Difference between revisions of "Solid state fermentation"

<onlyinclude>Solid state fermentation (SSF) is a type of fermentation with a low water content in the substrate. The solid substrate is inoculated with the culture and the cultivation is mostly performed under controlled conditions, such as controlled temperature, light and humidity. Nutrient levels, C/N ratio, feedstock-to-inoculum ratio, pH and mixing can also be controlled.<ref name=":0" /> SSF is "a traditional cultivation technique of food technology and involves all cultivations of microorganisms on a solid substrate without free liquid phase."<ref name=":0">{{Cite book|author=Dr. Susanne Steudler, Dr. Anett Werner, Dr. Jay J. Cheng|year=2019|book_title=Solid state fermentation : research and industrial applications|publisher=Springer International Publishing|place=Cham|ISBN=978-3-030-23675-5}}</ref> Besides traditional food processing methods, solid state fermentation is also used for the industrial production of a diverse range of other products, such as enzymes, biogas, pigments, and antibiotics. SSF can be applied in many different fields, such as food and aroma production, production of medicines, waste treatment or environmental technology. One example of a traditional solid state fermentation is the production of Sake (a Japanese alcoholic beverage from rice). The polished and cooked rice serves as the solid substrate of the first fermentation step in the Sake production process. It is inoculated with Kōji-kin (''Aspergillus oryzae'') spores.<ref>{{Cite journal|title=Genomics of Aspergillus oryzae: Learning from the History of Koji Mold and Exploration of Its Future|year=2008-8|author=Masayuki Machida, Osamu Yamada, Katsuya Gomi|journal=DNA Research: An International Journal for Rapid Publication of Reports on Genes and Genomes|volume=15|issue=4|page=173–183|doi=10.1093/dnares/dsn020}}</ref> ''A. orizae'' is a fungus which converts the starch from the rice to sugar. After this solid state fermentation, a liquid state fermentation step follows during which yeast converts the sugar to ethanol.</onlyinclude>

As described above, the feedstock can be a food product, such as rice or wheat bran. Another traditional food product produced by SSF is Tempeh, cooked soy beans fermented by different ''Rhizopus'' fungi. Soy beans are also fermented by ''Bacillus subtilis ssp. natto'' to create a food product called Nattō. Other substrates can also be used, such as tofu dregs (okara), coconut dregs, different cooked beans and peanuts.

Agro-industrial residues such as cassava bagasse are e.g. used for the production of citric acid via SSF.<ref name=":1">{{Cite journal|title=Citric acid production by solid-state fermentation on a semi-pilot scale using different percentages of treated cassava bagasse|year=2005-12|author=F. C. Prado, L. P. S. Vandenberghe, A. L. Woiciechowski, J. A. Rodrígues-León, C. R. Soccol|journal=Brazilian Journal of Chemical Engineering|volume=22|issue=4|page=547–555|doi=10.1590/s0104-66322005000400007}}</ref> Citric acid is industrially produced by using the filamentous fungus ''Aspergillus niger''. A wide range of agro-industrial residues can be used for the production of citric acid, such as apple and grape pomace, carrot waste, carob pod, orange and pineapple waste, cassava bagasse, coffee husk, kiwifruit peel, mussel processing wastes, okara (soy residue), rice and wheat bran.<ref name=":1" />

In SSF, the bioreactor provides suitable environment for microorganism growth and biological activity. Bioreactors must be able to hold the media and be sealed well which prevents harmful environment substances entering in to the bioreactor. Important parameters in bioreactor design include temperature, oxygen concentration, moisture gradients as well as mixing/agitation, aeration and heat transfer. SSF bioreactor design can be classified into four groups, which can be separated by aeration and mixing type:

Rotating drum bioreactors mix intermittently without forced aeration, operating on continuous or semi-continuous mode. A rotating drum bioreactor is a horizontal cylinder. The drum is semi-filled with a bed of substrate. The fermented bed cannot be too high and this creates good oxygen and carbon dioxide transfer. Temperature control also depends on the mixing effect on the solid substrate.

In order to maximize product yield, downstream strategies play an important role. The choice of downstream strategy is dependent of the type of product. For instance, if the product containts cellulase than the following downstream-based strategies can be used:<ref>{{Cite book|author=Darshan M. Rudakiya|year=2019|section_title=Strategies to Improve Solid-State Fermentation Technology|book_title=New and Future Developments in Microbial Biotechnology and Bioenegineering|publisher=Elsevier}}</ref>