SSF Technology for Solid Waste Management - Sunita Gaind

Solid state fermentation popularly referred as SSF is a process of growing microbes on moistened solid substrate, in which enough moisture is present to maintain microbial growth and metabolism, but there is no free moving water. Microorganisms grow on the surface of moist particles as well as inside and between them. The space between the particles is occupied by the gas phase. Due to its numerous advantages over submerged fermentation in terms of product recovery, production cost, low wastage, reduced energy requirement and simple operation; it can offer many viable alternatives to the disposal of agricultural/ agro-industrial waste, producing several new products in the process.
Large amount of waste generated by food, agricultural and forestry industries cause a serious disposal problem. Present scenario demands the effective management of this waste as a resource and microbial technology is significantly on demand. Bagasse and peels generated in the beverages and juice industries, coffee pulp obtained in the coffee industry, husks from the cereal industries, sweet potato residue, corn cob, wheat bran, rice bran, spent brewery grains, pomace of apple and grape, waste of oil processing mills as coconut cake, castor cake, pea nut cake, soybean cakes have a nutritional potential,. Due to the presence of carbon, nitrogen and fibre content they could be exploited microbiologically for production of industrially relevant compounds under solid-state fermentation. The application of SSF technology has been extended to biological detoxification of agro-industrial residues, bioconversion of biomass, bioconversion of crop residues for nutritional enrichment, production of antibiotics, biopesticides, biopulping, plant growth factors, enzymes, organic acids and pigments etc.

However, the selection of a suitable microorganism as well as the agro industrial residue that can support the growth of inoculated culture is a pre requisite for the process to be successful. Most important physical parameters to be considered include its texture. It should have small granular or fibrous particles that don't stick together or tend to break and have a solid porous matrix to absorb water more than its dry weight. Substrates with particle size of 425-500 micron have been reported to be best for enzyme production. It should have a relatively high water activity on solid gas interface to allow high rate of biochemical processes. Moisture is another critical factor on which the efficiency of any SSF process depends. The biological activity ceases at moisture content below 15 %; however, the upper limit varies with the substrate material type and may range from 30-80 %. For bacterial growth moisture of the solid matrix above 70 % is preferred. In case of yeast it can vary between 62-70 % and for fungal growth the range could be as wide as 20-70 %. For fungal growth, high water activity results in sporulation and low water activity favours spore germination and mycelial growth. Intermittent agitation can result in better product recovery. As each organism has its pH and temperature optima, they also need due consideration for SSF process to be successful.

A large number of microorganisms including bacteria, fungi and yeast can grow on solid substrates in SSF system. However, fungi and yeast are more acceptable and most exploited microorganisms that can tolerate low moisture content and can grow under adverse environmental conditions. Aspergillus niger is known to produce as many 19 different enzymes and several other value added compounds, such as citric acid and alcohols, by SSF of agro industrial residues. Several organisms may produce single enzymes and vice versa. The different enzyme produced using residues include both food and feed enzymes (phytase, amylase, cellulase, chitinase, protease, lipase, laccase, tannase) employing variety of microorganisms. Most widely used fungi include member of genus Penicillium, Aspergillus (A. flavus, A. fumigatus, A.awamori , A.oryzae, A.niger), Trichoderma (T.viride): bacteria of genus Bacillus (B. brevis, B. circulans, B.coagulans, B. amyloliquifaciens) used for production of alpha amylase. Pitchia anomala and Sporotrrichum thermophilum have been used for production of phytase. Bioconversion of wheat straw for protein enrichment can be carried out using Chaetomium cellulolyticum and Candida utilis. Bacillus thuringenesis has been grown on wheat bran for production of toxin.
SSF offers attractive alternative to add value to the residues besides to be of great economical advantage and environment friendly way for solid waste management.

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