LACTIC FERMENTATION: STEP BY STEP PROCESS AND EXAMPLES - BIOLOGY - 2025

The lactic fermentation, also known as lactic acid fermentation is the process of synthesis of ATP in the absence of oxygen that perform some microorganisms, including a type of bacteria called "lactic acid bacteria", which ends with acid excretion lactic.

It is considered a type of anaerobic "respiration" and is also carried out by some muscle cells in mammals when they work hard and at high speeds, greater than the oxygen transport capacity of the pulmonary and cardiovascular systems.

Lactic fermentation scheme (Source: Sjantoni / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0) via Wikimedia Commons and modified by Raquel Parada Puig)

The term "fermentation", in general terms, refers to obtaining energy (in the form of ATP) in the absence of oxygen, that is, in anaerobiosis, and lactic fermentation refers to the synthesis of ATP and the excretion of acid. lactic acid in anaerobiosis, as products of glucose metabolism.

Equation of lactic acid production from glucose.

Lactic acid bacteria

Man has been exploiting the benefits of lactic fermentation for the production and preservation of food for a long time and, without a doubt, lactic acid bacteria are a fundamental pillar for this purpose.

These belong to a quite heterogeneous group of bacteria that, usually, have the shape of cocci and bacilli; They are Gram-positive, non-catalase-producing, non-sporulating, immobile and anaerobic bacteria, capable of synthesizing lactic acid from pyruvate formed by the glycolytic pathway.

They belong to different genera, among them Pediococcus, Leuconostoc, Oenococcus and Lactobacillus, within which there are homofermentative and heterofermentative species.

Homofermentative lactic acid bacteria produce, for every glucose molecule they consume, two lactic acid molecules; heterofermentative lactic acid bacteria, on the other hand, produce one molecule of lactic acid and another of carbon dioxide or ethanol, for example.

Lactic fermentation process (step by step)

Lactic acid fermentation begins with a cell (bacterial or muscle) consuming glucose or some related sugar or carbohydrate. This "consumption" occurs through glycolysis.

- Glycolytic pathway

ATP investment

Initially, 2 ATP is invested for each glucose molecule consumed, as it is phosphorylated by the hexokinase enzyme to yield glucose 6-phosphate, which is isomerized to fructose 6-phosphate (glucose 6-P isomerase enzyme) and is phosphorylated back to fructose 1, 6-bisphosphate (phosphofructokinase enzyme).

Later, the fructose 1,6-bisphosphate is "cut" in half to release two triose phosphate known as glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, a reaction catalyzed by an aldolase enzyme.

These two 3-carbon phosphorylated sugars are interconvertible with each other by an enzyme triose phosphate isomerase, so it is considered that, up to this point, each glucose molecule that is consumed is converted into two glyceraldehyde 3-phosphate molecules that are phosphorylated to 1,3-bisphosphoglycerate.

The above reaction is catalyzed by an enzyme called glyceraldehyde 3-phosphate dehydrogenase (GAPDH), which requires the presence of the "reducing power" of the cofactor NAD +, without which it cannot function.

ATP production

At this point in the route, 2 ATP has been consumed for each molecule of glucose, but these two molecules are “replaced” by the reaction catalyzed by the enzyme phosphoglycerate kinase, by which each 1,3-bisphosphoglycerate is converted into 3-phosphoglycerate. and 2ATP are synthesized.

Each 3-phosphoglycerate is converted to 2-phosphoglycerate by an enzyme phosphoglycerate mutase and this, in turn, serves as a substrate for the enzyme enolase, which dehydrates it and converts it to phosphoenolpyruvate.

With each glucose molecule that is consumed, 2 pyruvate molecules and 2 ATP molecules are produced, since phosphoenolpyruvate is a substrate for the enzyme pyruvate kinase, which catalyzes the transfer of a phosphoryl group from phosphoenolpyruvate to a molecule of ADP, producing ATP.

- Lactic fermentation and regeneration of NAD +

Pyruvate, a 3-carbon molecule, is converted into lactic acid, another 3-carbon molecule, through a reduction reaction that consumes one molecule of NADH for each molecule of pyruvate, regenerating the "inverted" NAD + in the glycolytic reaction. catalyzed by GAPDH.

The replacement of the NAD + molecules used does not lead to an additional production of ATP molecules, but allows the glycolytic cycle to repeat itself (as long as there are carbohydrates available) and 2 ATP is produced for each glucose consumed.

The reaction is catalyzed by an enzyme called lactate dehydrogenase and goes something like this:

2C3H3O3 (pyruvate) + 2 NADH → 2C3H6O3 (lactic acid) + 2 NAD +

Examples of processes in which lactic fermentation occurs

- In muscle cells

Lactic acid fermentation in muscle cells is common after a workout after several days of inactivity. This is evident because muscle fatigue and pain experienced by the athlete are associated with the presence of lactic acid in the cells.

Image by 5132824 at www.pixabay.com

As muscle cells exercise and oxygen stores are depleted (the cardiovascular and respiratory systems cannot cope with the necessary oxygen transport), they begin to ferment (breathe without oxygen), releasing lactic acid that can accumulate.

- Food products

Lactic acid fermentation carried out by different species of bacteria and fungi is used by man worldwide for the production of different types of food.

This metabolism by which different microorganisms are characterized is essential for the economic preservation and production of large quantities of food, since the acid pH achieved by them generally inhibits the growth of other potentially harmful or pathogenic microorganisms.

These foods include yogurt, sauerkraut (fermented cabbage), pickles, olives, different pickled vegetables, different types of cheese and fermented milk, kefir water, some fermented meats and cereals, among others.

The yogurt

Yogurt is a fermented product derived from milk and is produced thanks to the fermentation of this liquid of animal origin by a type of lactic acid bacteria, generally of the Lactobacillus bulgaricus or Lactobacillus acidophilus species.

Yogurt (Image by kamila211 at www.pixabay.com)

These microorganisms convert the sugars present in milk (including lactose) into lactic acid, so the pH decreases (becomes acidic) in this liquid, modifying its flavor and texture. The firmer or liquid texture of different types of yogurt depends on two things:

1. From the concomitant production of exopolysaccharides by fermentative bacteria, which act as thickening agents
2. From coagulation that results from the neutralization of negative charges on milk proteins, as an effect of the change in pH generated by the production of lactic acid, which renders them completely insoluble

Fermented vegetables

In this group we can find products such as olives preserved in brine. Cabbage-based preparations such as sauerkraut or Korean kimchi are also included, as are pickled gherkins and Mexican jalapeno.

Fermented meats

Sausages such as chorizo, fuet, salami and sopressatta are included in this category. Products that are characterized by their particular flavors in addition to their high preservation capacity.

Fermented fish and shellfish

It includes different types of fish and shellfish that are usually fermented mixed with pasta or rice, as is the case with Pla raa in Thailand.

Fermented legumes

Lactic fermentation applied to legumes is a traditional practice in some Asian countries. Miso, for example, is a paste made from fermented soybeans.

Fermented seeds

In traditional African cuisine, there is a wide variety of products made from fermented seeds such as sumbala or kenkei. These products include some condiments and even yogurts made from cereals.

References

1. Beijerinck, MW, On Lactic acid fermentation in milk., In: KNAW, Proceedings, 10 I, 1907, Amsterdam, 1907, pp. 17-34.
2. Munoz, R., Moreno-Arribas, M., & de las Rivas, B. (2011). Lactic acid bacteria. Molecular Wine Microbiology, 1st ed.; Carrascosa, AV, Muñoz, R., González, R., Eds, 191-226.
3. National Research Council. (1992). Applications of biotechnology in traditional fermented foods. National Academies Press.
4. Nelson, DL, Lehninger, AL, & Cox, MM (2008). Lehninger principles of biochemistry. Macmillan.
5. Soult, A. (2019). Chemistry LibreTexts. Retrieved April 24, 2020, from chem.libretexts.org
6. Widyastuti, Yantyati & Rohmatussolihat, Rohmatussolihat & Febrisiantosa, Andi. (2014). The Role of Lactic Acid Bacteria in Milk Fermentation. Food and Nutrition Sciences. 05. 435-442. 10.4236 / fns.2014.54051.