Solid substrate fermentation: a biotechnological approach to bioconversion of wastes

Solid substrate fermentation processes have been used by man for many centuries. The term `solid substrate fermentation’ (SSF) describes the microbiological transformation on and/or within the particles of solid matrix (solid substrate) where the liquid content, bound with them, is at the level corresponding to the water activity (a<sub>W</sub>) assuring growth and metabolism of cells, but not exceeding the maximal water-holding capacity of the solid matrix (Cannel and Moo-Young, 1980a; Durand et al., 1988; Paredes-López and Harry, 1988; Pandey, 1992). In other words, in SSF the moist water-insoluble solid substrate is fermented by microorganisms in the absence or near-absence of free water, resulting in semisolid or solid fermentation systems (Hesseltine, 1977a). `Solid state’ and `solid substrate’ fermentations are terms used by different workers but they are essentially the same. On the other hand, in liquid state fermentations (LSF), the substrate is solubilized or suspended as fine particles in a large volume of water. SSF were used long before the microbiological or biochemical processes involved were understood. Various SSF processes with histories reaching far back in time are still practiced today (Moo-Young et al., 1983; Steinkraus, 1983a). These traditional SSF systems deal with fermented foods (e.g. tempeh in Indonesia, miso in Japan, pozol in Mexico), mold-ripened cheeses, starter cultures for fermented brews, and silage and compost. The use of soy sauce koji, a fermented oriental food preparation used in China, Japan and south-east Asia goes back as far as 1000 Bc and probably 3000 Bc in China (Cannel and Moo-Young, 1980a; Pandey, 1992).