Depending on the circumstance, the term
fermentation can have three meanings. In a
physiological sense, fermentation refers to a
process that produces energy by breaking down of energy-rich compounds under anaerobic conditions. This is the basis of
food fermentation, which is commonly used in the food industry in order to produce diverse food products and also as a food preservation method. In a
biotechnological context though, this term is used in a much broader sense. There, f
ermentation means a process in which microorganisms that are cultured on a large-scale under aerobic or anaerobic conditions, convert a substrate into a product which is useful to man.
This article attempts to provide an overview of the industrial fermentation process, mainly focusing on various types of fermentations, the basic steps in a typical fermentation process and the major industrial applications of fermentation.
Basic Steps of Industrial Fermentation
Any industrial fermentation operation can be broken down into t
hree main stages, viz,
upstream processing, the fermentation process and downstream processing.
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Upstream processing includes
formulation of the fermentation medium,
sterilisation of air, fermentation medium and the fermenter,
inoculum preparation and
inoculation of the medium.
The
fermentation medium should contain an
energy source, a
carbon source, a
nitrogen source and
micronutrients required for the growth of the microorganism along with
water and
oxygen, if necessary.
A medium which is used for a large scale fermentation, in order to ensure the sustainability of the operation, should have the following characteristics;
1. It should be cheap and easily available
2. It should maximise the growth of the microorganism, productivity and the rate of formation of the desired product
3. It should minimise the formation of undesired products
Usually, waste products from other industrial processes, such as molasses, lignocellulosic wastes, cheese whey and corn steep liquor, after modifying with the incorporation of additional nutrients, are used as the substrate for many industrial fermentations.
Sterilisation is essential for preventing the contamination with any undesired microorganisms.
Air is sterilised by memb
rane filtration while the
medium is usually
heat sterilised. Any nutrient component which is
heat labile is
filter-sterilised and later added to the sterilised medium. The fermenter may be sterilised together with the medium or separately.
Inoculum build up is the preparation of the seed culture in amounts sufficient to be used in the large fermenter vessel. This involves growing the microorganisms
obtained from the pure stock culture in
several consecutive fermenters. This process cuts down the time required for the growth of microorganisms in the fermenter, thereby increasing the rate of productivity. Then the seed culture obtained through this process is used to inoculate the fermentation medium.
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Inoculum preparation procedure
The fermentation process involves the
propagation of the microorganism and
production of the desired product. The fermentation process can be categorised depending on various parameters.
It can be either
aerobic fermentation, carried out in the presence of oxygen or
anaerobic fermentation, carried out in the absence of oxygen. Many industrial fermentation are carried out under aerobic conditions where a few processes such as ethanol production by yeast require strictly anaerobic environments.
The fermentation process can also be divided into three basic systems, namely
batch, continuous or fed-batch, depending on the
feeding strategy of the culture and the medium into the fermenter. Each of these processes has their own advantages and disadvantages. In a
batch operation, the medium and the culture are initially fed into the vessel and it is then closed. After that, no components are added apart from oxygen (in an aerobic process) and acid or alkali for the pH adjustment. The fermentation is allowed to run for a predetermined period of time and the product is harvested at the end. In a
continuous process, fresh medium is continuously added and the products, along with the culture is removed at the same rate, thus maintaining constant concentrations of nutrients and cells are maintained throughout the process. A
fed-batch system is a combination of these two systems where additional nutrients are added to the fermenter as the fermentation is in progress. This extends the time of operation but the products are harvested at the end of the production cycle as in a batch fermenter.
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The process can also be categorised as
solid state fermentation (SSF) or
submerged fermentation (SmF), depending on the
amount of free water in the medium. In a
solid state fermentation, the medium contains no free flowing water. The organisms are grown in a solid substrate which is moistened. This is used in certain industrial process such as ‘koji’ fermentation from soybeans, production of amylase and protease by Aspergillus oryzae on roasted soybeans and wheat, bioremediation, detoxification of agro-industrial wastes, etc.
Submerged fermentation is in which microorganisms grow submerged in a liquid medium where free water is abundant. This is the method of choice for many industrial operations over SSF although SSF is also rapidly gaining interest in the present.
A comparison of SSF and SmF
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Downstream Processing includes the
recovery of the products in a pure state and the
effluent treatment.
Product recovery is carried out through a series of operations including
cell separation by settling, centrifugation or filtration;
product recovery by disruption of cells (if the product is produced intracellularly);
extraction and
purification of the product. Finally, the
effluents are treated by chemical, physical or biological methods.
Fermentation Products
Commercially important products of fermentation can be described in five major groups as follows.
1.
Biomass (Baker’s yeast, SCP, Starter cultures, animal feed, etc.)
2.
Primary metabolites (amino acids, organic acids, vitamins, polysaccharides, ethanol, etc.) and
secondary metabolites (antibiotics, etc.)
3.
Bioconvertion or biotransformation products (steroid biotransformation, L-sorbitol etc.)
4.
Enzymes (amylase, lipase, cellulase, etc.)
5.
Recombinant products (some vaccines, hormones such as isulin and growth hormones etc.)
Sources
1. Food Biotechnology (2nd edition) edited by Kalidas Shetty, Gopinadhan Paliyath, Anthony Pometto and Robert E. Levin
2. Introduction to Biochemical Engineering By Dubasi Govardhana Rao
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