In anaerobic conditions, pyruvate undergoes fermentation. In humans and most animals, this fermentation leads to lactic acid as a product. In certain anaerobic organisms, like yeast, pyruvate will be converted into ethanol instead.
In humans, pyruvate, also known as pyruvic acid, is produced during glycolysis. Glycolysis is the process of breaking up glucose for energy that cells can use. Pyruvate can then go into the mitochondria and enter the citric acid cycle if oxygen is available. If this aerobic pathway is not possible, pyruvate undergoes fermentation with the help of two NADH molecules, which produces lactic acid. This acid can then be transported from the cell and stored in the liver for later use.