Fermentation is of different types and takes place under anaerobic conditions mostly in saprophytic microorganisms like certain bacteria and fungi. However, it may also take place in higher organisms under certain conditions.

The two most common types of fermentation are (1) alcoholic fermentation and (2) lactic acid fermentation.

(1) Alcoholic fermentation : the type of fermentation in which ethyl alcohol is the main end product .This is very common in yeast (unicellular fungus) and also seen in some bacteria. Yeast cells release enzymes called zymase complex which bring about the fermentation. The reactions are similar to anaerobic respiration.

(2) Lactic acid fermentation : The type of fermentation in which lactic acid is the end product.

It is carried out by some bacteria (e.g. lactic acid bacteria), and also by animals (muscle glycolysis in animals, under oxygen deficiency, results in the formation of lactic acid this is whay we experience in muscle cramps, or "Charley horse").

Lactic acid bacteria can ferment milk sugar lactose (C12H22O11) to lactic acid. The process is extracellular.

This brings about curdling of milk.

Commercial application of fermentation : There are various kinds of fermentation carried on by different microorganisms. Many of these result in highly useful end products. Such useful microbial activity is used on large industrial scale to obtain the useful end products for the benefit of mankind. Some of the industrial products of the microbial fermentation activities are (a) antibiotics (b) vitamins (c) industrial alcohol (d) bakery products (e) some dairy products (f) tanning of leather (g) curing of tea and coffee (g) lactic acid (h) butric acid, (i) acetic acid etc.