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Lactic Acid Fermentation: Definition, Products & Equation

Yash Solanki, Nicholas Gauthier
  • Author
    Yash Solanki

    Yash Solanki has taught post-secondary science for over four years. He has a bachelors degree in Biology from Iowa State University, and has worked as a scientific researcher at multiple global institutions.

  • Instructor
    Nicholas Gauthier

    Nicholas has a B.A. in Biology and Master of Secondary Education in Biology. He has New York State Permanent Certification in Biology, Earth Science, and General Science. He has over thirteen years of teaching experience. Prior to teaching, he gained hands on experience working in various medical labs.

What is lactic acid fermentation? Learn about the lactic acid fermentation equation and examples, and see a comparison of lactic acid vs. alcoholic fermentation. Updated: 05/20/2021

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What is Lactic Acid Fermentation?

Lactic acid fermentation is a type of anaerobic respiration (or fermentation) that breaks down sugars to produce energy in the form of ATP. It is called anaerobic because it occurs in the absence of oxygen. Lactic acid is generated as a byproduct of this reaction, which is what gives this type of fermentation its name. One well-known example of lactic acid fermentation occurs during the production of yogurt by Lactobacillus bacteria. It is the lactic acid generated during this process that gives the yogurt a sour flavor.

Where Does Lactic Acid Fermentation Occur?

The lactic acid fermentation occurs in certain animal cells and bacterial organisms. In animal cells, aerobic respiration is generally preferred - this process doesn't involve lactic acid fermentation. However, in conditions where the oxygen supply is insufficient - primarily in muscle tissues, during periods of strenuous exercise - cells must undergo anaerobic respiration in order to adequately meet the demand for energy. This process of anaerobic respiration produces lactic acid as a byproduct, which accumulates in the muscle tissues. The increased acidity is responsible for the burning sensation and weakness you feel in your muscles during exercise, and this is believed to help prevent you from overworking your muscles. Contrary to popular opinion, however, lactic acid is not the cause of delayed-onset muscle soreness (i.e. the soreness you might feel the day after an intense workout.)

After the muscle activity is over, lactic acid is removed from the muscle tissue through the bloodstream. It is then sent to the liver, where it undergoes chemical reactions to form pyruvic acid, which is used to produce additional energy as ATP.

Apart from muscle tissue, this process also occurs in red blood cells, because they lack mitochondria and thus cannot generate energy aerobically.

In bacterial organisms, lactic acid fermentation serves multiple functions. Bacteria that carry out lactic acid fermentation are commonly referred to as Lactic Acid Bacteria (LABs.) While fermentation is crucial in allowing LABs to generate energy in anaerobic conditions, it also helps them reduce the pH of their environment through the synthesis of lactic acid. The lowered pH helps make their environment unsuitable for most other microorganisms and thus reduces the competition faced by the LABs.

While some LABs are strictly anaerobic, others can switch between the two processes depending on the availability of oxygen.

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Lactic Acid Fermentation Products and Equation

The commonly used equation for Lactic acid Fermentation is:

Glucose + ADP + NADH -> C3H6O3 (Lactic Acid) + ATP + NAD+

Chemical structure of Lactic Acid (C3H6O3)

Lactic Acid Structure

While this equation highlights the lactic acid fermentation reactants and final products, the process actually occurs in two steps.

The first process is called glycolysis (which is also a part of the aerobic respiration pathway.) Glycolysis involves the conversion of glucose into 2 molecules of pyruvic acid, creating 2 ATP and 2NADH (from ADP and NAD+) in the process. Because glycolysis does not require oxygen, it is considered an anaerobic process.

In the next step, each molecule of pyruvic acid is converted into lactic acid. As a byproduct, 4 NAD+ is generated, 2 of which go back to continue the glycolysis pathway.

Based on these steps another way to write the Lactic Acid fermentation equation would be:

Glucose + 2 ATP + 2NAD+ -> 2 Pyruvic acid + 2 NADH + 2 ATP

2 Pyruvic acid + 4NADH -> 2 C3H6O3 (Lactic Acid) + 4NAD + ??????

Simplifying this equation by focusing on the net gain of products gets us back to the initial equation:

Glucose + 2 ADP + 2NADH -> 2 Lactic Acid + 2 ATP + 2 NAD+

This pathway of lactic acid fermentation is also called the homolactic pathway Through this process, glucose is broken down to generate energy and lactic acid. This pathway is most commonly employed by the lactic acid bacteria Pediococcus and Streptococcus, as well as some species of Lactobacilli.

Apart from glucose, other sugars such as lactose and maltose can also be used in lactic acid fermentation.

Other Pathways

There exist two other fermentation pathways, apart from the homolactic pathway, that produce lactic acid as their end product. These are the heterolactic pathway and Bifidium pathway. Their usage varies between different species of lactic acid bacteria, and each pathway produces different byproducts.

The equation for the heterolactic pathway is similar to the homolactic pathway. However, instead of 2 lactic acid molecules, only one lactic acid molecule is synthesized, along with a molecule each of ethanol and CO2 (Carbon Dioxide). This pathway also only generates 1 ATP molecule, rather than of 2. Lactic acid bacteria in the genus Leuconostoc and some species of Lactobascilli employ this method. The equation for the heterolytic pathway is:

Glucose + ADP -> Lactic acid + Ethanol + CO2 + ATP

The second alternative pathway is the Bifidium Pathway, or Bifidobacterium shunt. This pathway produces acetate and lactate as byproducts, and is employed by bacteria of the genus Bifidobacteria. Its equation is:

2 Glucose + 5ADP -> 3 Acetate + 2 Lactic acid + 5 ATP

Examples of Lactic Acid Fermentation Uses

Lactic Acid Fermentation has several culinary uses in human society, many of which derive from ancient culinary and preservation techniques. Some example of foods that utilize lactic acid fermentation are:

  • Yogurt
  • Fermented Pickles (e.g. Sauerkraut, Kimchi)
  • Sourdough
  • Soy Sauce

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