The General Steps of an SSF Process

At the most general level, the major processing steps of an SSF process are no different from those of a submerged liquid fermentation (SLF) process, with which we assume that the reader has a general familiarity. These processing steps include (Fig. 2.1):

• Inoculum preparation

• Substrate preparation

• Bioreactor preparation

• Inoculation and loading

• Bioreactor operation

• Downstream processing

• Waste disposal

Inoculum Preparation For Fermentation

Fig. 2.1. An overview of an SSF process operated in batch mode. Note that the details can vary from process to process. For example, the substrate might either be sterilized within the bioreactor or sterilized before being added. At this level of detail, a solid-state fermentation process is no different from an SLF process. However, the details of the bioreactor and how it is designed and operated vary significantly between SSF and SLF

Fig. 2.1. An overview of an SSF process operated in batch mode. Note that the details can vary from process to process. For example, the substrate might either be sterilized within the bioreactor or sterilized before being added. At this level of detail, a solid-state fermentation process is no different from an SLF process. However, the details of the bioreactor and how it is designed and operated vary significantly between SSF and SLF

In the development of a process, attention must be given to all these steps. Some of the issues that need to be addressed in the various process steps are briefly mentioned below:

Substrate preparation. The substrate may need to be cut, milled, cracked, or granulated in order to obtain particles of an appropriate size. It may be necessary to add water and nutritional supplements or to cook or pre-treat the substrate to increase the availability of nutrients. The substrate might be sterilized, or at least pasteurized, outside the bioreactor. Alternatively, it may be possible and preferable to do this step with the substrate inside the bioreactor.

Inoculum preparation. The type and method of inoculum preparation depends on the microorganism involved. Many SSF processes involve filamentous fungi and therefore spore-based inocula may be used. The aim of this step is to develop an inoculum of sufficient size and high viability. The inoculum can often be prepared in one of various forms. For a fungal fermentation it may be possible to produce a suspended mycelial inoculum by SLF, or to undertake a solid-state fermentation followed either by suspension of spores in a liquid or by drying and grinding of the solid to produce a powder than can be used as the inoculum.

Bioreactor preparation. The bioreactor must be cleaned after the previous fermentation, and may need to be sterilized before addition of the substrate, although, as noted above, in some cases it might be appropriate to sterilize the substrate inside the bioreactor.

Inoculation and loading. The inoculation step may occur either prior to loading or after loading. If the substrate bed cannot be mixed within the bioreactor, inoculation must be done outside the bioreactor. If the bed can be mixed, then the best method of inoculation might be to spray the inoculum as a mist over the bed as it is being mixed. If the substrate is pasteurized or sterilized and inoculated outside the bioreactor, it may be necessary to undertake the loading step quite carefully in order to prevent or at least minimize the entry of contaminants. At large scale, loading will need to be mechanically assisted.

Bioreactor operation. Much attention will be paid to this step later in the book. The details will depend on the specific bioreactor design, however, the general task is to manipulate various operating variables, such as the flow rate and temperature of the inlet air, the bed mixing speed, and the cooling water temperature, in order to control key fermentation parameters, such as bed temperature and water activity, at the optimum values for growth and product formation.

Unloading. In some cases a leaching or drying step is undertaken within the bio-reactor, in other cases the product recovery steps are undertaken outside of the bioreactor. In any case, solids must eventually be removed from the bioreactor. At large scale, unloading will need to be mechanically assisted.

Downstream processing. Depending on the process, either the whole of the fermented solids represents the product or a specific product is recovered from the solids and then purified. In the latter case, the extraction of the product from the solids represents a step in SSF processes that is not necessary in SLF processes. However, after extraction, the general principles of downstream processing are similar for both SSF and SLF.

Waste disposal. SSF is often suggested as a means of minimizing the impact of waste solid organic materials by preventing their being dumped in the environment. In some cases the whole solid is used as the product, for example, as a food or animal feed, but in others there will be a solid residue that must be disposed of adequately.

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