Improving liquefaction efficiency and ethanol titer for two-stage sulfite steam pre-treated softwood whole slurry by tuning the pre-treatment pH

The two-stage sulfite steam pre-treatment (TSSP), employing sodium sulfite in the first stage and SO2 in the second stage, has demonstrated significant potential in enhancing the bioconversion efficiency of softwood biomass, yielding a high ethanol titer. Importantly, it was emphasized that in this process, achieving high ethanol production did not necessitate hydrolysate concentration or detoxification. However, challenges emerged due to unique substrates characteristics such as large particle size, a high degree of polymerization, and elevated hemicellulose content, which limited the liquefaction efficiency during high solid loading hydrolysis. To address these limitations as well as further improve liquefaction efficiency and ethanol titer, the acidity of the TSSP was fine-tuned. This involved replacing SO2 with stronger acid H2SO4 and substituting alkaline sodium sulfite with acidic sodium bisulfite. Results indicated H2SO4 was more efficient than SO2 in decreasing the second stage pH, leading to a 50 % improvement in overall slurry liquefaction efficiency (reducing liquefaction duration from 48 hours to 24 hours). Furthermore, the use of H2SO4 as an acidic reagent in the TSSP resulted in a slightly higher ethanol titer of 74.6 g/L in the softwood whole slurry. In comparison to alkaline sodium sulfite, employing acidic sodium bisulfite as the sulfonation reagent in first stage of TSSP also helped to adjust the pH, resulting in a slightly higher ethanol titer of 76.6 g/L from the softwood whole slurry hydrolysate, during the simultaneous saccharification and fermentation. It was evident that the pH tuning approach successfully reduced the liquefaction time while achieving higher ethanol production efficiency from softwood biomass. [Display omitted] •pH tuning for two-stage sulfite steam pretreatment (TSSP) was carried out.•Sulfuric acid and bisulfite were used to tune the pH for TSSP.•Better substrates size reduction and higher hemicellulose removal were achieved.•Whole slurry liquefaction efficiency was largely improved by pH tuning.•Higher ethanol titer was achieved by pH tuning without substrates detoxification.