What Is Solid State Fermentation

Solid-state fermentation (SSF) involves the growth of microorganisms on moist solid particles, in situations in which the spaces between the particles contain a continuous gas phase and a minimum of visible water. Although droplets of water may be present between the particles, and there may be thin films of water at the particle surface, the inter-particle water phase is discontinuous and most of the inter-particle space is filled by the gas phase. The majority of the water in the system is absorbed within the moist solid-particles (Fig. 1.1(a)). More detail about the spatial arrangement of the system components is given in Chap. 2.

In fact, here we follow the nomenclature proposed by Moo-Young et al. (1983) where the more general term "solid-substrate fermentation" is used to denote any type of fermentation process that involves solids, including suspensions of solid particles in a continuous liquid phase and even trickling filters (Fig. 1.1(b)). Therefore solid-state fermentation is classified as one type of solid-substrate fermentation. In this book we concentrate specifically on solid-state fermentation systems, in the manner that we defined them in the first paragraph.

The aim of the present section is not to give an in-depth explanation of all the characteristics of SSF systems, nor to compare SSF with submerged liquid fermentation (SLF). The further reading section at the end of this chapter gives some sources of general background information for readers who do not have much familiarity with SSF systems. Here we will give only a very broad summary of some of the main points:

• The majority of SSF processes involve filamentous fungi, although some involve bacteria and some involve yeasts.

• SSF processes may involve the pure culture of organisms, or the culture of several pure strains inoculated simultaneously or sequentially, while in some processes a "self-selected" microflora arises from the original microflora (e.g., in composting) or from a specially prepared traditional inoculum.

• The majority of SSF processes involve aerobic organisms. Note that we use the word "fermentation" in this book in the sense of its more general meaning, that is, "the controlled cultivation of organisms" (the SSF literature uses the word fermentation in this sense).

• The substrates used in SSF processes are often products or byproducts of agriculture, forestry or food processing. Typically the source of nutrients comes from within the particle, although there are some cases in which nutrients are supplied from an external source. Usually a polymer gives the solid structure to the particle and this polymer may or may not be degraded by the microorganism during the fermentation. There are also some cases in which artificial or inert supports are used, with a nutrient solution absorbed within the matrix.

droplets of water in the inter-particle spaces

continuous gas phase water and nutrients absorbed within particle moist biofilm continuous gas phase water and nutrients absorbed within particle moist biofilm liquid with nutrients liquid with nutrients

biofilm suspended solid particle with attached biomass

Fig. 1.1. The defining features of solid-state fermentation (SSF) systems (following the terminology of Moo-Young et al. 1983). (a) The arrangement of moist solid particles and the continuous gas phase in SSF systems involving a filamentous fungus (left-hand side) and a unicellular organism (right-hand side). (b) Other systems that involve growth on solids, but which are not defined as SSF due to the large amount of water in the inter-particle spaces. The left-hand diagram represents a trickling-filter type system while the right-hand diagram represents a suspension or slurry system biofilm suspended solid particle with attached biomass

Fig. 1.1. The defining features of solid-state fermentation (SSF) systems (following the terminology of Moo-Young et al. 1983). (a) The arrangement of moist solid particles and the continuous gas phase in SSF systems involving a filamentous fungus (left-hand side) and a unicellular organism (right-hand side). (b) Other systems that involve growth on solids, but which are not defined as SSF due to the large amount of water in the inter-particle spaces. The left-hand diagram represents a trickling-filter type system while the right-hand diagram represents a suspension or slurry system

Much of this book will assume that we are working with pure cultures of aerobic filamentous fungi, to produce a specific product. In this case, there is a definite set of optimum conditions for growth of the process organism and product formation by it. Therefore this book does not consider composting, which is a specific application of SSF in which it is desirable for the temperature to vary during the process. Of course, with this and other important differences, such as the use of undefined mixed cultures, composting has its own literature, which is not directly relevant to the type of SSF process in which we are interested.

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