Lactic Acid Fermentation

Some bacteria produce lactic acid from pyruvic acid. This metabolic process is also seen in the muscle cells of animals. There can be inadequate oxygen supply for respiration in muscle cells during physical exercise. Thus, in anaerobic conditions, pyruvic acid is reduced to lactic acid. This happens in the presence of the enzyme lactate dehydrogenase. Also, a reducing agent like NADH+H+ is reoxidised to NAD+.

It is a fermentation process where much of the energy is not released. Here, less than seven per cent of the glucose energy is released. Also, not all of it is trapped as high energy bonds of ATP.

Anaerobic Respiration

Anaerobic respiration is a kind of cellular respiration that happens in the absence of oxygen. The two types of anaerobic respiration are – Lactic acid fermentation and alcoholic fermentation.


Fermentation usually happens in yeast cells and bacteria. Also, the animal muscle cells show lactic acid fermentation in case of inadequate oxygen supply.

In yeast, there is incomplete oxidation of glucose that happens in an anaerobic environment. Here, pyruvic acid is converted into carbon dioxide and ethanol. Some bacteria produce lactic acid from pyruvic acid by means of lactic acid fermentation.

Steps Involved in Lactic Acid Fermentation

Lactic Acid Fermentation

  • The glucose or 6-carbon molecule is broken down into Glyceraldehyde 3-phosphate, and then to 3-Phosphoglyceric acid.
  • During this, NAD+ is converted into NADH+H+.
  • The 3-Phosphoglyceric acid forms Phosphoenol pyruvic acid, which later forms the Pyruvic acid.
  • Net 2 ATP molecules are formed in this process (glycolysis).
  • This Pyruvic acid is reduced to Lactic acid with the help of reducing agent NADH+H+, which reoxidises to NAD+.
  • This process produces two lactate/lactic acid molecules from two pyruvate/pyruvic acid molecules. This reaction happens in the presence of the enzyme lactate dehydrogenase.


Lactic acid fermentation is the best method used for food preservation. Lactobacillus is the most commercially used bacteria for this process. They are used in the production of pickles, sour beer, fermented fish, yoghurt, etc.

Frequently Asked Questions

What is homolactic and heterolactic fermentation?

When one glucose molecule is converted into two molecules of lactate or lactic acid, it is termed homolactic fermentation. Here, only lactic acid is the final byproduct. In heterolactic fermentation, other byproducts like CO2 and ethanol are formed along with lactic acid.

What are facultative and obligate anaerobes?

Facultative anaerobes can produce energy with or without oxygen. Thus, they can also undergo aerobic respiration. Examples – E.Coli, Pseudomonas aeruginosa, etc. Obligate anaerobes cannot tolerate oxygen. They are strictly anaerobic. Examples – Clostridium, Actinomyces, etc.

Why do muscle cramps happen?

Inadequate oxygen supply in human muscle cells leads to anaerobic respiration. Thus, lactic acid fermentation takes place to produce energy molecules. Lactic acid is the byproduct of this process. Excessive buildup of lactic acid creates muscle discomfort called cramps.

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