Enhanced ethanol production via fermentation of rice straw with hydrolysate-adapted Candida tropicalis ATCC 13803

Abstract

Neutralized hydrolysate and pretreated rice straw obtained from a 2% (w/v) sulfuric acid pretreatment were mixed at 10% (w/v) and subjected to simultaneous saccharification and co-fermentation (SSCF), with cellulase, β-glucosidase, and Candida tropicalis cells at 15 FPU/g-ds, 15 IU/g-ds and 1 × 10 9 cells/ml, respectively. A 36-h SSCF with adapted cells resulted in YP/S and ethanol volumetric productivity of 0.36 g/g and 0.57 g/l/h, respectively. In addition to ethanol, insignificant amounts of glycerol and xylitol were also produced. Adapted C. tropicalis cells produced nearly 1.6 times more ethanol than non-adapted cells. Ethanol yield (Yp/s), ethanol volumetric productivity and a xylitol concentration of 0.48 g/g, 0.33 g/l/h and 0.89 g/l, respectively, were produced from fermentation of remaining hydrolysate with adapted C. tropicalis cells. The 0.20 g/g ethanol yield and 77% production efficiency from SSCF of pretreated rice straw indicate scale-up potential for the process. This study demonstrated that C. tropicalis produced ethanol and xylitol from a mixed-sugar stream, although cell adaptation affected ethanol and xylitol yields. Scanning electron microscopy indicated agglomeration of cellulose microfibrils and globular deposition of lignin in acid-pretreated rice straw.