Improvement of Enzymatic Hydrolysis of Whole Slurry from Bisulfite-Pretreated Furfural Residues with Low Enzyme Loading

The aim of this research was to compare the cellulose conversions of non-detoxified furfural residues after acid bisulfite-pretreatment at low temperatures (80, 100 and 120 °C). The whole slurry of furfural residues from bisulfite pretreatment was directly hydrolyzed for 72 h with various enzyme loadings (10, 16, and 22 FPU/g-glucan). The results showed that the enzymatic digestibility of non-detoxified whole slurry, at a relatively low enzyme loading of 10 FPU/g-glucan, increased from 71.0% to 93.8%, when furfural residues were pretreated with bisulfite at 100 °C for 3 h. Moreover, hydrolysis of furfural residues at 16 FPU/g-glucan reached a maximum substrate enzymatic digestibility of 99.9%. The surface tension of supernatants of hydrolysate decreased after acid bisulfite pretreatment, and the lowest surface tension (61.9 mN/m) was found after pretreatment at 100 °C. According to the results, mass balance quantificationally showed acid bisulfate pretreatment partially removes the lignin of furfural residues and FT-IR analysis revealed the sulfonation of lignin during acid bisulfate pretreatment, both enhancing the efficiency of enzymatic hydrolysis of cellulose.