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Solid State Fermentation.

Introduction.

Solid state fermentation (SSF)is the process that occurs in a solid substrate without or with negligible water but the substrate needs water to allow the micro-organisms to grow as well as for their metabolic activity. Solid State Fermentation has been widely discussed in recent times because it has myriad uses and is often a better alternative to the liquid variants of fermenters. The most vital act performed by SSF is the resistance of bacterial fungal cells to catabolic repression (inhibition of enzyme synthesis) in the presence of abundant substrates, such as glycerol, glucose or other carbon sources .Another important aspect is the usage of agro-industrial residues (A-IR) produced by current industrial processes as well as usage of unexplored biological resources as the solid substrate for production of metabolic products at relatively lower cost of production, allowing the process to be economically reasonable .Moreover, reduction of environmental issues such as A-IR, which results in soul pollution and production of foul odour, represents problems for the industry

SSF was originally used for production of enzymes, antibiotics, organic acids, biosurfactants and aroma compounds; however, Solid State Fermentation gained more popularity because of it’s ability to broaden the horizon of metabolite production and contribution to reduction of environmental pollution [9].Currently, SSF is being discussed with respect to ascribing value to untapped reserves of biomass. SSF addresses the issue of environmental pollution by providing an opportunity to utilise agro-industrial residues. In this way, a wide variety of applications can be achieved, such as bioremediation, production of lipids, biofuels (biodiesel, bioethanol, biobutanol, biohydrogen), aromas and flavours for the food industry, and production/extraction of bioactive compounds, among others.

Substrates used in SSF.

There are many biotechnological processes that involve the growth of organisms on solid substrates in the absence or near absence of free water. Solid state fermentation (SSF) deals with substrates that are solid and contain low moisture levels. Straws, sawdust or wood shavings, and a wide range of plant and animal materials. The most regularly used solid substrates are cereal grains (rice, wheat, barley, and corn), legume seeds, wheat bran and lignocellulose materials. Most of these compounds are polymeric molecules – insoluble or sparingly soluble in water – but most are cheap and easily obtainable and represent a concentrated source of nutrients for microbial growth. •It is defined as the growth of microbes without free-flowing aqueous phase.

•The SSF is alternative to submerged fermentation for production of value-added products like antibiotics, single cell protein, PUFA’s, enzymes, organic acids, biopesticides, biofuel and aroma production.

•The support used is especially grain brans, de-oiled oil seed cakes, and other substances alike.

•Initially, mostly fungi were used in such fermentation (as these microorganisms were considered to be very optimally active in very low water activity). Later, many bacterial species and yeasts were used to carry out such fermentation also.

•The microbiological process of SSF has generated great interest in recent years because it can be used for a variety of purposes, supported by some authors who have even indicated numerous advantages over their liquid counterparts (submerged fermentation).

Enzymes

SSF provides a superior mechanism for enzyme production since it gives high yield and can be used in industries. The nutrient components present in A-IR promote microbial growth and metabolic activity that leads to production of enzymes as a product or by-product. Various authors have reported that multiple enzymes can be produced using Solid State Fermentation at a small scale level and the literature indicates a large variety of enzymes that are produced by SSF. For large-scale production of enzymes in industries, the bioreactor being used should have the ability to control heat transfer, oxygen and moisture. While designing the bioreactor, the nature of both the substrate and the micro-organism must be considered. Interestingly, Japanese industries employ SSF technique for production of enzymes in industries. In SSF, the A-IR residue used varies with different micro-organisms. Since enzymes produced by SSF show significant market sales, they are considered an important aspect of biotechnology .The market of industrial enzymes has shown gradual growth: for 1998, sales were estimated at $1 billion but for 2015, they were estimated at $4.4 billion. This significant progress demonstrate the importance of SSF in the enzyme market, however, it is important to note that the titers of enzyme activity expressed in SSF are higher than those expressed in submerged fermentation (SF), for example: 5000 and 1600 U l−1 for a pectinase produced by Aspergillus niger, 7150 and 1714 UI l−1 for a exopectinase produced by A. niger C28B25 , 30 and 8 U/g of dry substrate for a protease produced by Aspergillus oryzae showed the importance of SSF in enzyme production.

Antibiotics

Antibiotics were originally produced by Submerged Fermentation but if fungal sample is taken from “near nature” sources, Solid State Fermentation is a better method. Also, A-IR acts a carbon and nitrogen source in antibiotic production using SSF. Moreover, in certain cases, nutrients present in the substrate may be inductors or supplementary nutrients for antibiotic production .To make more evident substrates used in antibiotics production by SSF, some examples are mentioned then: tetracycline by Streptomyces viridifaciens ATCC 11989 using sweet potato, rice bran, and soy meal, neomycin by Streptomyces marinensis using raspberry seed powder, wheat rawa, wheat bran, rice bran ; ), cephalosporin C by Acremonium chrysogenum C10 using sugarcane bagasse meroparamycin by Streptomyces sp. MAR01 using rice, wheat bran, quaker, bread, and ground corn , lovastatin by Penicillium funiculosum NCIM 1174 using green gram husk, black gram husk, wheat bran and orange peel rifamycin B by Amycolatopsis mediterranea MTCC14 and Nocardia mediterranei using coconut oil cake, groundnut oil cake, ground nut shell, rice husk and sunflower oil cake , rifamycin SV by Amycolatopsis mediterranei OVA5-E7 using ragi bran.

Organic Acids

Solid State Fermentation is preferred over Submerged Fermentation in the production of organic acids since the latter is a more expensive procedure .Since Solid State Fermentation uses agro-industrial waste as substrate, it is a cheaper and more viable option. SSF is also advantageous because it shows high efficiency when it comes to extraction of acids from fermented products. Various acids can be produced using agro-industrial substrate such as citric acid, lactic acid, gluconic and ellagic acid.

Bioactive compounds

Extraction of bioactive compounds from biotic materials has generally been realized using habitual extraction processes (solid–liquid/liquid–liquid). However, SSF has emerged as an alternative for the production/extraction of bioactive compounds The implementation of SSF for extraction of bioactive compounds using microorganisms (bacteria, yeast and fungi) is a suitable alternative, due to these microorganisms being able to produce enzymes required for bioactive molecule liberation into cell walls of plants or biotic materials (e.g. pectinases, cellulases, α-amylases, xylanases, β-glucosidase, β-galactosidase, and β-hesperidinase).

References