From ancient time people produced various products of fermentation using different types of fermentation. Although they didn’t know about these types of fermentation.
“Fermentation” is a word derived from the Latin word fervere, which means, to boil, explaining the fact that the carbon dioxide bubbles produced from the anaerobic reaction of sugar type substances by yeast. But in microbiology, it means the production of energy by the digestion of organic compounds, which gives a much-expanded resource to look at.
An oxidation process takes place after the digestion of sugar which produces reduced pyridine nucleotides. Then it must be reoxidized to resume the procedure.
Reoxidization of the reduced pyridine nucleotides completes by the electron transfer in the aerobic process through the cytochrome system, where oxygen is an electron acceptor. Nonetheless, reduced pyridine nucleotides get coupled with the reduction of the organic compound due to the anaerobic process.
In this context, we will discuss the types of fermentation. But the question is what is the real answer to types of fermentation. Some say there are two types of fermentation and others say 3 types of fermentation. I have also read several articles about the types of fermentation where they mention 5 types of fermentation. Some article also mentions 8 types of fermentation. But the exact answer is – there are 2 types of fermentation. They are alcohol fermentation and lactic acid fermentation.
Table of Contents
Type of fermentation:
Types of fermentation on the basis of the end products of the fermentation process
There are universally two types of fermentation processes like (1) Alcohol fermentation (2) lactic acid fermentation.
Ther are some other types of fermentation classified based on the end products of fermentation. (3) Propionic Acid fermentation (4) Butanoic Acid fermentation (5) Mixed Acid fermentation
It means the production of ethanol (CH3-CH2-OH). Generally, yeasts (Saccharomyces cerevisiae) are used for generating different types of alcohol, both as a beverage and as industrial use. Yeasts are aerobic organisms, but they can partially function as anaerobic.
The energy-gain under anaerobic conditions is very low so the growth takes time with much lower cell-yield but under aerobic conditions, the cell-yield increases exponentially but decreases the generation of alcohol. This phenomenon is called Pasteur-effect.
Making ethanol from pyruvic acid requires two steps, such as pyruvic acid to acetaldehyde and then to ethanol from acetaldehyde. Step one is to catalyze pyruvic acid decarboxylase which needs TPP as a coenzyme. Step two is to catalyze by alcohol dehydrogenase which needs NADH2 as a coenzyme.
Various types of yeasts (mostly Saccharomyces cerevisiae) have been nurtured and carefully selected for mass-scale production of alcohol for different aspects. Different products and catalysts are used depending on the type of product.
As we know, the bread industry uses a large amount of yeast which are strongly aerated cultures that makes large cell-yield with little or no alcohol. Malted barley extract is a substitute for beer production.
To produce malt, a large amount of maltose produced by hydrolysis of starch present in barley seeds are needed. Maltose divides itself into glucose from which we can make alcohol under anaerobic conditions.
To produce wine, grape juice is needed as a substrate. Certain type of strains is selected to divulge the characteristic taste and flavor of different alcoholic liquors.
For the generation of industrial alcohol, molasses is required as a starting material. Also, sulfite liquor, a waste product of the paper industry, can be used as a cheap substrate for industrial alcohol generation.
Some bacteria (Zymomonas mobilis, Pseudomonas saccharophila) play part in alcoholic fermentation which extracts glucose by EDP producing pyruvic acid which is then going through decarboxylation and dehydration to produce ethanol.
Figure regarding the reaction is given below:
Glucose → Pyruvic acid → Acetaldehyde → Ethanol
Glucose is converted to pyruvic acid in 10 steps of the glycolytic pathway.
Lactic Acid fermentation:
They are of two types: Homo-fermentative and hetero-fermentative.
In homo-fermentative, pyruvic acid is reduced by the enzyme lactic acid dehydrogenase and produces lactic acid. NAD and NADH2 are also produced which is again used for oxidation of GAP to DGPA in the glycolytic pathway.
Two moles of lactic acid is generated from each mole of glucose after being dissimilated through EMP.
In hetero-fermentative, the end products are lactic acid and ethanol or acetic acid and CO2. Here, glucose is dissimilated by lactic acid bacteria with the help of PPC. For one half of the glucose mole, lactic acid is produced, and acetic acid or ethanol is produced from the other half.
Examples of some homo-fermentative lactic acid bacteria are Lactococcus lactis, L. cremoris L. diacetilactis, L. thermophilus, Lactobacillus Brevis, L. acidophilus, etc. Some hetero-fermentative lactic acid bacteria are Lenconostoc mesenteroides, Lactobacillus bulgaricus, Bifidobacterium bifidum, etc.
These bacteria prefer anaerobic condition to grow because they do not have any cytochromes or catalase, but some study shows they can also be grown in a microaerophilic condition.
Figure regarding the reaction is given below:
Glucose → Pyruvic acid → Lactic acids
See More: Where does fermentation occur
It is the simplest form of fermentation as it only requires one step in which pyruvic acid is reduced to lactic acid.
In a hetero-fermentative lactic acid bacterium, two main enzymes are missing from the glycolytic pathway-aldolase and triosephosphate isomerase. So, it cannot use EMP, instead, they use the pentose phosphate pathway.
The hetero-fermentative bacteria produce xylulose-5-phosphate by a TPP-linked pentose phosphate ketolase into GAP and acetyl phosphate. Then, GAP is reduced to pyruvic acid by EMP enzymes and acetal acid is reduced either acetic acid or ethanol. From there, lactic acid is produced using lactate dehydrogenase activity.
Lactic acid is extensively used for producing fermented food all over the world. The bacteria mold lactose in the milk and produce lactic acid which curdles milk protein.
The products are yogurt, curd, buttermilk, etc. They are also used in fermented vegetables like sauerkraut (fermented cabbage), cucumber pickles, fermented olive (pickles). They are also widely used to produce sausage from beef and pork.
Propionic Acid fermentation:
It is produced by anaerobic bacteria such as Propionibacterium, Veillonella, Clostridium, Selenomonus, and many more. Propionibacterium acidipropionici and P. freudenreichi are generally used as the main propionic acid fermenters. Propionibacteria inhibits cytochromes and catalase which helps them to endure some amount of oxygen.
Propionic acid bacteria produce pyruvic acid by dissimilating glucose via EMP. Then pyruvic acid is converted into oxalacetic acid by a biotin-linked carboxylation reaction. Then it is reduced in two steps to succinic acid through reverse-TCA cycle reaction.
Succinic acid is reduced to succinyl-CoA, which is also a reverse TCA cycle. Next, succinyl-CoA generates methyl malonyl-CoA by vitamin B12-linked enzyme methyl malonyl mutase which creates an intramolecular rearrangement which further decarboxylated to propionyl-CoA.
Finally, propionyl-CoA is reduced into propionic acid where CoA is removed by the CoA-transferase enzyme are converted into succinic acid.
They are well known to produce Swiss cheese along with lactic acid. Propionic acid adds a special flavor to Swiss cheese.
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Butanoic Acid Fermentation
All the bacteria that carry out the butyric acid generally belong to the genus clostridium. And this type of fermentation is known as butanoic acid fermentation. Apart from this butyric acid, there are some other products including n-butanol, ethanol, isopropanol, acetic acid, and acetone as well.
But it depends on the species. For instance, butyric acid with acetic acid can produce C. butyricum, C. lactoacetophilum, C. pasteurianum, etc. whereas, C. butylicum and C. acetobutylicum can form butyric acid, acetic acid, and isopropanol or acetone.
However, CO2 is always present as a fermentation product in all cases. Moreover, acetyl-CoA is produced by decarboxylation in the formation of pyruvic acid, and clostridia here dissimilate glucose by EMP.
In the butyric-butyric fermentation acetyl-CoA is a the key-intermediate which ensures the rise to all the products by different pathways.
Here, acetyl-co-A is produced with the liberation of one CoA in the pathway that led to butyric acid in C. butyricum, two molecules of acetyl-CoA. This took place by the presence of the enzyme thiolase. After that, β-hydroxybutyryl-CoA has dehydrogenated Acetoacetyl CoA, and then formed P-hydroxbutyryl CoA with NADH2 acting as H-donor.
Through the active participation of the enzyme crotonase, the dehydrogenated product then forms crotonyl-CoA
Under alkaline conditions, butyryl CoA is converted by C. acetobutylicum to n-butanol through two steps catalyzed by butyryl-CoA dehydrogenase and butyryl aldehyde dehydrogenase as shown in the figure.
Besides, molecular hydrogen is one of the fermentation products which is always produced by clostridia. Hydrogen originates from phosphorolytic cleavage of pyruvate.
In this type of cleavage, at first, hydrogen is transferred to a protein that contains iron. It is known as ferredoxins, and it is then reduced. On the other hand, through the action of hydrogenase, molecular hydrogen is liberated; and then the ferredoxin is oxidized at the end.
Mixed Acid Fermentation:
Bacteria belonging to the family Enterobacteriaceae is mainly associated with this mixed acid fermentation. It is because these bacteria grow both aerobically ( which means carrying out oxygen respiration) or anaerobically( which means carrying out fermentation).
In this fermentation procedure, different kinds of organic acids and neutral compounds are produced and that is why it is called mixed-acid fermentation. And the major characteristic of this fermentation is formic acid but it is not the main product.
Acetic acid, succinic acid, lactic acid, ethanol, acetoin, butanediol, CO2, and molecular hydrogen are also formed in this fermentation depending on the species. Considering the products, the enterobacteria can be divided into two groups: 1. Escherichia coli-type fermentation, and 2. Enterobacter aerogenes type.
Moreover, mixed acid fermentation is also known as formic acid fermentation because of its function. Different products are produced like acetic acid, succinic acid, lactic acid, ethanol, acetoin, butanediol, CO2, and molecular hydrogen but the production depends on the different species.
Acetoin (acetyl methyl carbinol) and butanediol are usually produced by Enterobacter-type of fermentation products that are not formed by E. coli-type of fermentation. Moreover, the basis of the Voges-Proskauer reaction is formed by the detection of acetoin and butanediol forms.
Formation of acetoin and butanediol in Enterobacter proceeds via acetolactate pathway. The TPP- linked active acetaldehyde produced from pyruvic acid, described above, reacts with another molecule of pyruvic acid to form acetolactate.
Acetohydroxyl acid synthase catalyzes the reaction. As a result, Acetolactate so formed, is then decarboxylated by the enzyme acetolactate decarboxylase to produce acetoin. The second one is then reduced by butanediol dehydrogenase to 2,3-butylene glycol (butanediol), NADH2 acting as H-donor.
In enteric bacteria fermentation, the products that are produced include acetic acid, ethanol, lactic acid, and succinic acid. AcetyI-CoA can be used in several useful ways which are produced in pyruvic acid-formic acid lyase reaction in E.coli.
Types of fermentation on the basis of the type of substrate used during fermentation
There are mainly two types of fermentation on the basis of the type of substrate used during fermentation. They are:
- Submerged 2. Solid State
Submerged Fermentation plays a vital role in maintaining a controlled environment for adequate production. It ensures high-quality end products and helps to gain maximum productivity. Moreover, batch, fed-batch, or continuous mode are used to culture different microorganisms.
It raises productivity levels to a wide range. Somehow, the different types of food industry products are demonstrated by the proper cultivation of microorganisms in liquid media. However, Submerged fermentation is of three main types. They are – Batch, Fed-batch, and Continuous.
- Batch Fermentation:
It is a closed system. In this process, we add medium, nutrients, and inoculum to the bioreactor. But it has to be done under aseptic conditions. At the very beginning level, culture broth volume is kept constant and viable cells are inoculated into the bioreactor. After that, the classical growth curve is followed by cell culture.
This growth is evenly divided into 4 main phases. They are 1. Lag Phase, 2. Exponential Phase 3. Stationary Phase4. Death Phase. All the phases have a different function in the fermentation process. However, submerged batch cultivation is helpful in the production of many alcoholic beverages.
It is also used in the production of organic acids, acidifiers, preservatives, and amino acids or as flavor enhancers and sweeteners. For the overall process of submerged cultivation, stirred bioreactors (capacity 150–200 m3) or bubble columns (capacity up to 1000 m3) are highly recommended.
- Fed-batch Fermentation
It is a semi-open system which requires one or more nutrients to be added gradually to the bioreactor. In the meantime, the product is retained inside and thus the volume of the culture broth is increased as time passes by.
The most effective thing about fed-batch is that it can properly prolong product synthesis. Moreover, it increases the ability to have higher cell density which is very helpful for increasing productivity. Fed-batch cultivation is used mainly when substrate inhibition or catabolic repression is expected.
The food fed-batch fermentations are large-scale production of baker’s yeast, pure ethanol, which is further utilized for alcoholic beverages produced by mixing ingredients such as liquors or cordials Gradual feeding of the substrate and the production of ethanol by yeasts can be removed or decreased under aerobic conditions and fed-batch directly helps in the whole process.
And it is so far one of the most useful sides of fed-batch cultivation. The most vivid dissimilarity between batch and continuous fermentation is that during batch fermentation, the substrate is added only once, but during continuous fermentation, the substrate is added at a remittent rate with the withdrawal of the product at the same rate.
- Continuous Fermentation
Continuous fermentation is involved in the operation of fermentation at a required rate of growth. It does such activity by having control over substrate input and output rate. Its main benefit of this cultivation process is that it ensures a very high production level.
But the problem is that there is a high chance of contamination in this process and thus it makes the product a bit expensive because of the alteration needed to do because of the contamination. It makes the product expensive also because of the recovery.
However, a defined medium is used to conduct this process and other different steps are followed in the continuous fermentation.
So, the major difference between fed-batch and continuous process of fermentation is that in fed-batch the media used for bacterial or yeast growth may be complex or undefined media batch fermentation, but in continuous fermentation defined medium is necessary to conduct the procedure.
Although both of the systems are a helpful tool to learn more about the metabolic pathways of gene expression analysis.
Solid-state is way more different than the classical submerged cultivation process because very low water activity is needed in the whole system of solid-state.
Here, a concentrated medium is used for the fermentation and it ensures the lower risk of contamination of bacteria, yeast, or other harmful things. It happens because the moisture level remains low and balanced thus it eliminates yeast and bacteria effectively.
Thus, the product yield remains higher and makes the product recover way easier than the other process. It even saves money and keeps the product cheaper than the products of classical submerged fermentation.
SEE MORE: Details of Solid-state fermentation
The Types of fermentation process on the basis of commercial importance:
There are five major types of fermentation process which are commercially important. They are:
The fermentation that produces Microbial biomass
In this process, microbial cells are produced as products. It is divided into two major processes. One is the production of Yeast and the other is the production of the microbial cell.
Baker’s yeast is being produced since the early 1900 and from then it has gained popularity for its multiple uses in many different places, but yeast was produced as a portion of human food in Germany for the first first time during WWI. Then in the 1970s, continuous processes were established for animal food production.
After that, a process was of the production of fungal biomass for human food was established by animal feed biomass fermentation. And from that time, it has become a more stable and economic ground for the future production system.
The fermentation that produces Microbial enzymes
Enzymes are mainly produced from the plant, animal and, microbial sources for commercial purposes. The main advantage of the microbial enzymes is that it can be produced by using the established fermentation procedures and even on a very large scale.
Besides, it is very easy to improve the productivity of this microbial system. Usually, these enzymes are used enormously in food and other related industries.
Moreover, the production of enzymes is controlled by micro-organism and thus the production level might be uplifted. And this control is beneficial and has to be exploited or somehow modified for better results.
For example, induction may be exploited if inducers are used in the medium. Also, by using the recombinant DNA techniques, the number of gene copies coding may be increased for the enzymes.
The fermentation that produces Microbial metabolites
The proper growth of a microbial culture can be divided into different stages. The lag phase is one of the periods when growth does not seem to occur and it happens right after the inoculation of a culture into a nutrient medium.
The lag phase is also called the time of adaptation. In this phase, amino acids, nucleotides, proteins, lipids, nucleic acids are produced as primary products of metabolism. These elements ensure the growth of the cells accurately.
Secondly, during the exponential phase, the growth rate of cells increases gradually. When the growth seems to cease, it is known as the stationary phase.
In the course of deceleration and stationary phases, some compounds are synthesized which are not produced. These compounds do not even have any primary function in cell metabolism. Finally, after a certain period, the cell number declines, and this stage is known as the death phase.
However, the products produced in the phases are categorized in different ways named as primary or secondary for the convenience. Yet, it is sometimes very difficult to categorize the products as primary or secondary metabolism. They mainly perform in antimicrobial activity, growth promotion, and other pharmacological properties.
The fermentation that produces Recombinant product
The creation or discovery of recombinant DNA technology has increased the production of the fermentation products to a great extent by opening many doors which is potential in this field.
Genes are now easily transferred by this process. Moreover, microbial cells have become capable of synthesizing heterologous proteins. Many of the microbial cells are used as hosts for this type of system. Usually, insulin, interferons, human serum albumin, and many other products are produced by genetically engineered organisms.
In this type of process, the expression of foreign genes is maximized. Degradation of the product is minimized and the secretion of the product is ensured.
The most advantageous side of microbial cells is that these cells can convert a compound into a compound that is financially more valuable compound and structurally related as well. It happens because the microbial processes are better than purely chemical ones as they are far more specific.
Moreover, they can even enable the addition, removal, or modification as required; and they do it without the use of any other chemical protection. They are a very strong catalyst as well. The main thing about the fermentation process is that large biomass has to be produced just to catalyze a single reaction.
To catalyze the reactions, there are many other processes. However, the immobilized cells play a vital role by performing the task of catalysts or enzymes which can be used several times if needed.