Lactic Acid Fermentation
Your body needs energy, and it gets that energy through cellular respiration. Cellular respiration begins with glycolysis, where glucose, NAD+, and ADP become pyruvate, NADH, and ATP. If there is oxygen, this continues to the Krebs Cycle, where more ATP and NADHs are produced, as the pyruvates are stripped of more carbons. However, when there is no oxygen, lactic acid fermentation occurs.
After glycolysis, you are left with 2 Pyruvates and 2 NADHs and 4 ATP (2 ATP were needed to go through glycolysis). Generally, the idea of lactic acid fermentation is to enable glycolysis to occur over and over again.
In fermentation, these pyruvates help oxidize the NADHs into NAD+s, thus allowing glycolysis to keep going. These pyruvates turn into lactate, or lactic acid. Without NAD+, glycolysis cannot occur, so replenishing them is vital for the body to keep producing energy, albeit less efficiently than if it was aerobic.
Numerous foods use lactic acid fermentation to craete the taste. For example, the bacteria lactobacillus are used in yogurt. In addition, kimchi, sauerkraut, and kefir and given their sour taste through lactic acid fermenation.
When you exercise, lactic acid fermentation takes place, which causes your muscles to feel sore.
Alcohol fermentation is what creates alcohol, and it is pretty similar.
Pyruvate loses a carboxyl group and produces acetaldehyde and carbon dioxide. That acetaldehyde is reduced to ethanol, while NADH is oxidized into NAD+. So the products of alcohol fermentation are ethanol and CO2.
Many foods, such as bread, wine, and beer use yeast to perform fermentation. The yeast takes in glucose and goes through alcohol fermentation. The reason bread doesn't have alcohol is that the intense heat of the oven bakes it off.