Fermentation strategies for the efficient use of olive tree pruning biomass from a flexible biorefinery approach

Olive tree pruning (OTP) represents an attractive biomass feedstock in the Mediterranean countries and worldwide. In this work, OTP has been studied as raw material for the production of advanced biofuels (i.e. bioethanol) within a biorefinery perspective. After pretreatment by water extraction and phosphoric-acid-catalyzed steam explosion, the whole pretreated slurry was completely inhibitory to the tested Saccharomyces cerevisiae strains. Detoxification of the liquid fraction overcame such inhibition allowing complete fermentation of both glucose and xylose by the recombinant xylose-fermenting S. cerevisiae F12. When reaching sugar depletion, the fermentation broth was fed with the hydrolysate resulting from enzymatic saccharification of the solid fraction at high solid loadings. This process configuration increased ethanol concentrations up to 45 g/L, reaching 80% of the theoretical conversion yields. Overall, about 180 g of ethanol per kg of extracted OTP biomass could be obtained with this process, which increases previous conversion yields by 12.5%. This strategy also enables the use of the extracted fraction for antioxidant production and offers the potential utilization of the xylose-rich fraction to obtain alternative fermentation-based bioproducts (simultaneously obtaining 125-150 g of ethanol per kg of extracted OTP biomass), thus allowing adaptation of the process to the market needs.