Solid State Fermentation (SSF) – Definition, Steps, Bioreactor, Applications

What is Solid State Fermentation (SSF)?

• Solid State Fermentation (SSF) is a novel fermentation technology used in a variety of industries, including medicines, food, and textiles, to create metabolites from microbes by using solid support rather than a liquid medium. In contrast to standard fermentation procedures, SSF involves the development of microorganisms in the absence of a free-flowing aqueous phase.
• Antibiotics, single cell protein, polyunsaturated fatty acids (PUFAs), enzymes, organic acids, biopesticides, biofuels, and fragrance compounds are produced using SSF as an alternative to submerged fermentation. SSF solid support is often made out of grain brans, de-oiled oil seed cakes, and other similar materials.
• Initially, SSF mostly used fungus for fermentation processes since these microorganisms were thought to be extremely active in low water activity settings. However, various bacterial species and yeasts have been used in SSF over time, broadening the variety of microorganisms suited for this fermentation process.
• Because of its adaptability in a variety of applications, the microbiological process of SSF has piqued the interest of many people in recent years. SSF has a number of advantages over its liquid equivalent, submerged fermentation, according to certain authors. Higher product yields, greater product quality, easier downstream processing, lower water and energy requirements, and improved process economics are all possible benefits.
• Solid State Fermentation (SSF) is a fermentation technology used in sectors such as pharmaceuticals, food, and textiles to create microbial metabolites using solid support rather than a liquid medium. It is a viable alternative to submerged fermentation and can produce useful products like as antibiotics, proteins, enzymes, organic acids, and biofuels. SSF has gained popularity due to its adaptability and possible benefits over submerged fermentation, making it a desirable technique in a variety of industrial processes.

Definition of solid state fermentation

Solid State Fermentation (SSF) is a fermentation method that involves the growth of microorganisms on solid support, such as grain brans or oil seed cakes, without a free-flowing aqueous phase. It is used in various industries to produce valuable metabolites, including antibiotics, enzymes, and biofuels, among others.

Substrates are used for Solid State Fermentation (SSF)

There are a variety of biotechnological procedures that involve the development of organisms that live on solid substrates, in the absence or in close proximity to free water. SSF or solid state fermentation (SSF) is a process that involves substrates that are solid and have very low levels of moisture. The most frequently utilized material for solidification are cereal grains (rice wheat, barley and corn) and legume seeds. wheat branand lignocellulose material like straws, sawdust as well as wood shavings as well as many other animal and plant material.

The majority of these are polymeric molecules that are insoluble or only sparingly soluble in water, however the majority are easily available and cheap and provide a high concentration of nutrients that aid in the growth of microbial colonies.

• A solid, porous and porous structure that is biodegradable, or not, yet with an extensive surface area per volume with a range of up to the 106 m2/cm3 range that allows for an immediate for microbial growth at the gas/solid surface.
• The matrix must absorb water equivalent to at least one or more multiples of its weight dry, with an extremely high level of water activity on the gas/solid interface, in order to permit high levels of biochemical processes.
• The oxygen-oxygen mixture along with other aerosols and gases should flow at a moderate pressure and mix the mash that is fermenting.
• The gas/solid interface is an ideal environment for the rapid growth of certain strains of yeasts, molds or bacteria, whether mix or pure cultures.
• Its mechanical characteristics of the matrix need to resist either gently stirring or compression, in accordance with the fermentation process. This calls for small, granular or fibrous particles, that are not likely to separate or adhere to one another.
• The solid matrix must not be contaminated with the inhibitors of microbial activity. It should be capable of absorption or contain the microbial food sources that are available like carbohydrates (cellulose and starch), sugars (sugars) and nitrogen-rich sources (ammonia, the peptides, urea) as well as mineral salts.

Features of Solid Substrate Fermentation

Solid Substrate Fermentation (SSF) is distinguished from other fermentation processes by many characteristics:

• Absence of a free-flowing liquid medium: SSF does not use a free-flowing liquid medium. Microorganisms instead develop on solid substrates in the absence or near absence of free water. However, SSF requires a particular amount of moisture (usually approximately 15%).
• Solid substrates: Cereal grains, wheat bran, sawdust, wood shavings, and various plant and animal products are common solid substrates for SSF. These substrates are usually polymeric, which means they are insoluble or just slightly soluble in water. They are a concentrated source of nutrients that are required for microbial growth.
• Traditional and diverse applications: SSF is an ancient and traditional technology that has been employed in various nations for a variety of applications. It is used in the manufacturing of fermented products like as edible mushrooms, cheese, soy sauce, enzymes, and organic acids. SSF is also used to make fermented foods like idli, dosa, dhokla, bread, drinks, fermented fish, pork, yoghurt, cheese, and pickles. Fermentation improves these foods’ nutritional content, digestibility, and flavour.
• Different cultures can be used: SSF can use single pure cultures, mixed cultures, or a combination of microorganisms. The cultures used are determined by the fermentation procedure and the desired product. Substrate pretreatment may be used to improve nutrient availability.
• Non-aseptic process: SSF is often performed as a non-aseptic or non-sterile procedure, which helps to reduce sterilisation expenses. Adequate spacing between solid substrates is essential for proper air circulation, which facilitates gas exchange and heat dissipation. To ensure appropriate conditions during SSF, forced air circulation may be used.

SSF is distinguished by the absence of a liquid medium, the use of solid substrates, a wide range of traditional applications, the use of various cultures, and non-aseptic operation. It’s a versatile fermentation technique that’s been used for millennia to make a variety of fermented meals and other items.

Organisms Used in Solid State Fermentation (SSF)

• Solid State Fermentation (SSF) organisms can be isolated pure cultures, mixed recognisable cultures, or entirely mixed indigenous microbes. The specific organisms used are determined by the target result and the SSF process parameters.
• Certain SSF processes, such as tempeh and ontjom manufacturing, involve the selective growth of organisms, particularly moulds that thrive in low moisture settings. These moulds ferment with the help of extracellular enzymes released by the fermenting bacteria. These mould species’ development and metabolic activity are aided by the low moisture levels in SSF.
• However, despite their propensity for higher moisture content for successful fermentation, bacteria and yeasts can be used in SSF. Due to the reduced moisture level, when bacteria and yeasts are used in SSF, the fermentation process may produce worse results than when moulds are used. Nonetheless, bacteria and yeasts can contribute to the SSF process under certain conditions and with suitable substrate and process optimisation.
• Moulds, bacteria, yeasts, or a combination of these organisms can be employed in SSF. The organisms used are determined by the target result and the SSF process’s unique requirements. While moulds are frequently favoured because of their capacity to grow in low moisture situations and release essential enzymes, bacteria and yeasts can also be used in SSF, albeit their effectiveness may be diminished in lower moisture environments.

Solid State Fermentation (SSF) Steps

SSF is usually a multistep process comprising those steps as follows:

Solid State Fermentation (SSF) typically involves multiple steps to achieve the desired product:

1. Pre-treatment of substrate raw materials: The raw materials used as substrates undergo pre-treatment through mechanical, chemical, or biochemical processes. This pre-treatment aims to improve the availability of bound nutrients and reduce the size of components. Examples of pre-treatment include pulverizing straw or shredding vegetable materials. However, the cost-effectiveness of pre-treatment should be considered in relation to the final value of the product.
2. Hydrolysis of polymeric substrates: In this step, primarily polymeric substrates such as polysaccharides and proteins undergo hydrolysis. The hydrolysis process breaks down these complex molecules into smaller, more easily utilizable compounds.
3. Utilization (fermentation) of hydrolysis products: The hydrolysis products generated in the previous step are utilized by microorganisms for fermentation. This involves the metabolic transformation of the available compounds into desired end products.
4. Separation and purification of end products: After fermentation, the end products need to be separated and purified from the solid substrate. This step is crucial to obtain the desired final product in its pure form.
   • The low moisture content characteristic of SSF allows for a smaller reactor volume per unit mass of substrate compared to Liquid State Fermentation (LSF). It also simplifies the process of product recovery.
   • However, SSF presents challenges related to mixing, heat exchange, oxygen transfer, moisture control, and gradients of pH, nutrients, and products due to the heterogeneous nature of the culture.
   • The heterogeneity makes it difficult to measure and control these parameters accurately, limiting the industrial potential of SSF. To overcome these challenges, microorganisms selected for SSF are often more tolerant to a wide range of cultivation conditions.

In summary, the steps in SSF include pre-treatment of substrate raw materials, hydrolysis of polymeric substrates, utilization of hydrolysis products through fermentation, and separation/purification of end products. The low moisture content of SSF offers advantages but also presents challenges that need to be addressed for successful implementation.

Solid state bioreactors

Solid state bioreactors play a crucial role in solid state fermentation (SSF) processes by providing a suitable environment for microbial growth and biological activity. The design of these bioreactors is essential to maintain optimal conditions for the fermentation process, including temperature, oxygen concentration, moisture gradients, mixing/agitation, aeration, and heat transfer. There are different types of solid state bioreactors based on their aeration and mixing characteristics:

1. Tray Bioreactors (Group 1): Tray bioreactors consist of individual trays stacked on top of each other with spaces in between to enhance air availability. The trays, made of materials such as wood, bamboo, metal, or plastic, have open tops and perforated bottoms. Tray bioreactors are static beds that are infrequently or not mixed at all. Air is introduced into the chamber and circulated around the trays with controlled humidity and temperature.
2. Packed-Bed Bioreactors (Group 2): Packed-bed bioreactors are tubular containers filled with substrate particles and microorganisms. The packing materials are not typically mixed, and forced aeration is provided. The design of packed-bed bioreactors varies in terms of cross-section shape, orientation (vertical, horizontal, or angled), and aeration location (top or bottom). Additional aeration can be achieved by inserting a perforated tube inside the bioreactor.
3. Rotating Drum Bioreactors (Group 3): Rotating drum bioreactors operate on a continuous or semi-continuous mode with intermittent mixing and without forced aeration. They are horizontal cylinders partially filled with a substrate bed. Good oxygen and carbon dioxide transfer are facilitated by maintaining an optimal bed height, and temperature control relies on the mixing effect of the solid substrate.
4. Fluidized-Bed Bioreactors (Group 4): Fluidized-bed bioreactors consist of a vertical chamber with a perforated base plate. Forced aeration at the bottom chamber fluidizes the solid substrate particles and promotes mixing. An agitator is often included to prevent agglomerate formation and settling. The mixture of solid particles and gas behaves like a liquid, ensuring effective gas, solid, and liquid mixing.

Each type of solid state bioreactor offers unique advantages and is suitable for specific SSF applications. The choice of bioreactor design depends on the characteristics of the substrate, the microorganisms involved, and the desired fermentation outcomes. Proper bioreactor design and operation are crucial for optimizing SSF processes and maximizing product yields.

Applications of Solid State Fermentation (SSF)

Solid State Fermentation (SSF) finds numerous applications across various industries due to its advantages and versatility. Some of the key applications of SSF are:

1. Microbial Products: SSF is used for the production of a wide range of microbial products. It has been successfully applied in the production of enzymes, including amylases, cellulases, proteases, lipases, and many others. These enzymes find applications in various industries such as food processing, textile, detergent manufacturing, and biofuel production. SSF is also used to produce organic acids, flavoring compounds, and other valuable metabolites.
2. Feed Production: SSF is utilized for the production of feed additives and animal feed. Solid substrates, such as agricultural by-products, can be converted into nutritional supplements and feed ingredients using SSF. This helps in utilizing agricultural waste and improving the nutritional value of feed for livestock.
3. Biofuels: SSF has shown promise in the production of biofuels. Lignocellulosic materials, such as agricultural residues and forestry waste, can be fermented using SSF to produce bioethanol and other biofuels. This application contributes to the development of sustainable and renewable energy sources.
4. Food and Beverage Industry: SSF plays a crucial role in the production of various fermented food and beverage products. Examples include traditional fermented foods like idli, dosa, dhokla, and bread. SSF is also employed in the production of fermented beverages like sake, soy sauce, and certain types of beer. Fermentation enhances the nutritional value, digestibility, and flavor of these food products.
5. Bioremediation and Bioleaching: SSF has been used for bioremediation purposes, particularly in the degradation of pollutants in solid waste or contaminated soil. Microorganisms capable of degrading pollutants are grown on solid substrates to carry out the remediation process. SSF is also utilized in bioleaching, which involves the use of microorganisms to extract valuable metals from ores and industrial waste.
6. Industrial Chemicals and Pharmaceuticals: SSF is employed in the production of various industrial chemicals and pharmaceutical products. It offers an efficient and cost-effective method for producing compounds such as organic acids, antibiotics, bioactive compounds, and other high-value chemicals. SSF can be used to cultivate specific microorganisms that produce these compounds, which can then be extracted, purified, and utilized in the manufacturing of pharmaceuticals and other industrial products.

Overall, the applications of SSF span a wide range of industries, contributing to the production of microbial products, feed additives, biofuels, fermented foods, bioremediation, and industrial chemicals. Its versatility and potential for sustainable production make SSF an attractive option in various bioprocesses.

Advantages of Solid State Fermentation (SSF)

Solid State Fermentation (SSF) offers several advantages compared to other fermentation methods, making it a preferred choice in many applications. The advantages of SSF include:

• Environmental Friendliness: SSF produces minimal waste and liquid effluent, making it less damaging to the environment compared to liquid fermentation methods. This is beneficial for sustainable production and reduces the overall ecological footprint.
• Simple and Low-Cost Process: SSF employs natural solid substrates as media, often using agro-industrial residues or agricultural waste. It is a low-technology and low-energy process, requiring less capital investment compared to other fermentation techniques. The use of readily available and inexpensive raw materials contributes to cost-effectiveness.
• Reduced Contamination: SSF has a lower risk of microbial contamination compared to liquid fermentation since there is no need for sterilization. The solid substrate provides a physical barrier against contaminants, resulting in a cleaner fermentation process.
• Easy Downstream Processing: SSF facilitates easy downstream processing and product recovery. The concentrated nature of the solid substrate simplifies extraction and purification steps, reducing the need for large volumes of solvents or chemicals.
• Utilization of Agro-Industrial Residues: SSF allows for the utilization of underutilized or non-utilized agro-industrial residues as substrates. This provides an alternative avenue for value addition to these residues, contributing to waste reduction and resource efficiency.
• High Yield and Productivity: SSF can achieve reasonably high product yields due to the concentrated source of nutrients provided by the solid substrates. The controlled conditions and specific microorganisms used in SSF can lead to efficient conversion of substrates into desired products.
• Simple Bioreactor Design: Bioreactors used in SSF are typically small in volume and compact, resulting in a simpler design compared to large-scale liquid fermentation systems. Aeration and effluent treatment processes are also relatively straightforward in SSF.
• Versatile Substrate Options: SSF can utilize a wide range of domestic, industrial, and agricultural wastes as substrates. This flexibility in substrate choice allows for the utilization of various natural materials, providing an economical and sustainable approach to fermentation.
• Economic Feasibility: SSF has proven to be economically feasible due to its low-cost operation, utilization of inexpensive substrates, simplified processing steps, and high product yields. It presents an attractive option for industries looking for cost-effective fermentation processes.

Overall, the advantages of SSF, such as environmental friendliness, simplicity, cost-effectiveness, reduced contamination, and utilization of various substrates, make it a valuable and attractive technique in bioprocesses and fermentation applications.

Limitations/Disadvantages of Solid State Fermentation (SSF)

Solid State Fermentation (SSF) also has certain disadvantages that should be considered. These include:

• Limited Monitoring: Precise monitoring of SSF parameters such as oxygen (O2) and carbon dioxide (CO2) levels, as well as moisture content, is challenging. The heterogeneous nature of the solid substrate makes it difficult to obtain accurate and real-time measurements of these parameters, which can affect the control and optimization of the fermentation process.
• Slow Growth and Limited Product Formation: Microorganisms in SSF tend to grow more slowly compared to liquid fermentation. This slower growth rate can limit the overall productivity of the fermentation process and the formation of desired products. The extended growth period may result in longer production cycles and reduced efficiency.
• Difficulty in Controlling Growth Environment: The heat generated during SSF can cause challenges in regulating the growth environment. The accumulation of heat within the solid substrate can lead to temperature variations, affecting microbial growth and metabolic activity. Maintaining consistent and optimal conditions throughout the fermentation process becomes challenging.
• Limited Application for Moisture-Sensitive Organisms: SSF is more suitable for microorganisms that can tolerate low moisture levels. This restricts the choice of organisms that can be used in SSF, as many microbial species require higher moisture content for efficient growth and fermentation. Moisture-sensitive organisms may not thrive or produce desired products effectively in SSF.
• Lack of Detailed Monitoring: Due to the complex nature of the solid substrate and the absence of a liquid phase, it is difficult to monitor SSF parameters in detail. This includes the inability to measure CO2 and O2 levels accurately or precisely control moisture content throughout the process. This limitation can make it challenging to optimize and control the fermentation conditions effectively.
• Slower Process and Longer Production Cycles: The slower growth rate and limited product formation in SSF can result in longer production cycles compared to liquid fermentation methods. This extended timeframe may not be ideal for applications that require rapid and high-volume production.

Despite these disadvantages, SSF still offers several advantages in specific applications and can be a suitable choice for certain types of microorganisms and products. Understanding the limitations and optimizing the process parameters can help mitigate some of these challenges in SSF.

FAQ

References

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