ABSTRACT
Fermentation by microorganisms can be divided into two parts: liquid-state fermentation (LSF) and solid-state fermentation (SSF) (Pandey et al., 2000). SSF means that microorganisms grow on a solid medium with little or no free water, which is quite close to a natural state. SSF involves microorganism’s growth and utilization of insoluble medium. Compared to LSF, SSF has many advantages: (i) SSF does not need strictly anaerobic conditions; (ii) facilities and energy costs are much lower; (iii) the product yield is higher; and (iv) the after-processing is simpler and produces only little wastewater during the process (Singhania et al., 2009). Therefore, SSF has attracted a great deal of attention recently and has begun being used worldwide. However, due to the low water activity, less cell-growth uniformity, and the transfer of nutrient substance and products during fermentation, it is quite difficult to measure and control the parameters. These factors make industrialization of SSF difficult in large scale (Durand, 2003). The industrialization of penicillin in 1945 created the new era of modern industrial fermentation but also caused a trend away from SSF (Pandey, 2003). However, the production rate of some modern biological products, such as enzymes (Graminha et al., 2009) and organic acids (Vandenberghe et al., 2000), created through SSF is much higher than that of LSF. On the other hand, the production of wastewater, the high consumption of ventilation, and mechanical agitation have obstructed the development of LSF. Thus, researchers have done intensive research on very high gravity (VHG) fermentation (Soccol and Vandenberghe, 2003). However, the utmost form of VHG fermentation is SSF. Hence, with the recent advances in bioengineering, scientists should pay much more attention on SSF.
