Agriculture Reference
In-Depth Information
Assuming that enzymatic hydrolysis of only one component of sugar beet shreds,
cellulose, to glucose requires several classes of enzymes as well as that pretreatment and
enzymatic hydrolysis steps demand
large quantities of energy, water, and chemicals
producing large quantities of wastewater that should be purified before discharging back to
the nature
, one can not expect that commercialization of cellulose to ethanol would be
sustainable. Regarding these, one of the
promising options to meet this challenge, among the
others, could be the valorization and reuse of side-streams, especially wastewaters obtained
during pretreatment of sugar beet shreds particulary those after pectin removal [53].
Reduction of wastewater treatment costs and consequent improvement of overal economy of
whole process could be accomplished by recovering pectin from the wastewater from
depectinization step by membrane separation. The obtained permeate could be further
processed
either
by
conventional
method
of
aerobic
biological
treatment
or
by
nanofiltration/reverse osmosis.
Another possibility to increase economic fisibility of bioethanol production from sugar
beet shreds is to reuse waste materials obtained after their enzymatic conversion. Analysis of
chemical composition of solid residue after enzymatic hydrolysis indicated that these
materials can be used as animal feed. This finding could considerably contribute to
profitability of bioethanol production from sugar beet shreds [54].
With the aim to completely utilize carbohydrate polymers from sugar beet shreds
differently pretreated samples were characterized and used for an enzymatic saccharification
study to analyze the amounts of fermentable monosacharides produced [11]. Enzymatic
conversion of sugar beet shreds was performed by mix of β-glucosidase, β-xylosidase, α-
arabinofuranosidase, xylanase, cellulase, arabinanase, galactanase, polygalacturonase and
pectat lyase
allowing fast and efficient hydrolysis of substrate. More than 90% of all cellulose
could be hydrolyzed within 24 hours, using lower enzyme levels than those reported in earlier
studies.
Furthermore, the synergistic action of cellulolytic and pectinolytic enzymes in release of
total monosaccharides and of glucose, arabinose and galacturonic acid was also studied.
Three cellulases, one hemicellulase and one pectinases were used separately or in binary or
ternary combinations to hydrolyse dry sugar beet shreds, and concentrations and composition
of released sugar were determined [9]. Pectinase appeared to be the most important enzyme
since by hydrolysing the pectic surface of the lignocellulosic substrate it favoured degradation
of cellulose by the respective enzymes.
If enzymatic hydrolysis is employed, the fermentation and hydrolysis stages could be
combined in a single step. In this process, known as simultaneous sacharificatin and
fermentation (SSF), fermenting organisms consume the simple sugars as they are produced by
the enzymes effectively preventing end-product inhibition of the cellulases.
Mixture of cellulases, pectinase and cellobiase was used for enzymatic conversion of
sugar beet shreds to monosaccharides which can be utilized by genetically engineered
bacteria
Escherichia coli
strain KO11,
Klebsiella oxytoca
strain P2, and
Erwinia
chrysanthemi
EC 16 pLOI 555 to produce ethanol [47]. They fermented carbohydrates from
sugar beet shreds with varying efficiencies with
E. coli
KO11 which was the most efficient
being able to convert pure galacturonic acid to ethanol with minimal acetate production.
Similar results were obtained in study of enzymatic saccharification of sugar beet shreds with
pectinolytic and cellulolytic activities supplemented with cellobiase after which
successful
fermentation to ethanol by
E. coli
KO11 was achieved
[10].
