L. Yerushalmi

Effect of increased hydrogen partial pressure on the acetone-butanol fermentation by Clostridium acetobutylicum

SummaryElevated H2 partial pressure in the acetone-butanol fermentation increased the butanol and ethanol yields on glucose by an average of 18% and 13%, respectively, while the respective yields of acetone and of the endogenous H2 decreased by an average of 40% and 30%, and almost no effect was observed on the growth of the culture. The butanol to acetone ratio and the fraction of butanol in the total solvents were also increased with the H2 pressure. There were no major differences in the observed pattern of change with pressurization at either t=0 or t=18 h. The results demonstrate the importance of H2 partial pressure in the regulation of the C. acetobutylicum metabolism.

Variations of solvent yield in acetone-butanol fermentation

SummaryCalculation of a “theoretical yield” of microbial process is often a subject of controversy. Theoretical yield values of the solvents (butanol-acetone-ethanol) produced in the cultivation of Clostridium acetobutylicum on glucose have been calculated for 30 different culture conditions. Two different approaches were taken based on expressing the stoichiometric relationship between the substrate and the products of the process. The maximum theoretical yield under acceptable conditions was established ranging from 38.6% to 39.9%. It was considered for an ideal biosynthetic situation when no intermediate acids were left over in the system and no carbon was utilized in the production of biomass. The values of the solvent yield are dependent on the ratio between the solvent products. The coefficients of the process stoichiometric relationship and the ratios between hydrogen gas and butanol are presented for each set of process conditions. A three-dimensional plot of the yield versus the weight fractions of butanol and ethanol in the system has been developed reflecting the continuous variations of this parameter with the solvent ratio.

Culture conditions for growth and solvent biosynthesis by a modified Clostridium acetobutylicum

SummaryA modified strain of Clostridium acetobutylicum and the fermentation medium conditions for good growth of the culture and normal production of solvents are described. The pretreatment of the culture with butyric-acid-enriched medium increased the final solvent yield on sugar and lowered the residual butyric acid accumulation. In a complex medium, relatively high concentrations of yeast extract (7.5 g/l) and ammonium sulphate (3 g/l to 6 g/l) were required for normal solvent synthesis. The nitrogen requirements for cellular growth and solvent production were distinctively different. Production of solvents and growth of the culture were dependent on the concentration of para-aminobenzoic acid and relatively independent of the variations of the initial pH of the medium in the range of 4.6 to 6.3. Solvent production was obtained with initial glucose concentrations of 20.5 g/l to 70 g/l, resulting in a maximum solvent concentration of 22 g/l and a maximum yield on glucose of 32.7%.

Fermentation process diagnosis using a mathematical model

SummaryIntriguing physiology of a solvent-producing strain ofClostridium acetobutylicum led to the synthesis of a mathematical model of the acetone-butanol fermentation process. The model presented is capable of describing the process dynamics and the culture behavior during a standard and a substandard acetone-butanol fermentation. In addition to the process kinetic parameters, the model includes the culture physiological parameters, such as the cellular membrane permeability and the number of membrane sites for active tansport of sugar. Computer process simulation studies for different culture conditions used the model, and quantitatively pointed out the importance of selected culture parameters that characterize the cell membrane behaviour and play an important role in the control of solvent synthesis by the cell. The theoretical predictions by the new model were confirmed by experimental determination of the cellular membrane permeability.

Circadian rhythmicity in a fermentation process

SummaryAn idea is proposed for the role of the circadian rhythmicity in the control of the oscillatory behavior observed in the growth and product formation during the cell-retention continuous culture of Clostridium acetobutylicum. C. acetobutylicum is highly sensitive to the permeability of the cell membrane. A physical mechanism for the variability of the cytoplasmic membrane has been proposed suggesting that the performance of the cell membrane, due to its liquid crystalline structure, is influenced by the external forces (e.g. earths magnetic field). A previously developed Physiological State Model was extended by incorporating the effect of external forces on the cell membrane permeability. The new mathematical model could simulate the observed oscillatory behavior of the microbial culture. Some experimental results in support of the theoretical predictions have been presented.