Why Should We Be Interested in SSF

The environment that the organism experiences in SSF is different from that experienced in SLF. In SLF it is relatively easy to control the conditions to which the process organism is exposed:

• the fungal hyphae are bathed in a liquid medium and do not run the risk of desiccation;

• temperature control is typically not overly difficult, such that the organism is exposed to a constant temperature throughout its growth cycle;

• the availability of O2 to the biomass can be controlled reasonably well at a particular level of saturation of the medium (although this can become very challenging in high density cultures);

• the availability of the nutrients to the organism can be controlled within relatively narrow limits if desired, through the feeding of nutrient solutions (at least in those processes in which soluble carbon and energy sources are provided);

• although shear forces do occur within mechanically stirred bioreactors, the nature and magnitude of these forces are well understood and it is possible to use bioreactors that provide a low-shear environment, if the organism is highly susceptible to shear damage, such as bubble columns or air lift bioreactors;

• pH control is relatively easy to provide.

In contrast, the environment in SSF can be quite stressful to the organism. For example:

• fungal hyphae are exposed to an air phase that can desiccate them;

• temperatures can rise to values that are well above the optimum for growth due to the inadequate removal of waste metabolic heat. In other words, the temperature to which the organism is exposed can vary during the growth cycle;

• O2 is typically freely available at the surface of the particle, however, there may be severe restrictions in the supply of O2 to a significant proportion of the biomass that is within a biofilm at the surface or penetrating into the particle;

• the availability of nutrients to the organism may be poor, even when the average nutrient concentration within the substrate particle, determined after homogenizing a sample of fermenting solid particles, is high. In other words, there tend to be large concentration gradients of nutrients within the particles;

• movement of the particles of the solid substrate can cause impact and shear damage. In the case of fungal processes the hyphae can suffer severe damage;

• it may be difficult to provide pH control.

Also, due to the different physical natures of the two systems, namely the presence of solid-air interfaces in SSF, growth morphologies of mycelial organisms, in terms of hyphal extension and branching patterns, may be quite different between SSF and SLF. This can be linked to different patterns of expression of genes, including those for several potential biotechnological products (Ishida et al. 2000).

These, and other differences, mean that SLF is an "easier" system with which to work. The ease of using SLF is greater still when substrate handling is considered. For example, it is much simpler and cheaper to pump liquids from one place to another than to move solids and it is easier to sterilize a large volume of liquid than a large volume of solids (in either batch or continuous sterilization mode). Given all these potential difficulties, for both the operator and the microorganism, it would appear that SLF should be the fermentation method of choice. In fact, in the majority of cases it is! However, there are certain instances in which, despite being more problematic, SSF may be appropriate:

• when the product needs to be in a solid form (e.g., fermented foods);

• when a particular product is only produced under the conditions of SSF or, if produced in both SLF and SSF, is produced in much higher levels in SSF. For example, certain enzymes are only induced in SSF and some fungi only sporu-late when grown in SSF, in which the hyphae are exposed directly to an air phase. If it is desired to use genetically unmodified organisms in a process for the production of such a product, then SSF may be the only option;

• when the product is produced in both SLF and SSF, but the yield is much higher in SSF. For example, Monascus pigment and many fungal spores are produced in much higher yields in SSF;

• when socio-economic conditions mean that the fermentation process must be carried out by relatively unskilled workers. Some SSF processes can be relatively resistant to being overtaken by contaminants;

• when the product is produced in both SSF and SLF, but the product produced in SSF has desirable properties which the product produced in SLF lacks. For example, spore-based fungal biopesticides produced in SSF processes are usually more resistant to adverse conditions than those produced in SLF, and are therefore more effective when spread in the field;

• when it is imperative to use a solid waste in order to avoid the environmental impacts that would be caused by its direct disposal. This is likely to become an increasingly important consideration as the ever-increasing population puts an increasing strain on the environment.

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