SOLID SUBSTRATE FERMENTATION- THE ALTERNATIVE FERMENTATION

(PICTURE TAKEN FROM ARS.USDA.GOV)

The popularity of the fermentation process has always been represented by the image of large stainless steel fermentors and the production of fermented based beverages such as beers and wines. Yet, unknown to most and not keenly looked upon by scientists is the fermentation technology of developing countries known as Solid substrate fermentation. Solid substrate fermentation technology has all these while been operating quietly as a rural technology and playing very important roles in the preparation of food for the masses. Food such as tempe derived by SSF of tempe represents important source of protein and vitamins to the masses. The SSF process by the fungi involved helped also in increasing the digestibility of the substrate.

Composting too is a SSF process! It is widely used in generating valuable fertilizers out of organic refuse.

FUTURE POTENTIAL OF SSF
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Although SSF has always been considered a traditional fermentation used more in food industries such as Tempe , cheese and koji fermentation, it has lately been seen as a potential fermentation technology to be exploited in modern fermentation industries of today.

Research are now carried out to create novel SSF that will be able to produce high volume of industrial enzymes and fermentation acids such as itaconic acids and citric acids and even for pharmaceuticals.

SIMILARITY OF SSF WITH LIQUID SUBSTRATE FERMENTATION
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In principle, there is no real differences between the popular liquid based fermentation and solid substrate fermentation. Both fermentation processes still involved the same components of the typical fermentation process such as:

1 Presence of the fermenting microorganisms
2 The raw fermentation substate to be acted upon by the microorganisms
3 The end products of the fermentations produced which includes biomass product
4 A simple concept of bioreactor for which the whole fermentation process occur
5 Provision by the bioreactor of the ideal environment for the fermentation to occur

The most significant difference shown by the solid substrate fermentation is:
1 The presence of the nutrients for the microorganisms in the form of a solid phase
2 The very minimal requirement for the liquid or water phase as compared in liquid fermentation such as CSTR

MICROORGANISMS IN SSF
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Due to the nature of solid substrate and minimal content of water, microorganisms that dominate SSF are the fungi. The fungi are adapted physiologically to live within low water activity compared to bacteria and they can tolerate high osmotic conditions such in solution of high sugar concentrations.

The mode of growth of these fungi by hyphal extension with active hyphal growing tip that search and exudes hydrolytic enzymes work well in SSF.

THE SOLID SUBSTRATE IN SSF
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In the case of the SSF, the solid substrate which constitute the food or source of nutrients for the microorganisms could be viewed as a solid but porous matrix.

This solid porous matrix can absorb and retain a minimum layer or film of water or liquid which support a relatively high level of water activity. The film of liquid surrounding the various solid porous matrix is where all the activities of supporting the activities of the microbes occur.

Effective mass transfer of nutrients, oxygen and waste products occur within this thin film of liquid. Due to the high concentration of fermentation products such as hydrolysed sugars occurring within this thin film of water. This layer of liquid is often viscous, thus explaining more the importance of water activity rather than the presence of free water in the metabolism and biochemistry of SSF. Under such situations where free water is often limited and relatively high water activity, it is not surprising that moulds, yeasts and bacteria will dominate.

The nature of the solid substrate matrix is important in the success of any SSF. It should be soft enough structurally, porous enough to allow higher surface area to volume ratio to maximize mass transfer activities of nutrients , oxygen and gases. The SSF should allow a degree of mixings so as to optimize the fermentation process.

KEY PARAMETERS IN SSF
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When we compare SSF with the normal liquid fermentation, there are four key issues which could be discussed.
1 Moisture content and water activity
2 Temperature and heat transfer
3 Ph control and contamination
4 Oxygen uptake

WATER CONTENT
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As previously discussed although SSF do not need to have high amount of water, the presence of water as capillary water around the solid particles is important. These capillary water are very tightly bound to the particles and not easily available to the microorganisms resulting in water activity that support mostly fungi.

The optimum Aw for growth of a limited number of fungi used in SSF processes was at least 0.96 whereas the minimum growth Aw was generally greater than 0.9.The optimum Aw values for sporulation by Trichoderma viride were lower than those for growth. Maintenance of the Aw at the growth optimum would allow production of fungal and avoid sporulation.

TEMPERATURE
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Fungal respiration is highly exothermic. Highheat generation by fungal activity within the solids lead to thermal gradients due to the limited heat transfer capacity of solid substrates. Heat removal is a crucial factor in large scale SSF processes. Conventional convection or conductive cooling devices are inadequate for dissipating metabolic heat due to the poor thermal conductivity of most solid substrates. This will result in non acceptable temperature gradients.
Sufficient heat elimination is carried out through the use of evaporative cooling devices. Aeration helps in the transportation of heat out of the SSF. It should be cautioned that excessive aeration may result in dessication of the process.

PH
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The pH of a culture may change in response to microbial metabolic activities such as the production of acidic metabolites. These acids will cause the pH to decrease.

AERATION
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The four main functions of aeration in SSFare:
(i) to maintain aerobic conditions,
(ii) for carbon dioxide desorption,
(iii) to regulate the substrate temperature and
(iv) to regulate the moisture level.
Solid state process allows free exposure of atmospheric oxygen to the substrate and rapid mass transfer of oxygen occur easily through the thin liquid film surrounding the particles.