Upper secondary · 15 min

Alcoholic fermentation of glucose

Watch yeast turn a sugar solution into ethanol while carbon dioxide bubbles out of the fermenter — a living, anaerobic chemical factory.

Goal

Balance the fermentation equation C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂, link the gas bubbles to CO₂ production, and know why temperature matters.

Apparatus and reagents

Glucose or sugar solution, baker’s yeast (Saccharomyces cerevisiae), a flask or bottle, a fermentation lock or balloon, warm water bath and a thermometer.

Procedure

Read the diagram: glucose solution on the left is converted, in the yeast’s hands, to ethanol on the right while CO₂ leaves through the gas space above.
Set the temperature slider near 30 °C — the comfortable range for baker’s yeast. Much colder slows the enzymes; much hotter kills the cells.
Balance the equation yourself: one glucose gives two ethanol and two carbon dioxide molecules — count the carbons on both sides.
In a real setup, attach an airlock or balloon: the escaping CO₂ inflates it and keeps oxygen out, preserving the anaerobic pathway.

What to observe

The gas region above the liquid fills with CO₂ — in the fermenter this appears as steady bubbling through the airlock.
Fermentation stops on its own near 12–15 % ethanol: the product becomes toxic to the yeast that made it.

Explanation

Yeast first splits glucose through glycolysis into pyruvate, then decarboxylates and reduces it to ethanol, releasing CO₂. The cells gain only about 2 ATP per glucose — far less than respiration — which is why fermentation runs fast and hot. The bubbling seen in the simulation stands for the gaseous half of the equation.

History of the experiment

Fermented drinks are thousands of years old, but the chemistry only became clear when Louis Pasteur showed in 1857–1860 that fermentation is carried out by living yeast cells, not by spontaneous generation. Eduard Buchner then proved in 1897 that cell-free yeast extract still ferments sugar — the discovery of enzymes working outside cells, which earned the 1907 Nobel Prize.

Chemists behind it

Louis Pasteur Emil Fischer