Low Ethanol Production Research Articles

Production of ethanol by Clostridium thermosaccharolyticum: I. Effect of cell recycle and environmental parameters

The direct microbial conversion (DMC) process for the production of ethanol from lignocellulosic biomass is limited by low volumetric ethanol production rates due to the low cell densities of Clostridium thermosaccharolyticum which is a key organism for ethanol production in this process. Hence, this study focuses on the use of a continuous- culture cell recycle system to improve the volumetric ethanol productivity and yield of the fermentation of xylose by C. thermosaccharolyticum. Early experiments with the continuous-culture cell recycle system showed a two-fold improvement in volumetric ethanol productivity. However, the ethanol yield at the higher dilution rates suffered because of the large amount of lactate produced. The manipulation of two environmental parameters--iron concentration in the nutrient medium and the N(2) purge rate of the fermentor headspace--allowed a dramatic reduction in the lactate production and a simultaneous improvement in the ethanol titer and yield. Under the improved conditions of increased iron concentration (12.5 mg/L FeSO(4) x 7H(2)O) and decreased N(2) purge rate (0.1 L/min), a continuous culture of C. thermosaccharolyticum operating at a dilution rate of 0.24 h(-1) and 50% cell recycle produced 8.6 g/L ethanol and less than 1 g/L each of acetate and lactate. The volumetric ethanol productivity was 2.2 g/L/h, which is 8 times larger than obtained for a continuous culture operated with no cell recycle and the same specific growth rate.

Ethanol production in horizontal bioreactor

The rate of continuous alcohol fermentation by a mixture of free and immobilized yeast cells was found to be higher in a horizontal flow channel reactor than in a vertical column reactor under the same operational conditions. This higher fermentation rate in the horizontal reactor was attributed to accumulation of yeast cells in the reactor by free sedimentation and incomplete mixing in the direction of liquid flow. It was estimated that most of the ethanol in the horizontal bioreactor was produced by free cells in suspended or settled states. The relatively low ethanol production by the immobilized yeast cells on the ethanol production was considered due to higher product inhibition of fermentation rate within the support.

Mixed culture of Saccharomycopsis fibuliger and Zymomonas mobilis on starch-use of oxygen as a regulator

The conversion of starch to ethanol in a mixed culture of an amylolytic yeast, Saccharomycopsis fibuliger and an anaerobic bacterium, Zymomonas mobilis, was studied. Interactions between the component cultures were commensalism and competition for glucose. Control of oxygen supply to the culture was used as an external regulator of growth and competition. No accumulation of reducing sugars was observed in the mixed culture when compared to a monoculture of Saccharomycopsis fibuliger grown on starch. The glucose formed was instantly used by Zymomonas mobilis for ethanol production and the glucose inhibition of hydrolysis of non-glucose reducting sugars was released. The final concentration of ethanol, 9.7 g·l−1, produced from 30 g·l−1 of starch, shows out that all the glucose available from starch hydrolysis was converted to ethanol. Glucose production from starch was the rate-limiting reaction in the system, causing a lower ethanol production rate in the mixed culture than in the monoculture of Zymomonas mobilis, grown on glucose.