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 °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 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.
Highlights
The availability and low cost of cellulosic biomass has led to interest in its use as a raw material for bioenergy [1]
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
Gas production caused the pressure in the fermentation vessels to increase when C. thermocellum was inoculated into basal media with 5 g switchgrass as the substrate (Figure 1)
Summary
The availability and low cost of cellulosic biomass has led to interest in its use as a raw material for bioenergy [1]. Cellulosic biomass can be used to produce liquid fuels via fermentation. Solventogenic bacteria or fungi can ferment the sugars. Saccharification and fermentation can be done in a single step with cellulolytic, solventogenic bacteria, such as Clostridium thermocellum [2]. C. thermocellum is a thermophile, and the enzyme system retains ~25% activity at mesophilic temperatures [5]. This characteristic enables sequential culture with a non-cellulolytic mesophile. In this way, the complex, highly active cellulosome of C. thermocellum [6] can be used to liberate sugars for use by mesophilic organisms, such as C. acetobutylicum [7]. The hypotheses were: 1) C. thermocellum would liberate sugars and increase gas production by C. beijerinckii, and 2) the rate and extent of gas production would be surface area-dependent
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