Ethanol production from olive stone hydrolysates by xylose fermenting microorganisms

Abstract

AbstractOlive stones are the main solid byproducts obtained from olive oil production and from table olives production. As a lignocellulosic material, the use of olive stones for ethanol and other chemicals production has been proposed, particularly under the biorefinery concept. As part of such a process, this work deals with the fractionation of the lignocellulosic material by dilute acid autoclave pretreatment at 2% sulfuric acid, 130°C, 60 min and 1:1 liquid to solid ratio. Moreover, the work addresses the fermentation of the liquors obtained after pretreatment. The released sugars are composed mainly by xylose and other hemicellulosic sugars. The fermentation performance of three xylose-fermenting microorganisms, e.g. two Escherichia coli species and Scheffersomyces stipitis, are compared. The study analyzes in a first step the microorganism behavior on synthetic liquors, with a similar composition to that of the real liquors. Finally, and taken into account the results from the previous steps, the real liquor obtained from olive stones pretreatment is fermented. Results show that E. coli MM160 is the best ethanol producer out of the three microorganisms studied. Globally, the pretreatment produced a liquor containing 140 g hemicellulosic sugars/l and requiring firstly dilution by 50% and a detoxification step by overliming. The fermentation of this liquor by E. coli MM160 results in a 25 g ethanol/l solution equivalent to 50 g ethanol/kg olive stone, in spite of 20 g acetic acid/l also present. These results confirm both olive stones and E. coli MM160 as promising feedstock and microorganism for ethanol production.