rubber bung is secured to the flask and a test tube of limewater is attached to the conical flask to prevent air from entering the set-up. Carbon dioxide gas produced will form white precipitate in the limewater. Dilute solution of ethanol (up to about 15%) is produced.

Things to note

The optimum temperature for the reaction is at 37°C. The speed of fermentation decreases as temperature reduces. At higher temperatures, the enzymes will be denatured. The enzyme structure in yeast is destroyed by heat and no longer acts as catalyst.

The product of fermentation is a dilute solution of ethanol. It contains up to about 15% of ethanol. A higher alcohol content is not produced as it causes the yeast to die and fermentation to stop.

The ethanol is separated from the water by fractional distillation. The ethanol will be distilled first as it has lower boiling point of 78^oC than water, which has boiling point of 100^oC. This allows for higher concentrations of alcoholic beverages or pure ethanol to be produced.

Fermentation takes place in the absence of oxygen. In the presence of oxygen in the air, the ethanol in the alcoholic drinks will be oxidised to ethanoic acid, which is sour. Hence, fermentation can only take place in air tight apparatus. The rubber bung is secured tightly to the flask. The limewater in the test tube prevents air from entering the apparatus.