Ethanol production through simultaneous saccharification and fermentation of switchgrass using Saccharomyces cerevisiae D 5A and thermotolerant Kluyveromyces marxianus IMB strains

Abstract

Hydrothermolysis pretreated switchgrass at 200 °C for 10 min was used in a simultaneous saccharification and fermentation (SSF) process using five thermotolerant yeast strains Kluyveromyces marxianus IMB 1, IMB 2, IMB 3, IMB 4, and IMB 5 at 45 °C and Saccharomyces cerevisiae D 5A at 37 °C. SSF was carried out for 7 d using 5, 10, and 15 FPU/g glucan to determine the effect of decreasing cellulase loading on ethanol yield. The effect of initial pH on SSF by S. cerevisiae D 5A was also investigated. Fermentation by K. marxianus IMB 1, IMB 2, IMB 4, and IMB 5 ceased by 72 h and fermentation by K. marxianus IMB 3 ceased by 96 h, while fermentation S. cerevisiae D 5A continued for 7 d. At 96 and 120 h, IMB 3 and S. cerevisiae D 5A had similar ethanol yields while the other K. marxianus strains were lower at a 95% confidence level. Final ethanol yields for IMB 3, IMB 1, IMB 5 strains were similar to one another, however, ethanol yield for S. cerevisiae D 5A (92% maximum theoretical) was greater than all of the IMB strains except IMB 3 at a 95% confidence level. Reducing enzyme loading reduced ethanol yields for both K. marxianus IMB 3 and S. cerevisiae D 5A. Reducing buffer pH from 5.5 to 4.8 reduced ethanol yields for S. cerevisiae D 5A. This study shows that K. marxianus IMB 3 has potential for commercial use for ethanol production from cellulose in SSF processes with further improvement of its thermotolerance.