Bioconversion of sugarcane bagasse and dry spent yeast to ethanol through a sequential process consisting of solid-state fermentation, hydrolysis, and submerged fermentation

Sugarcane bagasse (SB), a by-product from the sugar industry, is considered a promising feedstock for ethanol production, owing to its high cellulose content and its ready availability. In this study, chemically pretreated SB was mixed with dry spent yeast (DSY) at a mass ratio of 60:40 before being fermented under solid-state fermentation (SSF) by Aspergillus niger TK1. The fermented solids at a concentration of 24 g/L (on a dry basis) was subsequently hydrolyzed to produce a hydrolysate, which was concentrated to contain 29.7 +/- 0.6 g/L of reducing sugar and 585.1 +/- 6.5 mg/L of free amino nitrogen. The concentrated hydrolysate was fermented by Saccharomyces cerevisiae TISTR 5339, yielding 8.98 +/- 0.2 g/L of ethanol. An ethanol yield on reducing sugar was 0.46 g/g, which was 90.33% of the theoretical yield. Based on the results, a bioconversion process consisting of SSF followed by hydrolysis of fermented solids and submerged ethanol fermentation is proposed. In this process, ethanol yield on SB-DSY mixed substrates was 0.08 g/g.