Ethanol Production from High Solid Loading of Rice Straw by Simultaneous Saccharification and Fermentation in a Non-Conventional Reactor

Simultaneous saccharification and fermentation (SSF) at high solid loading is a potential approach to improve the economic feasibility of cellulosic ethanol. In this study, SSF using high loading of rice straw was assessed using a vertical ball mill reactor. First, the conditions of temperature and number of glass spheres were optimized at 8% (w/v) initial solids (41.5 degrees C, 18 spheres). Then, assays were carried out at higher solid loadings (16% and 24% w/v). At 8% or 16% solids, the fermentation efficiency was similar (eta(F)similar to 75%), but the ethanol volumetric productivity (Q(P)) reduced from 1.50 to 1.14 g/L.h. By increasing the solids to 24%, the process was strongly affected (eta(F) = 40% and Q(P) = 0.7 g/L.h). To overcome this drawback, three different feeding profiles of 24% pre-treated rice straw were investigated. Gradual feeding of the substrate (initial load of 16% with additions of 4% at 10 and 24 h) and an inoculum level of 3 g/L resulted in a high ethanol titer (52.3 g/L) with Q(P) of 1.1 g/L.h and eta(F) of 67%. These findings demonstrated that using a suitable fed-batch feeding strategy helps to overcome the limitations of SSF in batch mode caused by the use of high solid content.