Fermentation is the incomplete breakdown of carbohydrates under anaerobic conditions. It is of two types:

A. Alcoholic fermentation : It occurs in fungi such as yeast, Rhizopus etc. and bacteria. It can be described in 2 steps :

Step I : Pyruvic acid is decarboxylated into acetaldehyde in the presence of enzyme pyruvate decarboxylase and CO₂ is liberated.

Step II : Acetaldehyde is reduced to ethyl alcohol in the presence of enzyme alcoholic dehydrogenase and NADH₂ is oxidised to NAD. There is loss of energy stored in ethyl alcohol.

Importance.

I. Alcoholic fermentation is common in yeasts which are used in industrial preparation of ethyl alcohol from molasses. It is the basic process underlying the production of beers and wines.

2. Yeasts are used in baking industry as CO₂ produced during alcoholic fermentation raises its dough.

B. Lactic acid fermentation : Lactic acid fermentation is common in animals and certain bacteria. In vertebrates, lactic acid formed in muscles in the anaerobic conditions.