Solid-state fermentation
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Solid state fermentation (SSF) is a
biomolecule A biomolecule or biological molecule is a loosely used term for molecules present in organisms that are essential to one or more typically biological processes, such as cell division, morphogenesis, or development. Biomolecules include large ...
manufacturing process used in the food, pharmaceutical, cosmetic, fuel and textile industries. These biomolecules are mostly
metabolite In biochemistry, a metabolite is an intermediate or end product of metabolism. The term is usually used for small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, c ...
s generated by microorganisms grown on a solid support selected for this purpose. This technology for the culture of microorganisms is an alternative to liquid or submerged
fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...
, used predominantly for industrial purposes.


Processes

This process consists of depositing a solid culture substrate, such as rice or wheat bran, on flatbeds after seeding it with
microorganisms A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
; the substrate is then left in a temperature-controlled room for several days. Liquid state fermentation is performed in tanks, which can reach at an industrial scale. Liquid culture is ideal for the growing of unicellular organisms such as bacteria or yeasts. To achieve liquid aerobic fermentation, it is necessary to constantly supply the microorganism with oxygen, which is generally done via stirring the fermentation media. Accurately managing the synthesis of the desired metabolites requires regulating temperature, soluble oxygen, ionic strength and pH and control nutrients. Applying this growing technique to
filamentous fungi A mold () or mould () is one of the structures certain fungi can form. The dust-like, colored appearance of molds is due to the formation of spores containing fungal secondary metabolites. The spores are the dispersal units of the fungi. Not ...
leads to difficulties. The fungus develops in its vegetative form, generating
hyphae A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...
or multicellular ramous filaments, while a
septum In biology, a septum (Latin for ''something that encloses''; plural septa) is a wall, dividing a cavity or structure into smaller ones. A cavity or structure divided in this way may be referred to as septate. Examples Human anatomy * Interatri ...
separates the cells. As this
mycelium Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrate (biology), substrates. A typical single ...
develops in a liquid environment, it generates abundant viscosity in the growing medium, reducing oxygen solubility, while stirring disrupts the cell network increasing cell mortality. In nature, filamentous fungi grow on the ground, decomposing vegetal compounds under naturally ventilated conditions. Therefore, solid state fermentation enables the optimal development of filamentous fungi, allowing the mycelium to spread on the surface of solid compounds among which air can flow. Solid state fermentation uses culture substrates with low water levels (reduced water activity), which is particularly appropriate for mould. The methods used to grow filamentous fungi using solid state fermentation allow the best reproduction of their natural environment. The medium is saturated with water but little of it is free-flowing. The solid medium comprises both the substrate and the solid support on which the fermentation takes place. The substrate used is generally composed of vegetal byproducts such as beet pulp or wheat bran. At the beginning of the growth process, the substrates and solid culture compounds are non-soluble compounds composed of very large, biochemically complex molecules that the fungus will cut off to get essential C and N nutrients. To develop its natural substrate, the fungal organism sets forth its entire genetic potential to produce the metabolites necessary for its growth. The composition of the growth medium guides the microorganism's metabolism towards the production of
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
s that release bio-available single molecules such as sugars or
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s by carving out macromolecules. Therefore, when selecting the components of the growth medium it is possible to guide the cells towards the production of the desired metabolite(s), mainly enzymes that transform polymers (cellulose, hemicellulose, pectins, proteins) into single moieties in a very efficient and cost-effective manner. Compared to submerged fermentation processes, solid state fermentation is more cost-effective: smaller vessels, lower water consumption, reduced wastewater treatment costs and lower energy consumption (no need to heat up water, poor mechanical energy input due to smooth stirring). Cultivating on heterogeneous substrates requires expertise to maintain optimal growth conditions. Air flow monitoring is key because it impacts temperature, oxygen supply and moisture. In order to maintain sufficient moisture content for the growth of filamentous fungus, waterlogged air is used and may require further addition of water. In most cases, solid state fermentation does not require a completely sterile environment as the initial sterilization of the fermentation substrate associated with the rapid colonization of the substrate by the fungous microorganism limits the development of the autochthonous flora.


Uses


Traditional food production

Traditionally, SSF has been used in Asian countries to produce Koji using rice to manufacture alcoholic beverages such as
Sake Sake, also spelled saké ( ; also referred to as Japanese rice wine), is an alcoholic beverage of Japanese origin made by fermenting rice that has been polished to remove the bran. Despite the name ''Japanese rice wine'', sake, and indee ...
or Koji using soybean seeds. The latter produces sauces such as
soy sauce Soy sauce (also called simply soy in American English and soya sauce in British English) is a liquid condiment of Chinese origin, traditionally made from a fermented paste of soybeans, roasted grain, brine, and '' Aspergillus oryzae'' or ''Asp ...
or other foods. In Western countries, the traditional manufacturing process of many foods uses SSF. Examples include fermented bakery products such as bread or for the maturing of cheese. SSF is also widely used to prepare raw materials such as chocolate and coffee; typically cacao bean fermentation and coffee bean skin removal are SSF processes carried out under natural tropical conditions.


Enzyme production

Enzymes and enzymatic complexes able to break down difficult-to-transform macromolecules such as cellulose, hemicelluloses, pectin and proteins. Solid state fermentation is well suited for the production of various enzymatic complexes composed of multiple enzymes. Enzymatic compounds generated by SSF find outlets in all sectors where digestibility, solubility or viscosity is needed. This is why SSF enzymes are widely used in the following industries: * fruit and vegetable transformation (
pectinase Pectinases are a group of enzymes that breaks down pectin, a polysaccharide found in plant cell walls, through hydrolysis, transelimination and deesterification reactions. Commonly referred to as pectic enzymes, they include pectolyase, pectozym ...
s) * baking (
hemicellulase Cellulase (EC 3.2.1.4; systematic name 4-β-D-glucan 4-glucanohydrolase) is any of several enzymes produced chiefly by fungi, bacteria, and protozoans that catalyze cellulolysis, the decomposition of cellulose and of some related polysacchar ...
s) * animal feeding (hemicellulases and
cellulase Cellulase (EC 3.2.1.4; systematic name 4-β-D-glucan 4-glucanohydrolase) is any of several enzymes produced chiefly by fungi, bacteria, and protozoans that catalyze cellulolysis, the decomposition of cellulose and of some related polysaccharide ...
s) * bio ethanol (cellulases and hemicellulases) * brewing and distilling (hemicellulases)


Outlook

Liquid, submerged and solid state fermentation are age-old techniques used for the preservation and manufacturing of foods. During the second half of the twentieth century, liquid state fermentation developed on an industrial scale to manufacture vital metabolites such as antibiotics. Economic changes and growing environmental awareness generate new perspectives for solid state fermentation. SSF adds value to insoluble agricultural byproducts thanks to its higher energy efficiency and reduced water consumption. The renewal of SSF is now possible thanks to engineering firms, mainly from Asia, that have developed a new generation of equipment. Fujiwara makes vessels able to transform substrate areas up to for the production of soy sauce or sake. Other companies use solid state fermentation for enzyme complexes. In France Lyven has manufactured Pectinases and Hemicellulases on beet pulp and wheat bran since 1980. The company (now part of Soufflet Group) is now involved in a global R&D programme focusing on SSF technology.


See also

*
Aspergillus oryzae ''Aspergillus oryzae'', also known as , is a filamentous fungus (a mold) used in East Asia to saccharify rice, sweet potato, and barley in the making of alcoholic beverages such as ''sake'' and '' shōchū'', and also to ferment soybeans for m ...
* Micro-organism


Notes


References

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External links

* * * * {{cite web, url=http://www.soufflet.com/ , title=Soufflet Groupe , publisher=Soufflet.com , date= , access-date=2012-11-04 Fermentation