Bioethanol production using the sequential acid/alkali-pretreated empty palm fruit bunch fiber

Empty palm fruit bunch (EPFB) contains a rigid and ordered fiber structure within a relatively high level of hemicellulose and lignin. To enhance poorly enzymatic digestibility and fermentability, in this a sequential acid/alkali pretreatment strategy was applied to increase the fermentable sugar in EPFB fiber. Cellulose content in the biomass was effectively increased by using dilute sulfuric acid treatment to eliminate hemicellulose followed by highly concentrated sodium hydroxide treatment for delignification. The pretreated biomass consisted of 85.2 ± 1.9% cellulose, 1.8 ± 0.5% hemicellulose, and 9.2 ± 0.3% lignin. Under optimal conditions for enzymatic saccharification, 10% (w/v) of pretreated EPFB fiber was hydrolyzed completely and converted to 70.8 ± 0.8 g/L glucose and 1.8 ± 0.1 g/L xylose with a 83.9% yield efficiency. Simultaneous saccharification and fermentation of the pretreated biomass by Saccharomyces cerevisiae W303-1A produced 37.8 g/L ethanol in 1.5 L fermented medium containing 10% (w/v) pretreated EPFB fiber after 60 h. The ethanol productivity was 0.378 g ethanol/g dry EPFB fiber and 0.45 g ethanol/g glucose after fermentation, with a low concentration of xylose and organic acid metabolites. Also, 88% of fermentable sugar was used by the yeast for ethanol fermentation. These results indicate that sequential acid/alkali pretreatment increased enzymatic digestibility and ethanol productivity.