Preparation and characterization of cellulose whiskers-reinforced silk fibroin sponge for yeast immobilization for ethanol production

Abstract

Silk fibroin (SF) is a natural protein produced by the mulberry silkworm (Bombyx mori). It was chosen as a matrix in bionanocomposite sponges for yeast cell immobilization. Cellulose whiskers (CLWs), having an aspect ratio of 80, were used as reinforcements. The bionanocomposite sponges at SF/CLWs weight ratios of 100/0, 90/10, 80/20, 70/30, 60/40 and 50/50 were fabricated by using a freeze-drying technique before being treated with an aqueous methanol solution. At any studied SF/CLWs weight ratios, Fourier Transform Infrared Spectrophotometer spectra indicated beta-sheet conformation of SF after the methanol treatment. The presence of CLWs not only increased the compression modulus but also reduced the shrinkage of the bionanocomposite sponges as well as enhanced the conformation transition of SF. The formation of beta-sheet structure of SF significantly increased water stability of the bionanocomposite sponges. The Field Emission Scanning Electron micrographs showed that the bionanocomposite sponges exhibited an interconnected porous structure, providing high surface area for immobilizing Saccharomyces cerevisiae burgundy KY l l yeast cells. The sponge with the SF/CLWs weight ratio of 50/50 showed the highest average number of yeast cell attachment at 3.1 x 1010 cells/g sponge. The maximun ethanol production of immobilized yeast cell was 65% higher when compared with free yeast cell fermentation system.