Simultaneous Saccharification and Fermentation of Switchgrass by Thermotolerant Kluyveromyces marxianus Imb3: Effect of Enzyme Loading, Temperature and Operating Mode

Abstract

The objectives of this research were to determine the optimum enzyme (Accellerase 1500, Genencor International, NY, USA) loading, temperature and operating modes (batch and fed-batch) on ethanol production by simultaneous saccharification and fermentation (SSF) of pretreated Kanlow switchgrass using thermotolerant Kluyveromyces marxianus IMB3. Hydrothermolysis pretreatment of switchgrass was used. Also, 8% solids (dry basis, db) loading was used for the effect of enzyme loadings and temperatures tests. Various loadings of Accellerase 1500 (0.3, 0.5 and 0.7 mL g^-1 glucan) were used in SSF at 45ﾰC (activity of enzyme was 82.2 FPU mL^-1). Then, SSFs were performed at 37, 41 and 45ﾰC with the optimum enzyme loading. Finally, four different batch and fed-batch strategies were evaluated at the optimum enzyme loading and temperature with a total solid loading of 12% (db). The optimum enzyme loading was 0.7 mL g^-1 glucan in SSF with 8% solids at 45ﾰC, which resulted in the highest ethanol concentration of 22.3 g L^-1. This was equivalent to 85% maximum theoretical yield (MTY). For the effect of temperature experiment, the highest ethanol concentration of 23.0 g L^-1 (86.3% MTY) was obtained in SSF with 8% solids at 45ﾰC compared to about 20 g L^-1 ethanol (77% MTY) obtained at both 37ﾰC and 41ﾰC. In addition, the results showed that about 32 g L^-1 ethanol (81% MTY) was produced with a total solid loading of 12% (db) in all batch and fed-batch SSFs strategies. No significant differences in the amount of ethanol produced in batch or fed-batch operating modes were observed in the range of solids and enzyme loadings used (p > 0.05). Moreover, the results also showed that with fed-batch mode and 12% solids (db), the enzyme loading can be decreased by 33% of the optimum loading for batch process.