| Summary: | 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% (<i>w</i>/<i>v</i>) initial solids (41.5 °C, 18 spheres). Then, assays were carried out at higher solid loadings (16% and 24% <i>w</i>/<i>v</i>). At 8% or 16% solids, the fermentation efficiency was similar (η<sub>F</sub>~75%), but the ethanol volumetric productivity (Q<sub>P</sub>) reduced from 1.50 to 1.14 g/L.h. By increasing the solids to 24%, the process was strongly affected (η<sub>F</sub> = 40% and Q<sub>P</sub> = 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<sub>P</sub> of 1.1 g/L.h and η<sub>F</sub> 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.
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