Environmental Engineering Reference
In-Depth Information
2.4. Fermentation Technologies
While fermentation of glucose into ethanol is a well-understood process that occurs
widely among microorganisms, the fermentation of pentoses such as xylose, which are
abundant in biomass, has posed significant challenges. Recently, however, this challenge has
been addressed by creating recombinant yeasts and bacteria [21, 22], although the solution
may not yet have been fully optimized.
A second important aspect limiting commercialization is the fermentation efficiency of
the microorganisms: in typical fermentation pathways for glucose and xylose to ethanol, one
contributor to the high cost of ethanol production is the loss of half of the fixed carbon to
products other than ethanol [5].
The ideal bioethanol-fermenting microorganism would therefore readily ferment all
biomass sugars, resist toxic effects of aromatic lignin subunits and other inhibitory
byproducts such as acetate, be thermostable and acid-tolerant, and produce a highly active
cellulase multienzyme complex [54].
2.4.1. Pentose fermentation.
As mentioned above, Escherichia, Klebsiella, and
Zymomonas have now been engineered to ferment not only glucose but also xylose and
arabinose sugars [5, 54, 55]. Some of these are already experiencing commercial use as well:
BC International Corporation (www.bcintl-corp.com) is using genetically engineered
Escherichia coli to produce ethanol from biomass sugars, and Arkenol Inc.
(www.arkenol.com) is using Zymomonas in its concentrated-acid process.
In another example, Zymomonas mobilis has been transformed with Escherichia coli
xylose isomerase, xylulokinase, transaldolase, and transketolase genes. Expression of the
added genes are under the control of Zymomonas mobilis promoters. This genetically
modified microorganism, patented by the Midwest Research Institute, is now able to ferment
mixtures of xylose, arabinose, and glucose to produce ethanol [56, 57].
2.4.2. Combined cellulolysis and fermentation
. Consolidated bioprocessing (CBP), in
which the production of cellulolytic enzymes, hydrolysis of biomass, and fermentation of
resulting sugars to desired products occur in one step, is currently envisioned as the most
promising and eminently achievable path toward optimally efficient bioethanol production
[54]. Efforts to develop such a culture through engineering fermentative capacity into
cellulolytic organisms, as well as the alternative, engineering cellulolytic capacity into
fermentative organisms, are both underway and have been reviewed extensively [54].
In one example, Ingram and colleagues cloned two Erwinia endoglucanase genes into an
ethanol-producing Klebsiella species, producing a new microbe that produced up to 22
percent more ethanol when fermenting crystalline cellulose synergistically with added fungal
cellulases [58]. Cellulase genes have also been introduced into Lactobacillus, although not
necessarily for biomass utilization [59], and cellobiose utilization capability has been
engineered into Saccharomyces cerevisiae [60]. In an alternative example, with the additional
goal of offering improved relief from product inhibition in SSF, in which cellobiose
inhibition of exoglucanase is problematic, ethanol-producing genes have been successfully
introduced into native cellobioseutilizing bacteria [61, 62].
2.4.3. Synergistic co-cultures
. In experiments involving the cellobio se-fermenting
recombinant, Klebsiella oxytoca P2, in co-cultures with ethanol-tolerant strains of
Saccharomyces pastorianus, Kluyveromyces marxianus, and Zymomonas mobilis, the
combinations produced more ethanol, more rapidly, than any of the constituent strains. This
