Lactic fermentation

Lactic acid fermentation is a metabolic process by which glucose and other six-carbon sugars (also, disaccharides of six-carbon sugars, e.g. sucrose or lactose) are converted into cellular energy and the metabolite lactate. It is an anaerobic fermentation reaction that occurs in some bacteria and animal cells, such as muscle cells.

If oxygen is present in the cell, many organisms will bypass fermentation and undergo cellular respiration; however, facultative anaerobic organisms will both ferment and undergo respiration in the presence of oxygen. Sometimes even when oxygen is present and aerobic metabolism is happening in the , if pyruvate is building up faster than it can be metabolized, the fermentation will happen anyway.

Lactate dehydrogenase catalyzes the interconversion of pyruvate and lactate with concomitant interconversion of NADH and NAD⁺.

In homolactic fermentation, one molecule of glucose is ultimately converted to two molecules of lactic acid. Heterolactic fermentation, in contrast, yields carbon dioxide and ethanol in addition to lactic acid, in a process called the phosphoketolase pathway.

Several chemists discovered during the 19th century some fundamental concepts of the domain of organic chemistry. One of them for example was the French chemist Joseph Louis Gay-Lussac, who was especially interested in fermentation processes, and he passed this fascination to one of his best students, Justus von Liebig. With a difference of some years, each of them described, together with colleague, the chemical structure of the lactic acid molecule as we know it today. They had a purely chemical understanding of the fermentation process, which means that you can’t see it using a microscope, and that it can only be optimized by chemical catalyzers. It was then in 1857 when the French chemist Louis Pasteur first described the lactic acid as the product of a microbial fermentation. During this time, he worked at the university of Lille, where a local distillery asked him for advice concerning some fermentation problems. Per chance and with the badly equipped laboratory he had at that time, he was able to discover that in this distillery, two fermentations were taking place, a lactic acid one and an alcoholic one, both induced by some microorganisms. He then continued the research on these discoveries in Paris, where he also published his theories that presented a stable contradiction to the purely chemical version represented by Liebig and his followers. Even though Pasteur described some concepts that are still accepted nowadays, Liebig refused to accept them until his death in 1873. But even Pasteur himself wrote that he was “driven” to a completely new understanding of this chemical phenomenon. Even if Pasteur didn’t find every detail of this process, he still discovered the main mechanism of how the microbial lactic acid fermentation works. He was for example the first to describe fermentation as a “form of life without air."

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