Increasing Ethanol Titer and Reducing Enzyme Dosage via Fed-Batch, Simultaneous Saccharification and Fermentation in a High Solids Bioreactor

Abstract- Fed-batch, solid state, simultaneous saccharification and fermentation (SSF) was evaluated as an approach to reduce enzyme use in converting cellulose to ethanol, and to maximize ethanol titer. Kraft pulp, an intermediate in paper production, was used to represent a fractionated cellulose feedstock. Following a literature survey, average dosages were determined as 34 FPU of cellulase (Celluclast 1.5L) and 135 CBU o f β -glucosidase (Novozyme 188) per gram glucan, and were set as 100% dosages. Initially, submerged fed-batch SSF trials were conducted in a traditional bioreactor using enzyme dosages of 17, 33, 67, and 133%, with a final solids loading rate (SLR) of 14%. Ethanol production was similar (77.3-83.4% of theoretical yield) for trials with 33-133% enzyme dosages, but fell to 36% of theoretical at 17% enzyme dosage. Fed-batch saccharification and fed-batch SSF were then performed in a solid state bioreactor, achieving a 34.8% SLR. This reduced the initial 133% enzyme dosage to 19%. In saccharification trials the glucose yield was only 35% of theoretical (103.6 g/L), due to feedback inhibition of enzymes. Companion SSF trials achieved an ethanol yield of only 20% of theoretical (30.1 g/L ethanol). While some ethanol was lost due to evaporation, yeast inhibition by low water activity was presumed to be the primary limitation. Performance at lower solid loading rates were evaluated, and even diluted the fermented slurry and conducted a secondary fermentation. These trials suggest that enzyme inactivation via irreversible binding may be the primary limitation, instead of low water activity or nutrient limitation.