Effect of Laccase Detoxification on Bioethanol Production from Liquid Fraction of Steam-Pretreated Olive Tree Pruning

During lignocellulosic bioethanol production, the whole slurry obtained by steam explosion is filtered, generating a water-insoluble fraction rich in cellulose which is used for saccharification and ethanol fermentation, as well as a liquid fraction containing solubilised glucose and xylose but also some inhibitory by-products (furan derivatives, weak acids and phenols), which limits its use for this purpose. Since utilization of this liquid fraction to ethanol is essential for an economically feasible cellulosic ethanol process, this work studied a laccase from Myceliophthora thermophila to detoxify the liquid fraction obtained from steam-pretreated olive tree pruning (OTP) and to overcome the effects of these inhibitors. Then, the fermentation of laccase-treated liquid fraction was evaluated on ethanol production by different Saccharomyces cerevisiae strains, including the Ethanol Red, with the capacity to ferment glucose but not xylose, and the xylose-fermenting recombinant strain F12. Laccase treatment reduced total phenols content by 87% from OTP liquid fraction, not affecting furan derivatives and weak acids concentration. Consequently, the fermentative behavior of both Ethanol Red and F12 strains was improved, and ethanol production and yields were increased. Moreover, F12 strain was capable of utilizing some xylose, which increased ethanol production (10.1 g/L) compared to Ethanol Red strain (8.6 g/L).