Fermentation is the act in which organic molecules such as glucose convert into acids, gases, or alcohol in an oxygen-free environment.While studying alcoholic fermentation of glucose by yeast, Pasteur (1860) discovered that it can respire in the absence of molecular oxygen. This process was termed as anaerobic respiration.In microorganisms, anaerobic respiration is often called fermentation. Fermentation is named after products like alcoholic fermentation, lactic acid fermentation. Buchner (1897) found that fermentation could be caused by mixing sugar solution with yeast extract, instead of living yeast cells. The enzyme complex present in the extract was named zymase. Because of the latter, fermentation is also called zymosisThe word ‘ferment’ arises from Latin ‘fervere,’ which means ‘to boil.’ Fermentation process requires assistance from microorganisms such as Enterococcus, Streptococcus, and Lactobacillus to complete. Yeasts and Moulds like Debaryomyces, Saccharomyces, Mucor, Penicillium and Rhizopus species help ferment food.
The fundamental truth of fermentation is to obtain energy from carbohydrates in the absence of oxygen.
Types of fermentation
Alcoholic Fermentation
Alcoholic fermentation mainly includes the production of Ethanol CH3 CH2 OH.
Various alcoholic beverages and the alcohol used for industrial purposes are produced by common yeasts like Saccharomyces cerevisiae, an anaerobic organism with the ability to grow as facultative anaerobes.
Under anaerobic conditions, the energy yield is less, and as a result, the growth is slow. However, the energy yield shoots up significantly under aerobic conditions, but the alcoholic production falls dramatically. The Pasteur effect occurs when oxygen prevents fermentation.
For Alcoholic Fermentation, Pyruvic acid converts into ethanol in two simple steps:
Pyruvic Acid to Acetaldehyde
In the first step, the enzyme catalyst used is acid decarboxylase alongside TPP as a coenzyme.
Acetaldehyde to Ethanol
The second step is catalysed by alcohol dehydrogenase alongside NADH2 as a coenzyme.
For large scale production of alcohol, various strains of the yeast Saccharomyces cerevisiae have been carefully grown.
Lactic Acid Fermentation
Lactic Acid Fermentation can be done in two ways:
Homolactic acid fermentation
In this, lactic acid is produced as the only product by reduction of Pyruvic Acid with the help of the enzyme lactic acid dehydrogenase. No gas is released during this process. NAD from NADH2 is regenerated and further reused in the glycolytic pathway for oxidation of GAP to DPGA. From one molecule of pyruvic acid, we get one molecule of lactic acid; when it is dissimilated through EMP, we get two lactic acid molecules from one glucose molecule.
Heterolactic acid fermentation
In heterolactic acid fermentation, we obtain multiple products like lactic acid, ethanol or acetic acid and carbon dioxide. In this process, the bacteria used for lactic acid formation is dissimilated via PPC, producing lactic acid from only one half of a glucose molecule. From another half, we obtain ethanol/acetic acid and carbon dioxide.
Homolactic fermentation is the simplest among the two types of lactic acid fermentation as it involves only one step. When we exercise strenuously, we get a sharp stinging sensation building in our muscles, and it is because lactic acid is being formed in our muscles.
Propionic Acid Fermentation
Propionic Acid, CH3 CH2 COOH, is produced by numerous anaerobic bacteria such as Clostridium and Propionic bacterium. The key propionic acid bacteria are Propionibacterium acidipropionici and P. freudreichii.
Propionibacteria can handle some exposure to oxygen as they have cytochromes and catalase. Naturally, Propionibacteria is found in the rumen of herbivorous cattle.
We obtain pyruvic acid when the Propionibacteria dissimilatie glucose via EMP. Then, pyruvic acid is converted to OAA (oxaloacetic acid), which is further reduced to succinic acid in two steps by reversal of the TCA cycle.
We get succinyl-CoA when succinic acid is converted, a step of the TCA cycle reversal process.
In the next step, Succinyl-CoA produces methyl malonyl-CoA by the action of vitamin B12-linked enzyme methyl malonyl mutase, which further catalyses an intramolecular rearrangement.
By the process of decarboxylation, Methyl malonyl-CoA is converted to propionyl-CoA. Propionic acid is yielded from propionyl-CoA in the last step. Succinic acid receives CoA by an enzyme, CoA-transferase. Both lactic acid bacteria and propionic acid bacteria are used to prepare Swiss cheese.
Butyric Acid- Butanol Fermentation
The bacteria that proceeds the butyric acid-butanol fermentation have to be necessarily anaerobic spore-forming bacteria that must be related to the genus Clostridium.
Among the main products, butyric acid and n-butanol obtained, depending on the bacteria species, we also get numerous other products such as acetic acid, ethanol, isopropanol and acetone. Since carbon dioxide is a fermentation product, it is generally always present.
For instance, C. butyricum, C. lacto –acidophilus, C. pasteurianum, produce butyric acid along with acetic acid and carbon dioxide.
C. butylicum and C. acetobutylicum produce butyric acid, isopropanol or acetone, acetic acid and carbon dioxide.
Clostridium dissimilates glucose via EMP to form pyruvic acid. By the process of decarboxylation, pyruvic acid produces acetyl-CoA.
Mixed Acid Fermentation
Mixed Acid Fermentation occurs specifically in the bacteria related to the family Enterobacteriaceae. Bacteria that belong to this category can thrive aerobically, where they perform oxygen respiration and anaerobically, where they carry out fermentation.
This fermentation is called Mixed Acid Fermentation because, along with the main product, several other organic and neutral compounds, such as acetic acid, succinic acid, lactic acid, ethanol, acetoin, butanediol, carbon dioxide and hydrogen, are also obtained. Formic acid is a special product obtained in fermentation, and sometimes it is also referred to as Formic Acid Fermentation.
The reactions occurring in this fermentation are shown below:
Pyruvic acid is converted into acetyl-CoA and formic acid,
Formic acid gives molecular hydrogen and carbon dioxide.
Conclusion
Fermentation is a process that has many different uses, from being involved in our food to being used for industrial purposes. Bread, Beer, Wine, Cheese, Yoghurt, Probiotics and Indian foods like Dosa, Idli, Naan, Pickles are all prepared with fermentation. Fermented food improves our digestion and immune system and has anticarcinogenic properties. Not only are the end products beneficial, but those obtained during the process, such as formic acid, are also useful. A unique condition is essential for fermentation to occur, and certain bacterias are needed for the reactions to proceed. Fermentation is a metabolic process where starch or sugar is converted to alcohol, and the science of fermentation is called “Zymology.”