Bioethanol production from pretreated palm empty fruit bunch (PEFB) using sequential enzymatic hydrolysis and yeast fermentation

Abstract

This study investigated the effectiveness of steam pretreatment using distilled water or waste glycerol as solvents to produce soluble sugars via enzymatic hydrolysis of palm empty fruit bunch (PEFB). Subsequently, two yeast species (S. cerevisiae and P. stipitis) were introduced to stimulate the fermentation of the soluble sugars to ethanol. The fermentable sugar (glucose plus xylose) concentration generated during hydrolysis of pretreated PEFB was found to be mainly a function of the pretreatment conditions (i.e. type of solvent, liquid-phase pH, and reaction time) as well as the hydrolysis time. Glucose was the major component of fermentable sugar in all tests; while the xylose content fluctuated from 3 to over 40% of that of glucose, with the higher percentages obtained at the shortest pretreatment time. The use of alkaline waste glycerol (pH 11) as a solvent during pretreatment resulted in optimum hydrolysis with a mean fermentable sugar concentration of 51.5 ± 4.1 g/L, while the corresponding mean value of the waste glycerol tests (pH 8.3) was 35.3 ± 4.9 g/L. Regarding fermentation, alkaline waste glycerol pretreatment enhanced ethanol production by over 50% compared to distilled water. It was also observed that fermentation using a S. cerevisiae monoculture was more effective than that of an equivalent co-culture.