High titer ethanol production from rice straw via solid-state simultaneous saccharification and fermentation by Mucor indicus at low enzyme loading

Highlights: • Sodium carbonate pretreatment and Mucor indicus have effective performance in SSSF. • The effects of the most important factors on SSSF were investigated. • Maximum ethanol concentration and yield were 99.4 g/L and 89.5% theoretical yield. • High concentration of ethanol (>40 g/L) was achieved even at low enzyme loading. • Nutritional value of residual biomass from SSSF was analyzed as an animal feed. -- Abstract: High titer ethanol production from rice straw using Mucor indicus fungus was investigated through the solid-state simultaneous saccharification and fermentation (SSSF) process. The straw was pretreated with 0.5 M sodium carbonate solution for 3, 5, and 10 h to improve the efficiency of the process. Effects of the pretreatment on the composition, structural morphology, cellulose crystallinity, swelling capacity, and buffering capacity of the straw were studied. Moreover, the effects of SSSF reaction time, enzyme loading, and solid loading on glucose and ethanol production were investigated. Additionally, the nutritional value of the residue from the SSSF process, as an animal feedstock, was determined in terms of lipid and protein contents. The highest total sugar concentration was 89.2 g/L, obtained from the straw pretreated for 10 h after hydrolysis with 10 FPU/g straw at 15% (w/w) solid loading. Total sugars concentration was not significantly improved by increasing the pretreatment time at low enzyme loadings, whereas it was significantly improved at high enzyme loadings. The highest ethanol concentration and SSSF yield were 99.4 g/L and 89.5% (71.8% based on the raw material), achieved from the straw pretreated for 10 h through the 72-h SSSF at 30% and 15% solid loading, respectively, using 20 FPU/g straw. The low enzyme loadings of 2.5 and 5 FPU/g straw yielded ethanol with concentrations as high as 66.3 and 90.9 g/L, respectively, after 120-h fermentation at 30% solid loading from the straw pretreated for 5 h.