Multistep Process to Produce Fermentable Sugars and Lignosulfonates from Softwood Enzymolysis Residues

The residual solids from enzymatic hydrolysis are usually burned to produce energy and have been explored as a feedstock for various products including activated carbon and lignin based polymers. These products require additional procedures unrelated to the existing biorefinery equipment. In the current study, we proposed successive sulfite treatments to utilize the enzymolysis residues for producing fermentable sugars and lignosulfonates. Two sulfite methods were compared as an initial pretreatment. It was found that the acid bisulfite treatment achieved higher hemicellulose removal (90%), while neutral sulfite resulted in higher lignin removal (50%) and a greater lignosulfonate yield (26.6 g from 100 g raw material). Overall, the acid bisulfite treatment resulted in 66% glucan conversion by enzymatic hydrolysis, whereas it was only 37% for the neutral sulfite method. In addition, the neutral sulfite pretreated wood reached 80% of the maximum sugar yield after 6 h of enzymatic hydrolysis, whereas it was 48 h for the acid bisulfite pretreated wood. The multistep treatment process achieved 84% total sugar conversion and 58% lignin removal with 32.4 g lignosulfonates being produced from 100 g raw materials. The results demonstrate that the multistep treatment process has a potential to maximize the yields of fermentable sugars and lignosulfonates.