Feasibility of simultaneous saccharification and juice co-fermentation on hydrothermal pretreated sweet sorghum bagasse for ethanol production

For a commercial breakthrough of ethanol production from lignocelluloses, one main challenge still exists in achieving high ethanol concentrations during fermentation. Using sweet sorghum as a feedstock for ethanol production has the advantage to have both, directly fermentable sugars in the juice as well as sugars from lignocellulose of the plant fibers. A novel concept is presented, which combines the fermentation of dewatered sweet sorghum bagasse after hydrothermal pretreatment and sweet sorghum juice in one process step. For the pretreatment of sweet sorghum bagasse, liquid hot water was used as a reagent. Combinations of different retention times and target temperatures (170–200 °C) were investigated for severities between log R0 = 3.51 and 4.09. Further, a mathematical function of first order was established to describe the solubility of hemicellulose with reference to the severity. The highest solubilization of hemicellulose obtained was 85% with the highest severity applied (log R0 = 4.09, target temperature 200 °C). Additionally the cellulose content was enriched from an initial 36% DM (raw bagasse) to 59% DM. For almost all the pretreatment conditions the pH in the supernatant remained higher than or equal to the critical value of 4. For simultaneous saccharification and fermentation trials of the dewatered pretreated but non-washed material, a maximum convertibility of cellulose of 74% was obtained at a severity of log R0 = 4.02 (190 °C). When sweet sorghum juice with a sugar concentration of 100 g/L was mixed with the pretreated and dewatered bagasse (log R0 = 4.0, 190 °C) to a water-insoluble solids content of 7.5%, 53 g/L ethanol was achieved after 168 h of fermentation. The convertibility of cellulose after this time remained unaffected by the juice addition.