Fermentation কী?

Some foods are made by putting living things too small to see — microbes — to work on a raw material. Their feeding transforms it: the transformation is called fermentation, and humans have run it for thousands of years, since long before anyone knew the workers existed.

Without oxygen, some cells break glucose down only partially, in the cytoplasm, into smaller organic molecules that still hold most of the energy: this is fermentation. Yeast performs alcoholic fermentation,

and muscle cells short of oxygen, as well as the bacteria of yoghurt, perform lactic fermentation, turning glucose into lactic acid. A fermentation yields about fifteen times less ATP per glucose than respiration.

Without oxygen the respiratory chain stops, NADH cannot be re-oxidised, and glycolysis would halt within seconds for lack of $\mathrm{NAD^+}$. Fermentation regenerates it by using pyruvate itself as the electron acceptor: in lactic fermentation (muscle, red cells, lactic bacteria) pyruvate is reduced to lactate; in alcoholic fermentation (yeast, some plants) it is decarboxylated to acetaldehyde, which is reduced to ethanol, releasing $\mathrm{CO_2}$. The yield is glycolysis’ own: 2 ATP per glucose, a fifteenth of respiration’s; the carbon is barely oxidised and the products still hold nearly all the fuel’s energy. Anaerobic respiration — an electron-transport chain ending on nitrate, sulfate or carbonate instead of oxygen, in many bacteria — is a different thing, treated in the Year 2 volume.