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In this article we will discuss about:- 1. Introduction to Fermentation 2. Historical Events in the Progress of Fermentation 3. Process.
Introduction to Fermentation Process:
Fermentation term for the first time was coined by Louis Pasteur for a phenomenon of bubbling of sugar solution. Later on, it has been applied for the phenomenon of production of different chemicals involving microorganisms.
Presently, the term is used solely to any phenomenon involving microorganisms. Many products are made by large-scale fermentation including amino acids, enzymes, organic acids, vitamins, antibiotics, solvents and fuels. The typical fermentation process is depicted in Fig. 2.1.
The advantages in producing materials by fermentation are as follows:
1. Complex molecules such as antibiotics, enzymes and vitamins are impossible to produce chemically.
2. Optically active compounds such as amino acids and organic acids are difficult to prepare chemically.
3. Though some of the products that can be economically derived by chemical processes, but for food purpose they are better produced by fermentation such as beverages, ethanol and vinegar (acetic acid).
4. Fermentation usually uses renewable feed stocks instead of petrochemicals.
5. Reaction conditions are mild, in aqueous media and most reaction steps occur in one vessel.
6. Byproducts of fermentation are usually chemicals. The cell mass and other major by products are highly nutritious and can be used in animal feeds.
However, it is beset with some drawbacks, which are as follows:
1. The products are made in complex solutions in low concentrations as compared to chemically derived compounds.
2. It is difficult and expensive to purify the product.
3. Microbial processes are much slower than chemical processes, increasing the fixed cost of the process.
4. Microbial processes, are subjected to contamination by competing microorganisms, requires the sterilization of the raw materials and the containment of the process to avoid contamination.
5. Most microorganisms do not tolerate wide variation in temperature, pH and are also sensitive to upsets in the oxygen and nutrient levels. Such upsets not only slow the process, but fatal to microorganism. Thus careful control of pH, nutrients, air and agitation require close monitoring and control.
6. Although nontoxic, waste products have high BOD and requires extensive sewage treatment.
Though microorganism belonging to bacteria, fungi and yeasts are extensively used in these fermentation, few fermentations are also based on algae, plants and animal cells.
Several cellular activities contribute to fermentation products such as:
1. Primary metabolites- Ethanol, lactic acid and acetic acid.
2. Energy storage compounds- Glycerol, polymers and polysaccharides.
3. Proteins- SCP, enzymes of both extra and intracellular nature and foreign protein.
4. Intermediate metabolites- Amino acids, citric acid, vitamins and malic acid.
5. Secondary metabolites- Antibiotics.
6. Whole cell products- SCP, bakers yeast, brewers yeast, bio-insecticides.
Some of the products such as ethanol, lactic acid and cell mass products are generally growth associated, while secondary metabolites, energy storage compounds, and polymers are non-growth associated. Other products, such as protein depends on the cellular or metabolic function. Unlike primary metabolites which are essential for growth and reproduction, secondary metabolites are not essential for the growth and development of reproducing organism and are produced only in luxuriant conditions.
The secondary metabolites are basically are:
1. Secondary metabolites are produced only by few organisms.
2. Secondary metabolites are needed depending on environmental conditions.
3. Secondary metabolites are produced as a group of closely related structures.
4. Some organisms forms a variety of different classes of substances such as secondary metabolites.
5. The regulation of biosynthesis of secondary metabolites differs significantly from that of primary metabolites.
6. Secondary metabolites are mostly produced in iodophase (Fig. 2.3)
Origin and production of different secondary metabolites are depicted in Fig. 2.2 and 2.2 a.
Fermentative products are in use by man since ancient times. Fermentation of grains or fruit produce, bread, beer and wine that retained much of the nutrition of raw materials, while keeping the product from spoiling. The natural yeasts that caused fermentation added some vitamins and other nutrients to the bread or beverage.
Lactic acid producing bacteria ferment milk to yogurt and cheese and extend the life of milk products. Other food products such as pickles, vegetables and the fermentation of tea leaves and coffee beans were preserved or enhanced in flavor by fermentation.
Historical Events in the Progress of Fermentation:
Fermentation was an art until the second half of the 19th century. A batch was begun with either a starter, a small portion of previous culture, or with culture residing in the products or vessel. Pasteur (1775) made it clear that fermentation needs, heat treatment to improve storage quality and thus formed the basis for sterilization of medium. Emil Christian Hansen (1883) used for the first time pure culture of yeast for production of yeast in Denmark.
During 1920-30 the emphasis in fermentation shifted to organic acids primarily lactic acid and citric acid. The discovery of penicillin in 1929 and commercialized in 1942, gave a boost to fermentation industry and led to the development of big fermenters and submerged cultivation. Success of penicillin inspired pharmaceutical companies to launch massive efforts to discover and develop many other antibiotics.
In 1960s amino acid fermentations were developed in Japan. Commercial production of enzymes for use in industrial process began on a large scale in 1970. The discovery of the tools of genetic engineering expanded the possibilities for products made by fermentation in situ, and the first genetically engineered fermentation product was developed and commercialized in 1977. The historical events developed in the progress of fermentations are précised in table 2.1.
Fermentation may be aerobic if it is operated in the presence of oxygen, while it may be anaerobic if carried out in the absence of oxygen. Anaerobic fermentations can be carried out either by use of fresh medium, covered with an inert gas such as nitrogen or argon or accumulation of CO2 or foam (Fig. 2.4).
The fermentation is called batch fermentation when it is operated for a definite period. On the other hand, fermentation which is operated for an indefinite period it is called continuous fermentation. Some of the organisms are sensitive to substrate concentration and they are inhibited when the substratum is in high concentration. Under such conditions, fermentation can be carried by addition of substrates in installments and the process is called Fed batch fermentation.
Fermentations can be carried out under non-aseptic conditions where the risk of contamination is not a major concern. However, fermenters must be designed for prolonged aseptic operation. The design rules for an aseptic bioreactor demand that there is no direct contact between the sterile and non-sterile sections to eliminate microbial contamination.
Similarly, fermentation based on number of organisms involved can be classified into simple fermentation when only one organism is involved to produce a product from substratum. On the other hand, in some fermentations two organisms are involved in order to get a fermentation product from a substratum. The product of first phase of fermentation serves as substratum for second phase in order to yield desired product.
In this type of fermentation, two organisms may grow simultaneously and product is formed instantly. Commercial growth of lichens involving algae and fungi is a good example for simultaneous fermentation. Production of glutamic acid from glucose firstly gets oxidized to ketoglutaric acid, which in turn get aminated to produce glutamic acid and production of lactic acid from glucose by yeast and Lactobacillus lactis, production of β-carotene jointly by (+) and (-) strains of either choaenophoracucurbitarum or Blakesleea trispora are three very good examples.
On the other hand, the two organisms involved in a fermentation are separated widely in time and space, such fermentation is called successive fermentation. For example-production of acetic acid from glucose. First glucose is acted by yeast to produce ethyl alcohol, which is oxidized to acetic acid by Acetobacter aceti. Similarly production of lysine from glycerol. Glycerol is fermented to Diaminopimelic acid (DAPA) by an auxotrophic mutant of E. coli which gets aminated to form L-Lysine by Aerobacter aerogenes.
When more than two organisms are involved in a fermentation it is called as mixed fermentation or multiple fermentation.
In this fermentation, the substratum is heterogeneous and organisms with different potentialities of producing enzymes are involved in the fermentation. For instance, degradation of municipal wastes and decomposition of dead plants and animals can be taken as mixed or multiple fermentation. Similarly, remediation of waste water comes under this fermentation.
Process of Fermentation:
Fermentation process can be conveniently divided into six stages regardless of the type of process.
They are:
1. The formulation media used for the growth of the microorganism to be employed as inoculum and also in the production of fermentation products.
2. The sterilization of the medium, fermenter and other associated equipment.
3. The preparation of adequate quantities of pure culture that is to be inoculated into the fermenter.
4. The creation of optimum conditions in the fermenter for optimum growth of the organism and for optimum output of the desired product.
5. The extraction of the product and its purification.
6. The disposal of effluents generated during fermentation.
The inter relationships among these six phases are diagrammatically illustrated in Fig. 2.10.
This process varies with the type of organism used and product to be produced.
The entire process can be discussed under two headings:
(a) Upstream Process:
It includes selection of organism and medium, medium sterilization, inoculation and ends with monitoring of fermentation process and product formation. This involves selection of microorganism. The selection of microorganisms for fermentation should be critically done. At first it should have potential to produce particular substance in an economic amounts. It should be nonpathogenic and non-hazardous. Further it should be amenable to growth in a fermenter and produce the product in good amounts,
(b) Downstream Process:
It includes the product separation and purification and effluent treatment.