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Solid State Fermentation (SSF)

Solid State Fermentation (SSF) is a process of fermentation utilized by a range of industries such as textile, food, pharmaceuticals and so...

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Solid State Fermentation (SSF)
Solid State Fermentation (SSF)

Solid State Fermentation (SSF) is a process of fermentation utilized by a range of industries such as textile, food, pharmaceuticals and so on. in order to create compounds of microorganisms that use solid supports instead of liquid medium. It’s defined as growing microbes that do not have a flowing aqueous phase that is free-flowing.

The SSF is a substitute for submerged fermentation to produce high-value products such as antibiotics, single cell proteins and PUFA’s as well as enzymes Organic acids, biopesticides biofuels and aroma production. The substrate used is primarily grain brans, deoiled oil seeds, and various materials similar to.

In the beginning, fungi were the primary organisms utilized in these fermentations (as the microorganisms considered to be highly active in conditions of low water activity). Later on, various yeasts and bacterial species were employed to perform this type of fermentation too. The microbiological process behind SSF has attracted a lot of attention in recent times because it is able to be utilized for many different purposes as outlined by some authors who have pointed out several advantages over their counterparts that are liquid (submerged fermentation).

Definition of solid state fermentation

SSF is described in various ways. Numerous researchers within the field have developed their own definitions of SSF.

  • SSF is defined as growing microorganisms using solid and moist supports, whether on carriers that are inert or insoluble substrates, which can be used as carbon as well as an energy source.
  • SSF as a method in which substrates in a solid particle state are utilized.
  • SSF is the process of microbial growth that takes place predominantly in the walls of substances which are able to absorb or store water, either with or without insoluble nutrients.
  • SSF refers to the growth of microorganisms in a moistened substrates, where sufficient moisture is available to sustain metabolic and microbial growth however, there is no moving water or air, it is the constant stage.
  • SSF is defined as a procedure which involves an increase in microorganisms onto the surface of solid particles in bed formations in which the spaces between particles have been filled by the continuous gas phase.
  • SSF is defined as a three-phase heterogeneous process that includes liquid, solid and gaseous phases. This process could offer advantages in the cultivation of microbial to develop bioprocesses and products.

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

  • There are certain fermentation processes that do not involve liquid medium. For these bio- technological processes, the growth of the microorganisms is carried out on solid substrates in the complete absence or almost complete absence of free water. The presence of some moisture (about 15%) is necessary for solid substrate (or solid state) fermentation (SSF).
  • The most commonly used solid substrates for SSF are cereal grains, wheat bran, sawdust, wood shavings and several other plant and animal materials. These solid substrates are polymeric in nature, insoluble or sparingly soluble in water, and contain concentrated source of nutrients for the growth of microorganisms.
  • SSF is a very old traditional technique carried out in many countries. It is used for the production of edible mushrooms, cheese, soy sauce and many other fermented products (including enzymes and organic acids). Composting is a good example of SSF.
  • Solid substrate fermentation has been very popular for the production of fermented foods (idli, dosa, dhokla, bread, beverages, fermented fish, meat, yogurt, cheese, pickles). Fermentation often makes the food more nutritious, easily digestable and better in flavour. For solid substrate fermentation, single pure cultures, mixed cultures or mixed organisms may be used. Pretreatment of substrate raw materials is sometimes done to facilitate the availability of nutrients.
  • Solid substrate fermentation is normally carried out as a non-aseptic process. This saves sterilization costs. It is important that the substrates used in SSF have adequate spaces in between to allow good air circulation. This facilitates adequate exchange of gases, besides promoting heat elimination. Forced air circulation may be done to maintain optimal conditions in SSF.

Organisms Used in Solid State Fermentation (SSF)

  • The microbiological elements of SSF can be found as pure, single species, mixed distinct cultures or as completely diverse indigenous microorganisms.
  • Certain SSF processes, e.g. tempeh, tempeh and ontjom production, require specific growth of organisms, like molds, which require low levels of moisture to perform fermentation using extracellular enzymes produced by microorganisms that ferment.
  • However, yeasts and bacteria that require a more moisture for effective fermentation, can be utilized to create SSF however, with lower yield.

Solid State Fermentation (SSF) Steps

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

Solid State Fermentation (SSF) Steps
Solid State Fermentation (SSF) Steps
  1. Pre-treatment of raw materials for substrates whether through mechanical, chemical or biochemical process to improve the absorption of bound nutrients as well as to minimize the dimensions of the components, e.g., pulverizing straw or shredding vegetable material to enhance the physical aspects to the method. But, the expense of the pre-treatment process must be weighed with the final value of the product.
  2. The hydrolysis of polymeric substrates is mainly, e.g., polysaccharides and proteins.
  3. Use (fermentation) for hydrolysis product.
  4. Separation and purification for end products.
    • SSF’s low moisture content SSF allows for a smaller reactor volume per mass of substrate than LSF and makes it easier for the process of recovering the product.
    • However, significant issues arise when mixing the control of moisture, oxygen transfer and the varying pH of the nutrient, and the product due to the diverse nature of the production.
    • The last characteristic of SSF makes the control and measurement of the mentioned parameters challenging as well as laborious and frequently inaccurate, which limits the potential industrial application of this method. Due to these limitations the micro-organisms which were chosen to be used in SSF are more adaptable to a broad range of conditions for cultivation.
Steps in Solid State Fermentation (SSF)

Bioreactors for SSF

Bioreactors designed for solid state fermentation are much simpler compared to liquid-state fermentation.

Bioreactors for SSF
Bioreactors for SSF | Image Source: https://www.biologydiscussion.com/notes/short-notes-on-solid-substrate-fermentation/10044

Solid State Fermentation (SSF) Applications

  • The use of bioprocesses in such areas as bioleaching, bio-beneficiation the bio-pulping process, bioremediation etc. has provided many advantages.
  • Solid-state fermentation has been identified as a promising technique for the production of products made by microbial organisms, such as fuel, food, feed industrial chemicals, as well as pharmaceutical products.
  • It is extensively used for produce a variety of organic acids, enzymes flavors, and other compounds. They need to be extracted and purified and then utilized in various products.

Advantages of SSF

1. Biological

  • Products produced in high volume
  • Higher productivity level of the products
  • Higher stability of products
  • Absence of catabolic repression
  • Tolerance to high substrate concentration
  • Natural, complex raw materials often provide a complete medium
  • Absence of rigorous control of fermentation process
  • Easier aeration
  • Low water demand helps to minimise contamination
  • Practically involves fungi-producing spores. Spores can be used as inoculum, can be preserved for a long time and can be used repeatedly
  • Absence of production of foam

2. Processing

  • Bioreactors are usually in small volume and are compact
  • The volume of the substrate loading is much higher
  • The growth substrates are from natural resources proportionally simple and unrefined
  • Pre-treatment and treatment of the natural resources can be very simple
  • Downstream processing might be simple since products are concentrated
  • Extraction of the products requires much less solvent (if necessary)
  • The process does not involve anti-foam chemicals
  • Minimise the generation of pollutants or harmful waste during products manufacture
  • Produce less liquid waste
  • Solving waste problem as biological detoxification
  • Substrates usually natural materials – Natural unusable carbon source which are extremely cheap, variable and abundant agro-industry and food waste
  • Modified bioreactors are simple, cheap and user-friendly
  • Low recovery cost in downstream processing
  • SSF process is very attractive from an economic point of view and proved to be economically feasible

Disadvantages of Solid State Fermentation (SSF)

  • The production of heat can cause problemsand it’s extremely difficult to control the conditions for growth. 
  • Monitoring in detail on SSF (e.g. CO2 and O2 levels and moisture content) is not possible.
  • The microorganisms which can tolerate moderate moisture levels are able to be utilized.
  • The organisms are slow to grow and as a result, there’s a limit to the production of new products.

References

  • Jai S Ghosh (2016) Solid State Fermentation and Food Processing: A Short Review. J Nutr Food Sci 6: 453. doi: 10.4172/2155-9600.1000453 
  • Ashok Pandey (2003). Solid-state fermentation. , 13(2-3), 81–84. doi:10.1016/s1369-703x(02)00121-3
  • Manan MA, Webb C. Design aspects of solid state fermentation as applied to microbial bioprocessing. J Appl Biotechnol Bioeng. 2017;4(1):511-532. DOI: 10.15406/jabb.2017.04.00094
  • https://www.biologydiscussion.com/notes/short-notes-on-solid-substrate-fermentation/10044
  • https://www.researchgate.net/figure/Steps-involved-in-solid-state-fermentation-process_fig1_337824396
  • https://edepot.wur.nl/200086
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