SSF of alkali-pretreated switchgrass was performed with a solid
loading of 5% and enzyme loading of 15 FPU/g-glucan (Fig. 2b). In
the first 24 h, glucose (10 g/L) and xylose (1.0 g/L) was released,
small amount of acetic and butyric acids (2 g/L) was produced.
No solvents were produced until 48 h, but the solvents production
quickly stopped at 72 h. The final total solvents was very low (3 g/
L). More than 9 g/L of glucose was present in the solution from 24
to 120 h. This indicated microbes could not actively consume sugars
in the SSF of Alkali-pretreated switchgrass, and enzyme hydrolysis
was not a rate-limiting step in SSF. It was most likely that
residual lignin (10.4%) and other degradation compounds in
alkali-pretreated switchgrass inhibited the microbial fermentation
step. Similar results have been reported on converting dilute acid
pretreated corn stover to butanol in a SSF process (Qureshi et al.,
2014), they found the pretreated corn stover still contained toxic
chemicals that inhibited cell growth and fermentation activity. A
further detoxification of pretreated solid was required to improve
the ABE yield