Lactic Acid Fermentation

To start off with, fermentation is an alternative energy yielding process for respiration preferred by organisms that are facultative or obligate anaerobes. We know that respiration is the 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. Another aspect we are aware of is that fermentation is commonly carried out by yeast cells or some bacteria to produce certain types of dairy products like cheese, yogurt and alcoholic beverages like wine, brandy, alcohol, rum etc. So let's get into the details of this process for more understanding.

Fermentation in organisms is mainly of three types. Very commonly it is classified under 2 categories; alcoholic and lactic acid fermentation. Alcohol fermentation occurs when the by-product pyruvate is converted to ethanol and carbon dioxide. On the other hand, in lactic acid fermentation, pyruvate is converted to lactic acid.

Aerobic Respiration
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, one is gylcolysis and other, citric acid cycle. These cycles involve the production of energy in the form of ATP (Adenosine Triphosphate) by breaking down the sugars mainly glucose (glucose 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 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 that set up a gradient/flux to generate ATP. The resultant electrons are accepted by oxygen and water is produced as a by-product.

Fermentation is a 2 step process, the first being anaerobic glycolysis, up till the formation of pyruvate. The pathways then change because of substrates and acceptors available and the environmental conditions prevailing. Lactic acid fermentation is generally carried out by anaerobic bacteria and yeast.

Homolactic Fermentation
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

Heterolactic Fermentation
This fermentation involves the use of pyruvate to produce lactic acid, ethanol and carbon dioxide as by-products under the aid of enzymes lactate dehydrogenase and pyruvate decarboxylase.

C₆H₁₂O₆ → CH₃CHOHCOOH + C₂H₅OH + CO₂

Lactic Acid Fermentation in Humans
Humans normally survive on cellular respiration. Though many times 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.

Lactic acid fermentation has wide applications in the food and beverage industries.

• Lactic acid production is commonly carried out by Lactic acid bacteria (Lactobacillus spps.) for production of cheese, yogurt, sauerkraut, bread and kefir which imparts a peculiar sour taste to these food items mainly due to lactic acid production.
• All beverage industries around use lactic acid 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 counter parts and thus healthy to consume.

I hope this article on lactic acid fermentation helped you get the gist of this energy yielding process.