Fermentation: life without air

Pasteur mentioned fermentation as “life without air” as he was the first one to understand that breakdown of organic compounds in absence of oxygen can be a method of obtaining metabolic energy for the microorganisms. There are two types of microorganisms regarding this i.e. that can live/survive without air: - Facultative anaerobes and Obligate anaerobes.

Facultative anaerobes have two types of energy yielding mechanisms. They can follow both aerobic and anaerobic pathways of obtaining metabolic energy. They can hence survive both in absence & presence of oxygen. So when sugar is fermented in absence of oxygen; alcohol & CO₂ are produced whereas when oxygen is present no alcohol formation takes place and CO₂ is the sole end product.

Significance of fermentation

After the discovery of anaerobic life Pasteur understood the actual reason behind fermentation i.e. why the microorganisms carrying out the chemical changes in the organic infusions. Until then this was a mystery. The discovery of anaerobic life just made it possible for him to understand the basis of these chemical changes. The reason is the need of energy to sustain life. And the microorganisms were making the chemical changes to derive energy from the organic infusions either by aerobic or via anaerobic oxidation. In that way they were performing the process similar to respiration in higher organisms.

Anaerobic life

Here comes another historical discovery of Pasteur, the Father of Microbiology – “The existence of forms of life that can live only in the absence of free oxygen” i.e. life in anaerobic form. This conclusion of presence of anaerobic life (at that time it was thought that life cannot sustain without oxygen) was drawn by him while observing the fermentation process of butyric acid. He was studying droplets collected from fermentation vats under microscope & found that the organisms at the margin of the flattened drop quickly became immotile (which were in close proximity of air) whereas that at the center remained motile. He thought that the immobility is due to the death of the organisms & the cause of it must be the contact with air. This was further confirmed when he passed air through fermentation vats of butyric acid fermentation. He found that the rate of fermentation retarded significantly & sometimes even stopped. So he at last concluded that some microorganisms can only live without oxygen and coined the names aerobes and anaerobes to designate them as life with or without oxygen.

For the next 20 years Pasteur studied various fermentation processes & came to the conclusion that all fermentation processes are accompanied by the formation of microorganisms. He also concluded that different fermentation processes contain different microbes. Moreover these occurrences of microbes are specific for specific fermentation (as evident by specific end products like alcohol and lactic acids). Pasteur differentiated these organisms microscopically according to shape & size and their favorable environmental conditions. For example, organisms from alcohol fermentation prefer acidic condition whereas that from lactic acid fermentation grows in neutral pH. So to grow lactic acid microbes one should use some neutralizing agents to prevent the acidic effect due to the formation of lactic acid (Pasteur used calcium carbonate i.e. chalk for this purpose).