Selection of thermotolerant yeast strains Saccharomyces cerevisiae for bioethanol production

The simultaneous saccharification and fermentation (SSF) process of lignocellulosic materials requires the utilization of microorganisms capable of working at high temperatures. The selection of Saccharomyces cerevisiae strains able to ferment sugars obtained from lignocellulosic material at temperatures above 35 °C with high ethanol yield has become a necessity. In this work, 11 S. cerevisiae strains were screened for their ability to grow and ferment glucose in a temperature range of 35–45 °C. All the strains grew (in agar plates) at 35 and 40 °C, only two strains grew at 42 °C, and no strain grew at 45 °C. The yeasts were placed in a liquid medium, and incubated at 35, 40 and 42 °C. Glucose-to-ethanol conversion yield was between 50% and 80% of the theoretical value. Strains that produced at least 70% of the theoretical ethanol yield were submitted to two acclimatization treatments. One pure yeast strain was isolated (IR2-9a). For bleached kraft pulp as well as organosolv-pretreated Pinus radiata chips, the ethanol yields by SSF using the IR2-9a strain were higher than those obtained using the control yeast.