Characterization of reduced carbohydrate solubilization during Clostridium thermocellum fermentation with high switchgrass concentrations

Abstract

Biological conversion of lignocellulosic biomass requires high solids concentration for economical recovery of end products. Clostridium thermocellum fermentations were conducted at various concentrations of switchgrass. Significant reduction of carbohydrate solubilization has been observed for switchgrass with increasing solids concentration. Tests were conducted to understand the cause of inhibition at high solids concentration. Avicel hydrolysis by cell-free C. thermocellum cellulase or C. thermocellum fermentation on cellobiose, supplemented with supernatant or solids from C. thermocellum culture on switchgrass, revealed that inhibition was from the supernatant but not from the solids. Culture of C. thermocellum on Avicel, supplemented with supernatant from C. thermocellum monoculture or mixed consortia, indicated that Avicel solubilization was reduced by almost two-fold with the monoculture supernatant while no significant reduction was observed for the mixed consortia supernatant. Adding 8 kg/m3 commercial xylooligomers during C. thermocellum culture on Avicel reduced Avicel consumption up to 90%. Simultaneous consumption of xylooligomers was suggested for industrial application of C. thermocellum culture on high solids concentration.