Direct conversion of cellulose to ethanol using clostridium thermocellum

Abstract

The degradation of cellulose by an anaerobic and thermophilic bacterium Clostridium thermocellum ATCC 27405, was studied. It has been found that this organism is able to convert cellulose directly to ethanol and other chemicals including sugars. The specific growth rate of this organism on Solka Floc was about 0.05 $hr^{-1}$. The final concentration of the accumulated reducing sugars was 3.4 g/liter representing a 58% accumulation of the degraded cellulose (5.84 g/liter). The ability of Cl. thermocellum to grow on glucose depends on yeast extract concentration : glucose was utilized in the medium containing 5g to 7g yeast extract per liter. When it was again transferred to cellulose medium, it retained the ability to utilize cellulose. When it was cultivated in the glucose medium containing 0.5% yeast extract, cell growth was observed after a lag of 30 hr to 35 hr and the specific growth rate was about 0.16$hr^{-1}$. The cellulolytic activity was expressed also in the glucose medium. Among the growth factors studied, biotin showed the most pronounced effect on glucose utilization and the effect of thiamine HCl was a little less than biotin. Riboflavin showed no effect. When both glucose and cellobiose were present in the medium containing 0.5% yeast extract, the culture utilizes cellobiose preferentially without an extended lag, but the glucose-adapted culture assimilated glucose preferentially without an extended lag. The glucose-adapted culture grew in the glucose medium containing 0.2% yeast extract. When Cl. thermocellum was cultivated on cellobiose, cellobiose was degraded and accumulated glucose. During active growth phase, the mole ratio of the accumulated glucose to the degraded cellobiose was less than one. But after cell growth was terminated, the mole ratio was between one and two. In case of the glucose-adapted culture, the specific growth rate of this culture on Solka Floc was about 0.155$hr^{-1}$ and 55% of cellulose was degraded. In addition...