Coproduction of hydrogen and volatile fatty acids via integrated two-step fermentation of sweet sorghum stalks by alkaline and enzymatic treatment

Coproduction of H₂ and volatile fatty acids from sweet sorghum stalks was successfully developed via two-step fermentation process involving alkaline and enzymatic treatment of the residual slurry obtained from the first step fermentation of the raw material. The optimum treatment conditions to obtain the highest yield of the end products included 2% (w/v) alkali at 120 °C for the residual slurry and 32 FPU cellulase per gram of the alkali-treated materials. The two-step fermentation using Clostridium thermosaccharolyticum increased the end product yields of H₂ (6.37 mmol/g-substrate) by 95%, acetic acid (2.33 g/L) by 97% and butyric acid (2.36 g/L) by 143% compared with those obtained in the single step fermentation without any treatment. As a result, the H₂ energy recovery efficiency of the two-step fermentation process with commercially attractive butyric acid as a coproduct reached 10.54%. These results provide a promising approach for sweet sorghum biorefining.