Fermentation of Sweet Sorghum Syrup to Butanol in the Presence of Natural Nutrients and Inhibitors

Sweet sorghum syrups represent a renewable raw material that can be available year-round for production of biofuels and biochemicals. Sweet sorghum sugars have been used as sources for butanol production in the past but most often the studies focused on sweet sorghum juice and not on sweet sorghum syrups. Therefore, we investigated the feasibility of using the syrups as feedstock. Initial studies showed that diluted sweet sorghum syrups, to 60 g/L of glucose equivalents, could not be used as a direct replacement of a synthetic growth medium for industrial butanol-producing strain Clostridium beijerinckii. Further studies revealed that supplemental nutrients (most notably, ammonium–nitrogen) were required for successful fermentation. This was found true for two manufacturing sources of sweet sorghum syrups from commercial cultivars and hybrids. Typically, 15 g/L of total acetone, butanol, and ethanol were produced with approximately half of that being butanol. Some minor statistical difference was noted between the production potential of the two sources of syrup. However, aconitic acid, which was present at similar levels in both syrups, was ruled out as a butanol fermentation inhibitor at the fermentation pH > 4.5.