Fermentation of lactic acid is a common cellular process, which takes place in many bacteria, yeast, and human muscle cells. This article provides information about lactic acid production, and how it helps to provide energy.
Fermentation is an alternative energy yielding process for respiration, which is preferred by organisms that are facultative or obligate anaerobes. Respiration is the most common energy yielding process in all organisms; the prerequisite being the presence of oxygen, and hence, referred to as aerobic cellular respiration. However, fermentation occurs totally in the absence of oxygen, and yields energy from oxidation of organic compounds (mainly sugars). This process is commonly carried out by yeast cells, or by some bacteria to produce certain types of dairy products like cheese and yogurt, and alcoholic beverages like wine, brandy, alcohol, rum etc.
Fermentation is mainly of three types, and commonly, it is classified under 2 categories; alcoholic and lactic acid. The former occurs when the byproduct pyruvate is converted to ethanol and carbon dioxide. On the other hand, in the latter type, pyruvate is converted to lactic acid.
Firstly, we have to understand the steps of aerobic respiration, since fermentation is a type of anaerobic respiration. In aerobic animals, respiration involves 2 pathways: gylcolysis and citric acid cycle. These cycles involve the production of energy in the form of ATP (Adenosine Triphosphate) by breaking down the sugars (mainly glucose – as it is the simplest form of sugar).
Glycolysis is a process involving a series of redox reactions to convert glucose into pyruvate or pyruvic acid; one of the products of glycolysis (end product). Pyruvic acid enters the Krebs cycle, and produces energy with the help of NADH molecules (co-factors that help to generate energy). Energy production actually occurs on the F1 particles situated on the cristae of mitochondria, wherein NADH is alternatively oxidized and reduced with the release of H+ ions/protons, which set up a gradient/flux to generate ATP. The resultant electrons are accepted by oxygen, and water is produced as a byproduct.
Steps of Lactic Acid Fermentation
Fermentation is a two step process, the first being anaerobic glycolysis, up till the formation of pyruvate. The pathways then change because of the available substrates and acceptors, and prevailing of specific environmental conditions. Fermentation of lactic acid is generally carried out by anaerobic bacteria and yeast. The following paragraphs explain this process along with the lactic acid fermentation formulas.
In this type, glucose is converted to pyruvate, which further generates 2 lactic acid molecules with the aid of the enzyme lactate dehydrogenase.
C6H12O6 → 2 CH3CHOHCOOH
This involves the use of pyruvate to produce lactic acid, ethanol, and carbon dioxide as byproducts, under the aid of the enzymes lactate dehydrogenase and pyruvate decarboxylase.
C6H12O6 → CH3CHOHCOOH + C2H5OH + CO2
Lactic Acid Fermentation in Humans
Man normally survives on cellular respiration. Lactic acid fermentation is an alternative pathway to produce energy under low oxygen conditions, especially due to severe straining or extreme exercising. The muscles get deprived of oxygen, causing the cells to undertake the lactic acid pathway for quicker energy requirements. The result is production of lactic acid in these parts leading to stiffness or cramps. The discomfort stays only for few days or hours eventually disappearing after a while.
Fermentation of lactic acid has wide applications in the food and beverage industries.
- Production of this acid is commonly carried out by the lactic acid bacteria, Lactobacillus spps., for production of cheese, yogurt, sauerkraut, bread, and kefir, and for imparting a peculiar sour taste to such food items.
- All beverage industries use the above described fermentation mechanism to produce wines, alcohol, beer, brandy, and other beverages.
- According to research, lactic acid products are high in vitamins and essential nutrients, contrary to their normal counterparts, and hence, are healthy to consume.