Preparation of silk fibroin/cellulose whiskers bionanocomposite sponges for yeast cell immobilization used in continuous ethanol production by packed bed bioreactor

Abstract

Silk fibroin (SF)/cellulose whiskers (CLWs) bionanocomposite sponges were prepared as a supporting material for yeast cell immobilization for further using in continuous ethanol production. The SF/CLWs bionanocomposite sponges with high porosity were fabricated by freeze-drying a silk fibroin solution containing cellulose whiskers at different SF/CLWs weight ratios. The freeze-dried SF/CLWs sponges were subjected to methanol treatment to increase water stability. Increasing the CLWs content resulted in higher water stability, less shrinkage, and better mechanical properties of SF/CLWs bionanocomposite sponges. In the continuous ethanol fermentation process, the bionanocomposite sponges with a SF/CLWs weight ratio of 50/50 were used to immobilize Saccharomyces serevisiae burgundy KY11 yeast cells. The bionanocomposite sponges with the immobilized yeast cells were loaded into the glass column of a packed bed bioreactor. Continuous ethanol fermentation was operated by varying D-glucose concentrations (100, 150, 200 g/l), dilution rates (0.15, 0.20, 0.25 hrˉ¹), and hydraulic retention times (6.87, 5.15, 4.12 hr). The continuous ethanol fermentation with a high concentration of D-glucose, low dilution rate, and high hydraulic retention time resulted in the production of a higher ethanol concentration. At 0.15 hrˉ¹ dilution rate and 200 g/I feed glucose concentration, the maximum ethanol production of 51.18 g/l with the volumetric ethanol productivity of 7.68 gIˉ¹hrˉ¹ was obtained under continuous ethanol fermentation.