Environmentally Sustainable Biofuels – The Case for Biodiesel, Biobutanol and Cellulosic Ethanol - Sustainable Biotechnology: Sources of Renewable Energy - page 57

CoA to acetate forming acetyl-CoA. Through a series of metabolic reactions,
butyryl-CoA is produced from acetyl-CoA, which is then converted to butanol in
the solventogenic pathway [33]. Acetyl-CoA can also produce ethanol and acetone
from acetoacetyl-CoA. A typical process produces acetone, butanol and ethanol in
the ratio 3: 6:1.
The butanol yield from the ABE fermentation of glucose is relatively low, about
15-25 wt% typically [33]. This is due to the buildup of acetic, butyric, and propionic
acids along with the products acetone, butanol, and ethanol, during the fermenta-
tion process. The solvents are toxic to C. acetobutylicum . The butanol destabilizes
the cell membrane of the microorganisms ultimately resulting in cell death. Higher
yields can be achieved by continuously removing the harmful solvents, mainly
butanol, and/or by genetically modifying strains of microorganisms that can tolerate
higher concentrations of butanol [33].
A butanol-tolerant mutant strain of C. acetobutylicum has been developed and
designated as SA-1 [34]. This strain shows a 121% improvement in butanol tol-
erance over the typical strain used in ABE fermentation. The enhancement of the
strain results in an overall increase in butanol production of 13.2%. Additional
advantages of the mutated strain are an increase in growth rate, more pH resistance,
more effective utilization of carbohydrates, and reduction in acetone concentration
by 12.5-40% [34]. Other studies using genetic and metabolic engineering have
modified strains, which have resulted in an increase of about 320% in the final
butanol concentration [35]. The antisense RNA process helps down-regulate genes
for butyrate formation by acidogenesis and increases the butanol yield through
solventogenesis. The process has resulted in strains with butanol yields of 35% [36].
Tetravitae Bioscience has combined a patented mutant strain of C. beijerinckii
and a continuous, integrated fermentation process that utilizes gas stripping. C. bei-
jerinckii is a species of rod-shaped anaerobic bacteria that is known for the synthesis
of organic solvents, and uses a broader substrate range and better pH range than
C. acetobutylicum . The solvent genes of C. beijerinckii are located on the chromo-
some, which is more genetically stable than on the plasmid for C. acetobutylicum .
The gas stripping process prevents the butanol concentrations from reaching toxic
levels by sparging oxygen-free nitrogen or fermentation gases through the fermen-
tation solution and the ABE captured in the gas are condensed [13]. The exhaust gas
is then recycled back to the reactor to collect more ABE for removal. Advantages
of this method are the low energy requirements, the fact that it does not remove
important acid intermediates, and that it allows for efficient recovery of butanol [37].
Environmental Energy Inc. (EEI) and Ohio State University (OSU) have devel-
oped a two-step anaerobic fermentation process in a joint project to produce butanol
from biomass. The first process converts the feedstock carbohydrates into butyric
acid through acidogenesis using C. tyrobutyricum . The second step converts the
butyric acid, using C. acetobutylicum , into butanol, which results in a significant
improvement from conventional processes. The butanol solution requires purifi-
cation from a recovery unit after the second step reactor. EEI's process uses
a purification process that takes advantage of the azeotrope formed by butanol