Agriculture Reference
In-Depth Information
E. aerogenes
to yield 4-vinnyl-guaiacol, a high-value additive for food and beverages and
medicinal applications [39]. Also, microbial conversion of the same substrate to natural
vanillin was conducted by simultaneous action of
Aspergillus niger
and
Pycnoporus
cinnabarinus
[40].
Moreover, sugar beet shreds appeared to be a suitable cost-effective supplemental
substrate for bacterial conversion by
Xanthomonas campesytris
in order to produce xanthan
gum [41].
3.
E
NZYMATIC
C
ONVERSION OF
S
UGAR
B
EET
S
HREDS
3.1. Bioethanol
World energy crisis encourages the development of alternative energy sources. Among
others, as a promising direction seems to be the production of bioethanol from different raw
materials.
Lignocellulosic biomass is readily available
and has no competing value as food;
however, there are no known organisms that can rapidly and efficiently convert all biomass
sugars into ethanol with high productivities.
For fermentation, the cell-wall material needs to
be degraded into fermentable monosaccharides. If degradation is performed by enzymes,
before this lignocellulosic feedstocks are often structurally modified by different
pretreatments [42]. Actually, the presence of hemicellulose and lignin along with crystalline
nature of cellulose makes lignocellulosic biomass recalcitrant for enzymatic hydrolysis and
represents one of the major problems to be solved in the bioethanol production from this
source [43]. Moreover, lignin causes unproductive binding of cellulolytic enzymes resulting
in reduction of hydrolysis rate and increase in enzymes consumption [44].
Enzymatic
processes are currently expensive, but they can operate with high yields and generate few
inhibitory side products.
As it was already mentioned, sugar beet shreds contain significant amount of
carbohydrates and insignificant amounts of lignin.
The low dry matter content makes
combustion of wet sugar beet shreds for heat and power production unfavorable although
their pyrolysis into solid char and bio-oil [45] and co-pyrolysis with lignite [46] were
investigated. On the other side, their low lignin and high sugar content make them an
interesting candidate for bioethanol production.
Enzymatic hydrolysis of pectin-rich
processing residue such as sugar beet shreds appears to be much more amenable to enzymatic
degradation than lignified cellulosic substrates because they do not require chemo-mechanical
treatments for effecient enzymatic hydrolysis as other lignocellulosic substrates [47]. In
addition, using pectin-rich residues from industrial processes is beneficial because the
material is already collected and partially pretreated to some extent to facilitate enzymatic
deconstruction of the plant cell wall polysaccharides [48].
It is essential that during the planning of enzymatic degradation of lignocellulosic waste
methabolic capabilities of ethanol producing microorganism are taken into consideration.
Fermentations of pectin rich materials such as sugar beet shreds have been conducted with a
variety of ethanologens, including yeasts and bacteria.
Escherichia coli
can ferment a wide
range of sugars including galacturonic acid, the primary component of pectin. However, the
mixed acid metabolism of
E. coli
can produce unwanted side products.
Saccharomyces
