Pretreatment of spruce and oak by N-methylmorpholine-N-oxide (NMMO) for efficient conversion of their cellulose to ethanol

Pretreatment of softwood spruce and hardwood oak with an industrial cellulose solvent, N-methylmorpholine-N-oxide (NMMO), was investigated prior to enzymatic hydrolysis and fermentation to ethanol. The pretreatments were carried out at 90, 110 and 130 °C for 1-3 h with 85% NMMO solution, followed by non-isothermal simultaneous saccharification and fermentation (NSSF). This NSSF included hydrolysis with cellulase and β-glucosidase for 24 h at 45 °C, followed by continuous saccharification and fermentation with Saccharomyces cerevisiae at 37 °C for 3 days. The NSSF of untreated oak and spruce resulted in 18.6% and 6.8% ethanol compared to the maximum theoretical yield. However, the pretreatment of oak and spruce at 130 °C resulted in almost total conversion of cellulose to ethanol and improved ethanol yield up to 85.4% and 89%, respectively. These numbers are comparable with ethanol from pure glucose with the same strain, which yielded between 84% and 90% of the theoretical ethanol yield.