In its broadest sense, fermentation refers to any process by which large organic molecules are broken down to simpler molecules as the result of the action of microorganisms. The most familiar type of fermentation is the conversion of sugars and starches to alcohol by enzymes in yeast. To distinguish this reaction from other kinds of fermentation, the process is sometimes known as alcoholic or ethanolic fermentation.

Ethanolic fermentation was one of the first chemical reactions observed by humans. In nature, various types of spoil decompose because of bacterial action. Early in history, humans discovered that this kind of change could result in the formation of products that were enjoyable to consume. The spoilage (fermentation) of fruit juices, for example, resulted in the formation of primitive forms of wine.

The mechanism by which fermentation occurs was the subject of extensive debate in the early 1800s. It was a key...