Glycolysis — the metabolic pathway that converts glucose (a type of sugar) into pyruvate — is the first major step of fermentation or respiration in cells. … In glycolysis, a single molecule of glucose (with six carbon atoms) is transformed into two molecules of pyruvic acid (each with three carbon atoms).Humans have taken advantage of the metabolism in a tiny fungus yeast and bacteria to create fermented foods from grains, vegetables and fruits such as kimchi and kombucha. Glucose is converted into pyruvic acid during glycolysis. When oxygen is available, pyruvic acid enters a series of chemical reactions (known as the tricarboxylic acid cycle) and proceeds to the respiratory chain. As a result of respiration, cells produce 36–38 molecules of ATP for each molecule of glucose oxidized. In the case of a ferment such as kombucha which is an aerobic process (first stage of fermentation is done with oxygen present) which has become known as the crabtree effect due to fermentation happening even when oxygen is available.
In the absence of oxygen (anoxygenic conditions), pyruvic acid can follow two different routes, depending on the type of cell. It can be converted into ethanol (alcohol) and carbon dioxide through the alcoholic fermentation pathway, or it can be converted into lactate through the lactic acid fermentation pathway.
Since Louis Pasteur’s work, several types of microorganisms (including yeast and some bacteria) have been used to break down pyruvic acid to produce ethanol in beer brewing and wine making. The other by-product of fermentation, carbon dioxide, is used in bread making and the production of carbonated beverages. Other living organisms (such as humans) metabolize pyruvic acid into lactate because they lack the enzymes needed for alcohol production, and in mammals lactate is recycled into glucose by the liver (Voet & Voet 2004).