Production of Fuel Ethanol from Pentose
Monosaccharides from Polymeric Encapsulated
Saccharomyces cerevisiae and D-Xylose Isomerase
Brian Frederick
Department of Chemistry and Biomolecular Science; Clarkson University
Executive Summary:
Rising prices and increasing demands for transportation fuel have resulted in a
heightened interest in alternatives to petroleum based products. A need exists for an
alternative that is currently available at a scale large enough to make a full market impact,
until the infrastructure is in place and technology developed enough to implement better
alternatives such as hydrogen fuel cell powered cars. Ethanol is at the forefront of the
current options as it is a liquid fuel fully blendable with current gasoline supplies and
functional in modern combustion engines. In fact, it is currently being used in gasoline
supplies as an additive to lower the cost and raise the octane rating. In New York State,
ethanol blends of up to 10% are normally sold, with 85% ethanol pumps becoming more
common.
The only alteration necessary to use ethanol is to produce it at greater amounts to
make it cheaper and easily accessible to the entire transportation fleet in the United
States. Ethanol is currently produced through fermentation of monosaccharides and
starches using industrial strains of yeast. The crop most used in the US is the food crop
corn. However, a significant amount of the sugars in biomass are in cellulose and
hemicellulose, polysaccharides that are highly resistant to degradation and break down by
the yeast that are used. By using dilute acid to hydrolyze these polysaccharides, or
enzymes isolated from various other organisms, these chains can be broken down into
monosaccharides that are easily fermented. However, an additional limitation of these
yeast is that they are limited to hexose sugar fermentation. There is no biochemical
pathway available to efficient industrial yeast for the fermentation of pentose sugars.
This still leaves a significant amount of plant matter that cannot be used for producing
ethanol.
Various approaches have been taken to fermenting pentose sugars, including
genetic modification, forced adaptation, and yeast hybridization. However, each of these
have significant drawbacks. What has been explored in this study is the use of an
efficient industrial strain of yeast combined with specific enzymes, trapped in an artificial
membrane. It has been shown that in the presence of the enzyme D-xylose isomerase, the
yeast Saccharomyces cerevisiae will produce ethanol from the most naturally abundant
pentose sugar, xylose1. The high concentrations necessary and cost of enzyme make this
approach economically impractical in free solution, but by using an artificial membrane
less enzyme can be used while maintaining the desired concentration. Additionally, the
capsules are reusable, further decreasing the costs.
Encapsulation has
been used for many
Encapsulated
yeast ( ) and
enzyme ( )
applications, including
biosensing, drug delivery,
and enzyme protection. In
+ +
+ + ++
-+ ++ +
+ ++
- -- + +
+ -- ++
-
this study, polymeric
Shell
- + +++ +
- + ++ - -
D-xylose
-
D-xylulose
capsules were formed as
outlined in Figure 1. A
liquid core was produced
containing S. cerevisiae and
enzymes, done in a manner
D-glucose
Ethanol for Biofuels
Figure 1: Formation of polymeric capsules with
liquid cores through electrostatic interactions.
that has been scaled up so as to produce hundreds of consistent capsules at a time. The
fermentation capabilities of these capsules were then tested on glucose- and xylosecontaining solutions, with pH, concentration of sugar, and concentration of ethanol
tracked over time. Some optimization of shell porosity, yeast and enzyme concentrations
has taken place, including the elimination of enzyme leaching. Furthermore, the ability
of these capsules to produce ethanol from hexose and pentose sugars at appreciable
amounts has been demonstrated. Additional research will focus on further optimization
of enzyme to yeast ratios within the capsules, as well as testing the fermentation
efficiency on hydrolysate mixtures derived from biomass such as willow and maple.
Reference:
1. Gong, C.; Chen, L.; Flickinger, M.; Chiang, L.; Tsao, G. Production of ethanol from
D-xylose by using D-xylose isomerase and yeast. Appl. Env. Microbiol. 1981, 41,
430-436.