fermentation

Chemistry

2022

We explain what fermentation is, what are the types of fermentation that can be used and the different uses it has.

The fermentation process was discovered by the French chemist Louis Pasteur.

What is fermentation?

Fermentation is a process of oxidation incomplete, requiring no oxygen to take place, and producing an organic substance as a result. It is a catabolic type process, that is, the transformation of molecules complex to simple molecules and generation of chemical energy in form of ATP (Adenosine Triphosphate).

Fermentation consists of a process of glycolysis (breakdown of the glucose molecule) that produces pyruvate (pyruvic acid) and that lacks oxygen as a receptor for the electrons leftover NADH produced (nicotin adenine dinucleotide), uses an organic substance that must be reduced to reoxidize the NADH to NAD +, finally obtaining a derivative of the initial substrate that is oxidized. Depending on the final substance, there will be different types of fermentation.

This process was discovered by the French chemist Louis Pasteur, who described it as "Life without air" (La vie sans l’air), since it can be carried out in the absence of oxygen by microorganisms like the bacteria, yeast, or some metazoans and protists. In this process, then, neither the mitochondria nor the structures linked to the process of cellular respiration.

Compared to the aerobic respiration, fermentation is not a method very efficient energy source: only 2 ATP molecules are produced per glucose molecule consumed, while breathing is obtained from 36 to 38.

However, this process is carried out by various cells of our body to cover the moments of absence of oxygen, as occurs in the muscle cells that ferment glucose when the oxygen input is not enough to continue breathing.

Types of fermentation

According to the substance obtained at the end of the fermentation process, we can classify it into:

  • Alcoholic fermentation. It is a process carried out mainly by yeasts, in which a quantity of alcohol ethanol is produced from certain sugars, carbon dioxide and ATP. This is the process used to produce alcoholic beverages.
  • Acetic fermentation. Own of the bacteria of the genus Acetobacter, transforms ethyl alcohol into acetic acid, that is, the alcohol in vinegar. It is, however, an aerobic process, so it can occur in wines exposed to the air.
  • Lactic fermentation. It consists of a partial oxidation of glucose, carried out by lactic bacteria or by animal muscle cells (when they run out of oxygen to breathe). This process generates ATP but produces lactic acid as a by-product, which produces the painful sensation of muscle fatigue when it accumulates.
  • Butyric fermentation. It consists of the conversion of glucose into butyric acid and gas, the latter giving it a typically unpleasant odor. It is characteristically carried out by bacteria of the genus Clostridium and requires the presence of lactose.
  • Butanediol fermentation. It is a variant of lactic fermentation, carried out by Enterobacteriaceae that release carbon dioxide and they generate butanediol, a colorless and viscous alcohol.
  • Propionic fermentation. In this process intervene the acetic acid, carbon dioxide and succinic acid. It is obtained from all of them propionic acid, a substance corrosive with a pungent odor.

Uses of fermentation

The wines require a production process in which alcoholic fermentation is used.

Many man-made industries take advantage of fermentation to obtain certain substances. For example, in food industries of cheese, propionic fermentation processes are carried out, or in the preservation of many types of edibles the presence of lactic acid is used, which acts as a preservative, due to lactic fermentation.

Something similar happens with the alcoholic industry, both for wines, beers or other types of liquors, which require a production process in which alcoholic fermentation intervenes. On the other hand, if some liquors such as wine are left uncovered for a long time, the added oxygen will start the acetic fermentation and the drink will start to become vinegary.

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