The use of agricultural residues for ethanol production constitutes one of the most promising alternatives from an environmental point of view for substituting fossil fuels in the transport sector. This work deals with the fermentability of hydrolysates obtained from olive tree pruning biomass and the influence of the pH of the culture medium. Hydrolysates of olive tree biomass were obtained by dilute acid pre-treatment of the raw material at 180 °C and 1% (w/v) sulfuric acid concentration. After pretreatment, solid residue and liquid were separated by filtration. The liquid fraction (hydrolysate) was then submitted to detoxification (overliming) before being used as fermentation medium. Pichia stipitis and Pachysolen tannophilus were compared as fermenting microorganisms. Different initial pH values were also tested. The best results in terms of ethanol yield were obtained by P. tannophilus with values as high as 0.44 g ethanol g -1 sugar, and all liquids were fermented, to a different extent. P. stipitis could not ferment hydrolysates with initial pH below 6.5. It was also determined that ethanol production did not improve once glucose in the medium was totally converted, even if other sugars (xylose) were also consumed.Additional key words: bioethanol; olive tree biomass; pentoses; Pichia stipitis; Pachysolen tannophilus. ResumenLa poda de olivo como residuo agrícola para la producción de etanol. Fermentación de hidrolizados procedentes de pre-tratamiento con ácido sulfúrico diluido El aprovechamiento de residuos agrícolas para la obtención de bioetanol es una de las alternativas más prometedoras desde el punto de vista medioambiental para la sustitución de los combustibles fósiles en el transporte. En este trabajo se estudia la fermentabilidad de hidrolizados procedentes de poda de olivo y se comprueba la influencia del pH del medio de cultivo. Se realizó un pre-tramiento con ácido sulfúrico diluido (1% p/v) a 180 °C de biomasa procedente de poda de olivo. Tras el pre-tratamiento, el residuo sólido y la fracción líquida (hidrolizado) fueron separados por filtración. El hidrolizado fue sometido a una etapa de detoxificación (overliming) antes de ser empleado como medio de fermentación. Como microorganismos fermentativos se usaron las levaduras Pichia stipitis y Pachysolen tannophilus. También se evaluaron distintos valores de pH inicial de fermentación. Los mejores rendimientos de etanol se obtuvieron para P. tannophilus con valores de 0,44 g etanol g -1 azúcar, y todos los líquidos fueron fermentados, en diferente extensión. P. stipitis no consiguió fermentar los hidrolizados con un pH inicial inferior a 6,5. Asimismo se detectó que la producción de etanol no mejoró una vez que la glucosa fue totalmente consumida, incluso cuando se consumieron otros azúcares como la xilosa.

Bioconversion of hemicellulose sugars is essential for increasing fuel ethanol yields from lignocellulosic biomass. We report for the first time with rape straw, bioethanol production from hemicellulose sugars. Rape straw was pretreated at mild conditions with sulfuric acid to solubilize the hemicellulose fraction. This pretreatment allows obtaining a prehydrolysate, consisting basically in a solution of monomeric hemicellulosic sugars, with low inhibitor concentrations. The remaining water insoluble solid constitutes a cellulose-enriched, free of extractives material. The influence of temperature (120ºC and 130ºC), acid concentration (2-4% w/v) and pretreatment time (30-180 min) on hemicellulose-derived sugars solubilisation was evaluated. The highest hemicellulosic sugars recovery, 72.3%, was achieved at 130ºC with 2% sulfuric acid and 60 min. At these conditions, a concentrated sugars solution, 52.4 g/L, was obtained after three acid consecutive contacts, with 67% xylose and acetic acid concentration above 4.5 g/L. After a detoxification step by activated charcoal or ion-exchange resin, prehydrolysate was fermented by ethanologenic Escherichia coli. An alcoholic solution of 25 g/L and 86% of theoretical ethanol yield was attained after 144 h when the prehydrolysate was detoxified by ion-exchange resin. The results obtained in the present work show sulfuric acid pretreatment under mild conditions and E. coli as an interesting process to exploit hemicellulosic sugars in rape straw.Additional key words: agricultural residue; acid pretreatment; xylose; E. coli; bioethanol production

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