Fractionation methodology to improve ethanol yields from olive tree pruning

Lignocellulosic biomass obtained from olive tree pruning is an abundant residue without any practical application which has been investigated for ethanol production with different pretreatment methodologies, such as liquid hot water, steam explosion, organosolvent or acid pretreatments (Romero-García et al. Biorefinery based on olive biomass. State of the art and future trends. Bioresource Technology 159 (2014) 421-432).

In this work a fractionation strategy was applied to improve ethanol yields. First, a water extraction step at 120ºC for 60 min was employed to remove the most part of extractives, followed by mild acid pretreatment at optimized conditions for hemicellulose solubilization (130ºC, 2.4% H₂SO₄, 84 min). Then, the pretreated solids (enriched in cellulose and lignin) were subjected to alkaline peroxide delignification at different conditions. A central rotatable experimental design was applied varying H₂O₂ concentration (1-7 % w/v), and treatment time (30-90 min). Temperature and liquid to solid ratio were maintained constant at 80ºC and 1:10, respectively.

The water extraction improved the performance in subsequent steps allowing the fermentability of the hemicellulose prehydrolyzate by ethanologenic Escherichia coli strain (8 g ethanol/100 g raw material). Concerning the delignification step, up to 80 % of lignin removal was achieved with a significant increase in enzymatic hydrolysis yield reaching values around 85%. Considering the ethanol production from delignified solids by simultaneous saccharification and fermentation (SSF), 15.5 g ethanol/100 g raw material were obtained. The fractionation methodology investigated resulted in the improvement of ethanol yields comparing with previous works.