Efficient butanol production from sweet sorghum stem juice by a co-culture system at an optimum temperature: Insights from oxidation-reduction potential monitoring and pilot scale validation

• Co-culture of Arthrobacter sp. and C. beijerinckii for butanol production at 37 °C. • Arthrobacter sp. creates anaerobic conditions, eliminating need for gas flushing. • ORP monitoring confirms anaerobic conditions and provides fermentation insights. • High butanol concentration, yield, and productivity achieved from sweet sorghum juice. • Successful scale-up to 30-L pilot-scale bioreactor for butanol production. A two-stage co-culture approach was employed in an acetone-butanol-ethanol (ABE) fermentation. An obligate aerobic bacterium, Arthrobacter sp., was first grown for 6 h at 30 °C to create anaerobic conditions. Subsequently, Clostridium beijerinckii TISTR 1461 was inoculated and a fermentation was performed at 37 °C. To identify an intermediate temperature suitable for both microorganisms, their growth was examined at 30, 34, and 37 °C. C. beijerinckii exhibited the highest specific growth rate at 37 °C, while Arthrobacter sp. displayed similar growth rates at all tested temperatures. Butanol production from a synthetic medium (P2 medium) by C. beijerinckii at different temperatures using oxygen-free nitrogen (OFN) gas flushing as a control treatment revealed that fermentation at 37 °C gave the highest butanol concentration ( P B , 9.98 g/L). Consequently, 37 °C was chosen for butanol production from sweet sorghum stem juice (SSJ) by co-culture of these two microorganisms in 1-L screw–capped bottles. Compared to the control treatment, higher P B (11.38 g/L), yield ( Y B/S , 0.37 g/g) and productivity ( Q B , 0.24 g/L·h) were achieved using the co-culture system. These results were further confirmed by monitoring the oxidation–reduction potential (ORP) during ABE fermentation in a 2-L stirred-tank bioreactor (STR). Moreover, when the co-culture fermentation at 37 °C was scaled up in a 30-L STR, the P B , Y B/S and Q B values were comparable to those obtained in the 2-L STR. Therefore, co-culture fermentation of Arthrobacter sp. and C. beijerinckii TISTR 1461 at 37 °C represents a promising method for large-scale butanol production.