Enzymatic hydrolysis of biomass at high-solids loadings through fed-batch operation

Highlights: • Fed-batch approach reduces operational problems caused by poor mixing. • Fed-batch approach enables fast adjustment of pH and temperature. • Fed-batch experiments of 5, 10, and 15% w v⁻¹ reach a holocellulose yield over 90%. • Fed-batch experiment of 20% w v⁻¹ reaches a holocellulose yield of 75%. • Fermentations from hydrolysates reach ethanol concentrations higher than 40 g L⁻¹. A fed-batch approach in stirred tank bioreactors for achieving enzymatic hydrolysis with a high load of lignocellulosic mass is described in this paper. This approach allows ease of mixing, increasing the load of a higher concentration of biomass without performance decrease, in comparison with the typical batch process, and consequently it provides a higher concentration of reducing sugars for a down-stream fermentation process; additionally, the fed-batch process times are shorter than the ones that have been reported so far. The work basically consisted of experiments in fed-batch operation with final sorghum straw loadings of 5, 10, 15 and 20% w v⁻¹, using a commercial enzyme complex, and conventional batch experiments for comparison purposes. Without typical operation problems of batch processes such as the lack of effective initial mixing and difficult adjustment of pH and temperature, hydrolysate broths up to a reducing sugars concentration of 130 g L⁻¹ were obtained. Finally, hydrolysates were fermented to verify effective bioethanol production, reaching concentrations of bioethanol of 48 g L⁻¹.