Ethanol is produced through the fermentation process, which involves the action of yeast on glucose in an anaerobic environment.

Fermentation is a biological process in which sugars are converted into alcohol and carbon dioxide by yeast or bacteria. In ethanol production, the primary sugar utilized is glucose, which can be derived from various sources such as corn, sugarcane, and even cellulose. The yeast commonly employed for this process is typically a species of the genus Saccharomyces.

The fermentation process begins with yeast breaking down glucose under anaerobic conditions, meaning there is no oxygen present. Under these conditions, yeast converts glucose into ethanol and carbon dioxide. This process is distinct from aerobic respiration, where glucose is metabolized into water and carbon dioxide – but no ethanol is produced.

The chemical equation representing this fermentation process is:

This reaction is exothermic, meaning it releases heat.

Following fermentation, the ethanol produced is distilled to increase its concentration. This step is necessary because the fermentation process typically yields a solution containing only about $10\%$ to $15\%$ ethanol. Beyond this concentration, the toxicity of ethanol can inhibit yeast activity, leading to a decline in fermentation efficiency.

It’s important to recognize that the fermentation process is not $100\%$ efficient. Not all glucose is converted into ethanol and carbon dioxide; some glucose is utilized by the yeast for growth and reproduction, and a portion may be transformed into other by-products.

In summary, the production of ethanol via fermentation involves the anaerobic breakdown of glucose by yeast, resulting in the formation of ethanol and carbon dioxide. This process is widely utilized in the production of alcoholic beverages and biofuels.