Olive tree pruning as an agricultural residue for ethanol production. Fermentation of hydrolysates from dilute acid pretreatment

Abstract

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.

Valorization of winery and oil mill wastes by microbial technologies

“…Romero et al (2007) carried out the fermentation of olive tree biomass hydrolysates using P. tannophilus as fermenting microorganism. The ethanolic fermentation of liquid fractions from pretreated olive tree biomass utilizing P. stipitis and P. tannophilus have been studied by Díaz-Villanueva et al (2012) and Romero et al (2007). Results reported show that ethanol yields obtained from hydrolysates using P. stipitis as the fermenting microorganism, ranging from 0.35 to 0.42 g/g although the process proceeds at lower rates.…”

A holistic engineering approach for utilization of olive pomace

“…Sulfuric acid is reported to be the most commonly used acid in olive tree residues [28,29]. Both unprocessed olive pomace and de-oiled pomaces were hydrolyzed using dilute sulfuric acid at concentrations of 2.0%, 4.0%, 6.0% for 30, 60, 90 min at 121 • C by autoclaving.…”

Hemicellulose-derived sugars solubilisation of rape straw. Cofermentation of pentoses and hexoses by Escherichia coli

“…However, when the fermentation was extended to 192 h, only an increase of 5% in ethanol production was reached and the productivity dropped to 0.14 g/L h. Therefore 144 h could be considered as the final time of fermentation. Díaz-Villanueva et al (2012), from olive tree (Olea europaea) pruning pretreated also by dilute sulfuric acid (180ºC, 1% w/v H 2 SO 4 , 10 min), obtained a sugar solution about 55 g/L. They were able to ferment this prehydrolysate after overliming at pH 6.5 with maxima ethanol yields of 0.40 (g ethanol/g consumed sugar) by Pachysolen tannophilus and Pichia stipitis.…”