Response surface optimization of ammonium sulfite pretreatment for fermentable sugar production from wheat straw

Abstract

The development of a clean and sustainable pretreatment is of great importance for the production of fermentable sugars. In this study, an ammonium sulfite (AS) pretreatment of wheat straw was optimized based on response surface methodology with a three-level, three-factor Box-Behnken design. The investigated factors were AS dosage, pretreatment time, and pretreatment temperature. The effectiveness of the AS pretreatment was evaluated using the standard enzymatic hydrolysis procedure. A second-order polynomial fit was performed to fit the experimental data, and the model analysis showed that the effect of the AS dosage on the final total sugar yields was much more significant than that of the other two factors. Under the optimum pretreatment conditions (27% of the AS dosage (based on the dry wheat straw) at 160 ºC for 63 min), the final total sugar yield achieved was 74.4% after saccharification, which was in agreement with the predicted value (76.5%). Furthermore, it was found that pre-impregnation with acetic acid before AS pretreatment or the post-mechanical refining after AS pretreatment could further increase the fermentable sugar yields to approximately 77%. In addition, the spent liquor containing nitrogen could be used for the production of lignin-based fertilizer, thus making the whole process clean and sustainable.