The following points highlight the four main types of industrial fermentations processes. The types are: 1. Solid State 2. Anaerobic 3. Aerobic 4. Immobilized Cell Fermentations.
Type # 1. Solid State Fermentation:
In industrial fermentations, microbial growth and product formation occur at the surface of solid substrates. Examples of such fermentations are mushroom cultivation, mould-ripened cheeses, starter cultures etc. More recently, this approach has been used for the production of extracellular enzymes, certain valuable chemicals, fungal toxins, and fungal spores (used for biotransformation).
Traditional substrates are several agricultural products, rice, wheat, maize, soybean etc. The substrate provides a rich and complex source of nutrients which may or may not need to be supplemented.
Such substrates selectively support mycelial organisms which can grow at high nutrient concentrations and produce a variety of extracellular enzymes, e.g., a large number of filamentous fungi, and a few bacteria (actinomycetes and one strain of Bacillus).
According to the physical state, solid state fermentations are divided into two groups:
(i) Low moisture solids fermented without or with occasional/continuous agitation, and
(ii) Suspended solids fermented in packed columns through which liquid is circulated.
The fungi used for solid state industrial fermentations are usually obligate aerobes (Table 39.5).
Solid-state industrial fermentations on large scale use stationary or rotary trays. Temperature and humidity controlled air is circulated through the stacked solids. Less frequently rotary drum type fermenters have been used. Solid state fermentations offer certain unique advantages but suffer from some important disadvantages. However, commercial application of this process for biochemical production is chiefly confined to Japan.
Type # 2. Anaerobic Fermentation:
In anaerobic fermentation, a provision for aeration is usually not needed. But in some cases, aeration may be needed initially for inoculum build-up. In most cases, a mixing device is also unnecessary, while in some cases initial mixing of the inoculum is necessary. Once the fermentation begins, the gas produced in the process generates sufficient mixing.
The air present in the headspace of the fermentor should be replaced by CO2, H2, N2 or a suitable mixture of these; this is particularly important for obligate anaerobes like Clostridium. The fermentation usually liberates CO2 and H2, which are collected and used, e.g., CO2 for making dry ice and methanol, and for bubbling into freshly inoculated fermenters.
In case of acetogens and other gas utilizing bacteria, O2-free sterile CO2 or other gases are bubbled through the medium. Acetogens have been cultured in 400 1 fermenters by bubbling sterile CO2 and 3 kg cells could be harvested in each run.
Recovery of products from anaerobic fermenters does not require anaerobic conditions. But many enzymes of such organisms are highly O2-sensitive. Therefore, when recovery of such enzymes is the objective, cells must be harvested under strictly anaerobic conditions.
Type # 3. Aerobic Fermentation:
The main feature of aerobic fermentation is the provision for adequate aeration; in some cases the amount of air needed per hour is about 60-times the medium volume. Therefore, bioreactors used for aerobic fermentation have a provision for adequate supply of sterile air which is generally sparged into the medium.
In addition, these fermenters may have a mechanism for stirring and mixing of the medium and cells.
Aerobic fermenters may be either of the:
(i) Stirred-tank type in which mechanical motor-driven stirrers are provided; or
(ii) Of airlift type in which no mechanical stirrers are used and the agitation is achieved by the air bubbles generated by the air supply.
Generally, these bioreactors are of closed or batch types but continuous flow reactors are also used; such reactors provide a continuous source of cells and arc also suitable for product generation when the product is released into the medium.
Type # 4. Immobilized-Cell Fermentation:
Industrial fermentations of this type are based on immobilized cells. Cell immobilization is advantageous when:
(i) The enzymes of interest are intracellular,
(ii) Extracted enzymes are unstable,
(iii) The cells do not have interfering enzymes or such enzymes are easily inactivated/removed and
(iv) The products are low molecular weight compounds released into the medium.
Under these conditions immobilized cells offer the following advantages over enzyme immobilization:
(i) Enzyme purification is not needed,
(ii) High activity of even unstable enzymes,
(iii) High operational stability,
(iv) Lower cost, and
(v) Possibility of application in multistep enzyme reactions.
In addition, immobilization permits continuous operation of bioreactor which reduces the reactor volume and, consequently, pollution problems. Obviously, immobilized cells are used for such bio-transformations of compounds which require action of a single enzyme.
Cell immobilization may be achieved in one of the following ways:
(i) Cells may be directly bound to water insoluble carriers, e.g., cellulose, dextran, ion-exchange resins, porous glass, brick, sand etc., by adsorption, ionic bonds or covalent bonds,
(ii) They can be cross-linked to bi-or multifunctional reagents, e.g., glutaraldehyde etc.
(iii) Polymer matrices may be used for entrapping cells; such matrices are polyacylamide gell, ĸ-Carrageenan (a polysaccharide isolated from a seaweed), calcium alginate (alginate is extracted from seaweed), polyglycol oligomers etc.
Out of these approaches calcium aliginate immobilization is the most commonly used since it can be used for even very sensitive cells, e.g., plant cells; ĸ-Carrageenan is also a useful entrapping agent.
Cell immobilization has been used for commercial production of amino acids, e.g., E. coli cells entrapped in polyacrylamide gel for production of L-aspartic acid, L-alanine production using a mixture of E. coli and Pseudomonas dacunhae immobilized in ĸ-Carrageenan, organic acids, e.g. L-malic acid from furmaric acid using Brevibacterium ammoniagenes cells immobilized in polyacrylamide gel/ĸ-carrageenan (subsequently, B. flavum was used in place of B. ammoniagenes), NADP production by B. ammoniagenes and yeast (Saccharomyces Cerevisiae) cells immobilized together in polyamide gel.