Fermentation

Fermentation is a kind of anaerobic respiration, carried out primarily by fungi and bacteria.

Although people had been using this process in the preparation of wines since prehistoric times but failed in their attempts to understand the alcoholic fermentation.

Gay Lussac was the first to discover fermentation.

Louis Pasteur concluded that fermentation occurred only when the living yeast cells were present and called this process as zymosis or fermentation.

Fermentation is of following types:

Types of anaerobic respiration and fermentation (Also called intramolecular respiration):

1. Alcoholic fermentation/anaerobic respiration:

It is the most common type of fermentation taking place in yeast and certain bacteria.

Buchner (1897) extracted zymase complex (enzymes) from yeast cells.

Fermentation takes place in the solution of large numbers of sugars, like fructose, galactose, mannose and disaccharides.

In fermentation by yeast, when percentage of alcohol in sugar solution is about 13%, yeast cells are killed and fermentation stops.

 At the time of germination of seeds the amount of sugar increases, so that fermentation takes place in germinating grains of barley.

Alcoholic fermentation takes place in two steps.

In the first step, pyruvic acid is decarboxylated resulting in the formation of acetaldehyde and CO2.

In the second step, acetaldehyde is reduced to alcohol by 2NADH + 2H+

The overall equation is as follows:

2. Lactic acid fermentation: Pyruvic acid formed at the end of glycolysis is converted to lactic acid by Homofermentative lactic acid bacteria (Lactobacillus lacti)

3. Lactic acid and ethyl alcohol fermentation

4. Acetic acid fermentation

(i)

(ii)

Comparison of fermentation and aerobic respiration

(i) Partial breakdown of glucose occurs in fermentation, whereas in aerobic respiration it is completely degraded to CO2 and H2O.

(ii) Net gain if only 2ATP molecules in fermentation, whereas it is very high in aerobic pathway.

(iii) NADH is oxidised to NAD+ rather slowly during fermentation, however, the reaction is very vigorous in case of aerobic respiration.