Enhanced ethanol production by simultaneous saccharification and fermentation of pretreated olive stones

Abstract

Olive stones are an agro-industrial by-product in Mediterranean countries that can be regarded as a potential source of bio-fuels and bioactive compounds. In this work, olive stones were pretreated with liquid hot water (LHW), dilute sulphuric acid (DSA) and organosolv delignification at temperatures in the range 190 °C–230 °C. The resulting solids from pretreatments were subjected to simultaneous saccharification and fermentation (SSF) with the yeast Saccharomyces cerevisiae IR2-9a for fuel-ethanol production. The composition of prehydrolysates from LHW and DSA pretreatments were strongly dependent of reactor temperature. The highest yields of d-xylose (0.21 kg kg−1) and d-glucose (0.11 kg kg−1) were achieved in DSA assays at 200 °C and 230 °C, respectively, while LHW processes provided the maximum oligosaccharide recovery (0.17 kg kg−1) at 210 °C. The addition of sulphuric acid in organosolv pretreatments was particularly effective to increase the cellulose content of the pretreated solids till percentages close to 83%. Finally, the maximum ethanol concentration achieved by SSF was 47.1 kg m−3, and 13 kg ethanol were obtained from 100 kg of raw material.