Phenolic compounds removal from sweet sorghum grain for efficient biobutanol production without nutrient supplementation

Industrial scale production of biobutanol has been hampered by substrate cost and availability. Sweet sorghum grain is an inexpensive substrate for acetone-butanol-ethanol (ABE) production by Clostridium acetobutylicum. Amylolytic activity of C. acetobutylicum eliminates the need for the hydrolysis of starchy grain prior to fermentation. However, untreated grain contains phenolic compounds, i.e. tannins, which exhibit inhibitory effects against amylolytic activity and ABE fermentation. Less than 3 g/L ABE was obtained from untreated sweet sorghum grain at different substrate concentrations. Concentration of 0.2 mM gallic acid equivalent (GAE) of sorghum tannins was detected as the critical concentration which inhibits severely ABE fermentation. Applying a multi-stage hot water treatment resulted in tannins removal and significant enhancement in total ABE production up to 18 g/L. For efficient butanol production from 40, 60, and 80 g/L sorghum grain, hot water treatment with two, five, and six stages were found to be essential for efficient butanol production, respectively. Moreover, the amylolytic activity of C. acetobutylicum was inhibited by sorghum grain tannins, more than twice as high as the effects on the ABE fermentation pathway. Furthermore, unlike most substrates, sweet sorghum grain could provide all nutrients required for ABE fermentation, eliminating the need for supplementing expensive additional nutrients.