Bioconversion in batch bioreactor of olive-tree pruning biomass optimizing treatments for ethanol production

Abstract

Processes efficiency for second-generation ethanol production depends mainly on the type of lignocellulosic raw material. Therefore, the optimization (considering a central composite design) for each step involved in olive-tree pruning biomass valorization was studied: (1) alkaline pretreatment of the original feedstock, (2) diluted acid hydrolysis of pretreated solids and (3) fermentation of the hemicellulosic hydrolyzates for ethanol production by Scheffersomyces stipitis. The recommended alkaline pretreatment conditions were 30 min, 90 ◦C and 0.5% w/v NaOH, with losses of 88.3% of acetyl groups from starting biomass, but only 6.9% of D-xylose. Comparing both, in natura and previously treated acid hydrolyzates at the most effective conditions (2.0% w/v H2SO4 and 60 min) revealed more inhibitory effect for non-treated liquor, with 4.8, 2.1 and 1.6 times higher concentrations of acetic acid, furans and phenolic compounds, respectively. A significant improvement in ethanol production was observed in treated hemicellulose liquor (20.4 g dm-3, YP/S = 0.20 g g-1 and QP = 0.21 g dm-3 h-1). In contrast, the yeast could not satisfactorily ferment the reference hydrolyzate. Biomass pretreatment with alkali previously to dilute acid hydrolysis was a suitable strategy for olive-tree pruning biomass biotransformation, substantially decreasing the hydrolyzate toxicity, without requiring an additional detoxification step.