Solid State Fermentation: Definition, Process and Application
What is solid state fermentation?
The production of microbial metabolites is mostly accomplished by solid-state fermentation, or SSF. By conducting SSF on a solid substrate with low moisture content, a high product concentration may be achieved with significantly less energy being used. There are a number of advantages to microorganism growth and oxygen transport in SSF, which is achieved when the substrate absorbs the required water content inside a solid matrix. Agricultural byproducts such as rice straw, wheat straw, sugarcane bagasse, rice hulls, and corn cobs are being utilized as SSF substrates. Submerged fermentation (SmF) is the method of choice for metabolite synthesis in industry due to its easier downstream process as compared to solid-state fermentation (SSF). Although the SSF process is poorly understood, there is room for robust and simple manipulation of the many components in the context of SmF. Although SmF has less complicated operational parameters than SSF, SSF's microbial secondary metabolite production is more suited to its mycelial structure. Thus, SSF exhibits a number of advantages, including effective gaseous conveyance, decreased water consumption, use of cellulosic waste, management of pH, and the need for smaller fermenters, leading to less strain for further processing.
SFF is the biological process when microbes proliferate on solid substrates without the existence of a liquid phase. Practically speaking, an aerobic bioreactor is generally filled with a solid substrate and then infected with the specific strain of interest in order to generate the intended bioproduct. Following production, this bioproduct can be retrieved, however in certain instances the resulting fermented solid can serve as the ultimate product. Various reactor types have been employed in the generation of solid-state fuel: packed bed reactors, mechanically stirred reactors, tray reactors, and plug flow topologies. All these topologies share a common goal in the scale-up process: to address the constraints of mass and heat transport in solid organic matter, which can lead to elevated temperatures that cause damage to the desired strain. The emergence of these issues is often observed as the size of SSF is increased, and they provide a significant challenge for the complete use and commercialization of SSF. In recent times, several models have been released to observe the mass dynamics in small-scale reactors. These models employ both conventional methods such residence time distributions and more sophisticated ways like computational fluid dynamics.
General overview of organic waste revalorization, with a special emphasis on solid-state fermentation (SSF). (Oiza N., et al., 2022)
Cultures of microorganisms used for solid-state fermentation to produce metabolites. (Srivastava N., et al., 2019)
Solid fermentation vs liquid fermentation
Both SSF and liquid fermentation are fundamental techniques employed in industrial microbiology to grow microorganisms and manufacture diverse bioproducts.
Substrate and medium: As a medium for seed growth, SSF makes use of solid materials like agricultural byproducts (such as rice husks and wheat bran). The substrate typically contains very little moisture. In solid substrate fermentation, germs are physically supported by solids, while in liquid fermentation, microbes are grown in a suspended or dissolved state utilizing a liquid nutrition medium. A nutrient-rich aqueous solution that facilitates effective absorption of nutrients is the medium in microbiological systems.
Moisture content: With a moisture level ranging from 20% to 70%, SSF is quite dry. Liquid fermentation integrates a considerable moisture content ranging from 95% to 100%, simulating natural circumstances like soil or decaying material with a damp but not saturated substrate. The complete medium is liquid, which makes it ideal for microbial growth since it creates a uniform environment.
Aeration: SSF typically needs enough aeration to guarantee the presence of oxygen, given that most metabolic activities are aerobic. The inherent solidity of the substrate facilitates the process of oxygen diffusion, whereas liquid fermentation generally needs mechanical aeration and agitation to sustain oxygen levels and guarantee even distribution of nutrients.
Advantages: SSF is cost-effective when using inexpensive agricultural residues, has a higher product concentration, and is ideal for producing products that are sensitive to high water content. In contrast, liquid fermentation is more straightforward downstream processing, has better homogeneity and ease of scale-up, and is easier to control environmental conditions such as pH, temperature, and nutrient levels.
Submerged fermentation versus solid-state fermentation. (Nanda S., et al., 2023)
Fermentation technology at BOC Sciences
- Aerobic Fermentation
- Anaerobic Fermentation
- Continuous Fermentation & Fed-Batch Fermentation
- Solid State Fermentation (SSF)
- Submerged Fermentation (SmF)
- Fermentation CDMO
What is the effect of solid fermentation?
SSF can break down complex organic molecules like lignocellulose into simpler ones, making nutrients more accessible. This is especially useful in agricultural waste valorization, as the substrate is high in complex carbohydrates, proteins, and fibers. In animal feed production, SSF can improve digestibility by breaking down antinutritional components and increasing the concentration of digestible minerals. SSF is particularly useful for manufacturing industrial enzymes such cellulases, proteases, and amylases. These enzymes are essential for a variety of sectors, including biofuels, food, and textiles. SSF can stimulate the creation of bioactive substances such as antibiotics, vitamins, and organic acids. These molecules have potential uses in medicines, food preservation, and nutritional supplements.Traditional fermented foods produced by SSF, such as tempeh, koji, and miso, benefit from increased tastes, textures, and nutritional profiles as a result of the fermentation process.
Effect of solid-state fermentation on plant-sourced proteins. (Nanda S., et al., 2023)
Solid state fermentation process
Selection of substrate: The substrate is chosen based on the intended final product. Common substrates include agricultural leftovers (e.g., wheat bran, rice bran, soybean meal), food manufacturing byproducts, and other organic materials.The substrate should include the nutrients required for microbial development, such as carbon sources (carbohydrates), nitrogen sources (proteins), minerals, and vitamins.
Preparation of substrate: The substrate is frequently physically treated (e.g., grinding or milling) to reduce particle size, increasing the surface area for microbial growth and improving substrate accessibility.
Inoculum preparation: The selected microorganisms are cultivated to a suitable cell density before being put into the substrate as an inoculum (microbial culture). The inoculum might take the shape of spores, cells, or mycelium. Under regulated conditions—including temperature, humidity, and aeration—the inoculated substrate is incubated. The particular microbe and product determine these optimal values.
Product harvesting: Drawn from the fermenting substrate is the intended product. The product will dictate whether this requires filtering, centrifugation, solvent extraction, or another kind of separation.
Solid-state fermentation process for production of value-added product. (Srivastava N., et al., 2019)
Application of solid-state fermentation
Industrial enzymes like cellulases, proteases, amylases, lipases, and pectinases are made from SSF most usually. Industries include textiles, food processing and biofuels depend critically on these enzymes. Many traditional dishes, like idli (a fermented rice and lentil meal), tempeh (fermented soybeans), and koji (used in sake, soy sauce, and miso manufacture), are built on SSF. These techniques improve food's flavor, texture, and nutritional worth. Penicillin, generated by Penicillium species, and other antimicrobial compounds are among antibiotics made from SSF. Pharmaceutical companies depend on these bioactive substances absolutely. Some pharmacological molecules, including immunosuppressants like cyclosporine and other secondary metabolites with therapeutic action, are produced using SSF. Applied in bioremediation of waste products and polluted soils, microorganisms cultivated may break down toxins like heavy metals, insecticides, and hydrocarbons.
References
- Srivastava N., et al., Solid-state fermentation strategy for microbial metabolites production: An overview, New and future developments in Microbial Biotechnology and Bioengineering, 2019: 345-354.
- Oiza N., et al., Solid-state fermentation from organic wastes: A new generation of bioproducts, Processes, 2022, 10(12): 2675.
- Nanda S., et al., A review of liquid and gaseous biofuels from advanced microbial fermentation processes, Fermentation, 2023, 9(9): 813.
