Solid feeding and co-culture strategies for an efficient enzymatic hydrolysis and ethanol production from sugarcane bagasse

The feasibility of second-generation (2G) processes still highly depends on improvements in the hydrolysis and fermentation steps, ensuring the whole use of biomass sugars in an integrated approach. The main bottlenecks are the differences between the optimal conditions for both steps, limitations for high solid load (HSL) hydrolysis, and hexoses and pentose co-fermentation. To overcome these problems, the present work explored a feeding strategy to improve HSL enzymatic hydrolysis of hydrothermally pretreated sugarcane bagasse (PSB), at optimal and suboptimal temperatures, further integrated with a non-recombinant yeast co-culture (Saccharomyces cerevisiae and Kluyveromyces marxianus) for sugar co-fermentation. The entire PSB fraction was used without conditioning. Efficient enzymatic hydrolysis was achieved using a feeding strategy in a helical stirred bioreactor (64 and 75% cellulose conversion with 22.5% of solids, 72 h at 35 and 50 °C, respectively), obtaining hydrolysates with high glucose and xylose concentrations (100 g/L). Simultaneous saccharification and co-fermentation (SSCF) provided high glucose (98%) and xylose (52%) conversion in 24 deep-well plates, with ethanol yield of 0.23 getOH/gS, but faced problems of cell viability loss in the bioreactor. Sequential SCF (SqSCF) co-culture stood out as a promising strategy for the use of biomass sugars. The results of this study provide significant contribution to the intensification and feasibility of 2G production, in addition to highlighting the difficulties faced for process integration aiming at the full conversion of sugars coming from high solid load enzymatic hydrolysis.