Switchgrass (<i>Panicum virgatum</i>) Fermentation by <i>Clostridium thermocellum</i> and <i>Clostridium beijerinckii</i> Sequential Culture: Effect of Feedstock Particle Size on Gas Production

Abstract

Fermentation of cellulosic biomass can be done in a single step with cellulolytic, solventogenic bacteria, such as Clostridium thermocellum . However, the suite of products is limited in consolidated bioprocessing. Fortunately, the thermophilic nature of C. thermocellum can be exploited in sequential culture. Experiments were conducted to determine the effect of feedstock particle size on fermentation by sequential cultures and to demonstrate this effect could be shown by gas production. Dual-temperature sequential cultures were conducted by first culturing with C. thermocellum (63 ° C, 48 h) before culturing with C. beijerinckii (35 ° C, 24 h). Switchgrass (2, 5 or 15 mm particle size) was the feedstock in submerged substrate (10% w/v) fermentation. The extent of fermentation was evaluated by gas production and compared by analysis of variance with Tukey’s test post hoc . C. thermocellum alone produced 78 kPa cumulative pressure (approx. 680 mL gas) when the particle size was 2 or 5 mm. The C. thermocellum cultures with 15 mm feedstock particles had a mean cumulative pressure of 15 kPa after 48 h, which was less than the 2 and 5 mm treatments ( P < 0.05). When the culture vessels were cooled (to 35 ° C) and inoculated with C. beijerinckii , and the cumulative pressures were reset to ambient, cumulative pressure values as great as 70 kPa (equivalent to an additional 670 mL gas) were produced in 24 h. Again, the longer (15 mm) particle size produced less gas ( P < 0.05). When the substrates were inoculated with C. beijerinckii without previous fermentation by C. thermocellum , the mean cumulative pressures were approximately 10 kPa. These results indicate that biological pretreatment with C. thermocellum increased the availability of switchgrass carbohydrates to C. beijerinckii , and that gas production is suitable method to show the effectiveness of a pretreatment.