ABSTRACT
Solid-state (substrate) fermentation (SSF) has been defined as the fermentation process occurring in the absence or near absence of free water. SSF processes generally employ a natural raw material as the carbon and energy source. SSF can also employ an inert material as the solid matrix, which requires supplementing a nutrient solution to contain necessary nutrients as well as a carbon source. The solid substrate (matrix), however, must contain enough moisture (1-6). Depending upon the nature of the substrate, the amount of water absorbed could be one or several times more than the dry weight, which leads to relatively high water activity (aw) on the solid-gas interface, in order to allow a higher rate of the biochemical process. Lower water activity conditions result in low diffusion of nutrients and metabolites, and higher water activity causes compaction of the substrate. Thus, maintenance of an adequate moisture level in the solid matrix, along with suitable water activity, are essential elements for SSF processes (2,4,7-10). Solid substrates should generally have a large surface area per unit volume (in the range of 103-106 m2/cm3), to allow for ready growth on the solid-gas interface (11). Smaller substrate particles provide larger surface area for microbial attack, but pose difficulty in aeration and respiration due to the limitation in interparticle space availability. Larger particles provide better aeration and respiration opportunities, but provide less surface area. In bioprocess optimization, it may sometimes be necessary to use a compromised size of particles (usually a mixed range) to maximize cost effectiveness. For example, wheat bran, which is the most commonly used substrate in SSF, is obtained in two forms, fine and coarse. The former contains particles mostly smaller than 500-600 µ, and the latter contains particles mostly larger than these. Most SSF processes use a mix of these two forms, at different ratios, for optimal production (1,2,12-14).
