Yeast fermentation refers to the process in which yeast cells decompose carbohydrates (such as glucose, fructose, etc.) into ethanol and carbon dioxide. The principle is as follows:

1. First, yeast cells decompose glucose into two molecules of three-carbon compounds (ie, pyruvate) through enzyme action.

2. Next, the yeast cells further break down pyruvate into ethanol and carbon dioxide.

3. Ethanol will be accumulated in the liquid as a product, and carbon dioxide will be released into the atmosphere.

The whole process requires energy, which is why yeast fermentation requires certain temperature and nutrient support. When the temperature is too high or too low, the activity of yeast cells will decrease; and if there is a lack of sufficient nutrients, the yeast cells will not be able to carry out decomposition reactions.

Yeast fermentation is widely used in food processing such as wine making, bread, and yogurt. At the same time, some special properties of yeast are also used in fine chemical synthesis, genetic engineering and other aspects.