Solid State Fermentation (SSF) is a fermentation technique utilized across industries such as pharmaceuticals, food, and textiles for the production of microbial metabolites using a solid support instead of a liquid medium.
SSF is defined as the growth of microorganisms in the absence of a free-flowing aqueous phase.
This method serves as an alternative to submerged fermentation for producing valuable products such as antibiotics, single cell proteins, polyunsaturated fatty acids (PUFAs), enzymes, organic acids, biopesticides, biofuels, and aromatic compounds.
Solid supports typically used in SSF include grain brans, de-oiled oilseed cakes, and similar materials.
Initially, fungi were primarily used in SSF due to their high efficiency under low water activity conditions; however, over time, many bacterial species and yeasts have also been employed in this fermentation method.
SSF has gained significant attention in recent years within the microbiological field because of its wide range of applications and numerous advantages compared to liquid-based (submerged) fermentation, as highlighted by several researchers.
Solid State Fermentation (SSF) Substrate
Solid State Fermentation (SSF) involves the cultivation of microorganisms on solid substrates with little to no free water.
SSF utilizes solid substrates that maintain low moisture content.
Common substrates include cereal grains (such as rice, wheat, barley, and corn), legume seeds, wheat bran, lignocellulosic materials (like straw, sawdust, or wood shavings), and various plant and animal-derived materials.
These substrates are mostly polymeric compounds that are insoluble or only sparingly soluble in water, yet they are inexpensive, readily available, and rich in nutrients that support microbial growth.
SSF substrates are characterized by several key properties:
A solid porous matrix, either biodegradable or non-biodegradable, with a large surface area per unit volume (typically ranging from 10³ to 10⁶ m²/cm³), enabling microbial growth at the solid/gas interface.
The matrix must be capable of absorbing water equal to or greater than its dry weight while maintaining high water activity at the surface to support active biochemical reactions.
Adequate air (oxygen and other gases/aerosols) must flow through the system at low pressure to ensure mixing and aeration of the fermenting material.
The solid/gas interface must provide a suitable environment for the rapid growth of specific microbial cultures, including molds, yeasts, or bacteria, in either pure or mixed forms.
The solid matrix must possess mechanical strength to endure compression or gentle stirring without breaking apart or clumping, favoring small granular or fibrous particles.
The matrix must be free of microbial inhibitors and capable of absorbing or containing key nutrients such as carbohydrates (cellulose, starch, sugars), nitrogen sources (ammonia, urea, peptides), and mineral salts.
Organisms Used in Solid State Fermentation (SSF)
In Solid State Fermentation (SSF), the microbial component may consist of pure single cultures, mixed known cultures, or completely mixed indigenous microbial populations.
Certain SSF processes, such as tempeh and ontjom production, require the selective growth of molds that thrive in low-moisture environments and perform fermentation by secreting extracellular enzymes.
Although bacteria and yeasts generally require higher moisture levels for effective fermentation, they can still be employed in SSF, though they typically produce lower yields compared to molds.
Steps in Solid State Fermentation (SSF)
Solid State Fermentation (SSF) typically involves multiple steps for the efficient conversion of lignocellulosic biomass into useful products such as enzymes.
The key steps in SSF include:
Pre-treatment of substrate raw materials through mechanical, chemical, or biochemical methods to improve nutrient availability and reduce particle size (e.g., pulverizing straw or shredding vegetable waste); however, the cost of pre-treatment must be weighed against the value of the final product.
Hydrolysis of complex polymeric substrates such as polysaccharides and proteins into simpler compounds.
Fermentation (utilization) of hydrolysis products by microorganisms.
Separation and purification of the final end products.
The low moisture content in SSF allows for a reduced reactor volume per unit of substrate mass compared to liquid state fermentation (LSF) and simplifies the recovery of products.
Despite these benefits, SSF presents challenges such as difficulties in mixing, heat transfer, oxygen distribution, moisture regulation, and the creation of gradients in pH, nutrients, and product concentrations due to culture heterogeneity.
These complexities make monitoring and controlling key parameters in SSF difficult, time-consuming, and often imprecise, which limits its industrial scalability.
Therefore, microorganisms used in SSF are generally selected based on their ability to tolerate a broad range of cultivation conditions.
Applications of Solid State Fermentation (SSF)
Solid State Fermentation (SSF) has emerged as a promising technology for producing various microbial products, including feed, fuel, food, industrial chemicals, and pharmaceuticals.
SSF is widely used for the production of enzymes, organic acids, and flavoring compounds, which are subsequently extracted, purified, and utilized in various applications.
The technology is also applied in important bioprocesses such as bioleaching, bio-beneficiation, bioremediation, and bio-pulping, offering multiple benefits in each case.
Advantages of Solid State Fermentation (SSF)
SSF generates minimal waste and liquid effluents, making it environmentally friendly.
It uses simple, natural solid materials as the fermentation media.
The process is low-tech, consumes little energy, and requires less capital investment.
Sterilization is often unnecessary, contamination risks are lower, and downstream processing is simpler.
Agro-industrial residues can be effectively used as substrates, offering value-added use for otherwise underutilized or wasted materials.
SSF typically yields reasonably high product output.
The design of bioreactors, aeration systems, and effluent treatment processes is straightforward.
A wide range of domestic, industrial, and agricultural wastes can be efficiently utilized in SSF processes.
Limitations of Solid State Fermentation (SSF)
Only microorganisms that can tolerate low moisture content are suitable for use in SSF.
Precise monitoring of critical parameters such as oxygen (O₂), carbon dioxide (CO₂), and moisture content is challenging in SSF.
Microbial growth tends to be slower, leading to limited product formation.
Excessive heat generation during the process poses challenges, and regulating the growth environment is difficult.
References
Pandey, A., Soccol, C.R., & Mitchell, D. (2000). Solid-state fermentation and its applications. Materials Today, ScienceDirect.
BiologyDiscussion. (n.d.). Short Notes on Solid Substrate Fermentation. Retrieved from BiologyDiscussion.com
Omics Online. (n.d.). Solid State Fermentation and Food Processing: A Short Review. Journal of Nutrition & Food Sciences, Retrieved from OmicsOnline.org
Mitchell, D. et al. (2006). Solid-State Fermentation – An Overview. ResearchGate, Retrieved from ResearchGate.net
Nandanam, A. (n.d.). Solid State Fermentation. SlideShare Presentation, Retrieved from SlideShare.net
Soccol, C.R. et al. (2017). Recent developments in solid-state fermentation. National Center for Biotechnology Information (NCBI), Retrieved from NCBI.nlm.nih.gov
University of KwaZulu-Natal. (n.d.). Solid State Fermentation – Course Material. Retrieved from UKZN Microbiology Department
%20-%20Process,%20Types,%20Organisms,%20Applications,%20and%20Limitations.webp "Solid State Fermentation (SSF): Process, Types, Organisms, Applications, and Limitations")
%20Substrate.webp "Solid State Fermentation (SSF) Substrate")
.webp "Steps in Solid State Fermentation (SSF)")
