Second generation bioethanol production at high gravity of pilot-scale pretreated wheat straw employing newly isolated thermotolerant yeast Kluyveromyces marxianus DBTIOC-35

Abstract

Second-generation bioethanol production by a newly isolated thermotolerant yeast strain was studied at 42 °C and above using pilot-scale dilute acid pretreated wheat straw (WS) as feedstock. This strain was identified as Kluyveromyces marxianus DBTIOC-35 by biochemical characterization as well as molecular phylogenetic analysis of the ITS-5.8S rRNA gene and D1/D2 domain of the 26S rRNA gene after PCR amplification and sequencing. Simultaneous saccharification and fermentation (SSF) at 42 °C and 45 °C using 10% biomass loading resulted in ethanol titers of 29.0 and 16.1 g L⁻¹, respectively. At 42 °C ethanol productivity was higher during SSF (0.92 g L⁻¹ h⁻¹) than separate hydrolysis and fermentation (SHF) (0.49 g L⁻¹ h⁻¹) at 20% biomass loading. The results indicated that at 20% biomass loading, SSF without pre-saccharification led to more ethanol production (66.2 g L⁻¹ with 83.3% yield) at a faster rate than SSF with pre-saccharification (PSSF) which produced an ethanol titer of 61.8 g L⁻¹, 77.7% yield and productivity of 0.86 g L⁻¹ h⁻¹. Based on these findings, application of newly isolated yeast K. marxianus DBTIOC-35 in SSF of lignocellulosic biomass can eliminate the pre-saccharification step which is a novel advantage of thermotolerant yeasts in terms of cutting down the overall biomass to bioethanol process time and enhancing bioethanol titer, yields and productivities.