Biomedical Engineering Reference
In-Depth Information
added value, such as xylooligosaccharides, arabinose, mannose, galactose, and so
on. The traditional
Saccharomyces cerevisiae
and
Zymomonas mobilis
can easily
convert glucose to ethanol, but they cannot convert xylose and arabinose to ethanol
by fermentation.
Pachysolen tannophilus
,
Pichia stipitis,
and
Candida shehate
can ferment xylose to ethanol. There are many disadvantages if these yeasts are
applied commercially for fuel ethanol production, such as low ethanol tolerance,
slower fermentation rate, and difficulty of oxygen control in the fermentation
process. Moreover, they are sensitive to the inhibitor produced in the lignocellulose
hydrolysis and pretreatment process. Although xylose can be converted to xylulose
by sugar isomerase, which makes it fermentable to traditional yeast, the fermenting
cost is too high. Almost no naturally occurring yeast can ferment some other
pentose, such as arabinose. Some bacteria can make use of these mixed sugars
(e.g.,
Escherichia coli
,
Klebsiella
,
Erwinia
,
Lactobacillus
,
Bacillus
spp.,
Clostridia
spp.). They can produce a small amount of ethanol, which is mixed acid and organic
solvent. Bacteria and yeast recombinant bacteria can be used to ferment cellulosic
material through metabolic engineering to produce fuel ethanol [
150
].
(2) Fermentation to produce xylitol
Based on current the research, yeast is the main natural microorganism with the
good performance to producte xylitol. Most others belong to
Candida
spp.,
Debary-
omyces
spp. and tube capsule
Saccharomyces
. Hemicellulose hydrolysate should be
detoxified first, then it can be inoculated and fermented. In the xylitol fermentation
process, the important impact factors include medium composition, inoculum size
and seed age, pH, temperature, and ventilation in addition to the nitrogen source. In
general, the growth of cell biomass positively relates to ventilation, and the xylitol
conversion rate is negatively correlated with ventilation. Under aerobic conditions,
the consumption of sugar is mainly used for cell growth. With increased ventilation,
the cell amount also increases. Xylose consumption is mainly used to generate
biomass if the growth of bacterial cells were too many. Simultaneously, increased
ventilation is conducive to consumption of inhibitory substances such as glucose
and acetic acid. Under microaerophilic conditions, most xylose converts into xylitol,
and ethanol production is small. Nakano et al. [
151
] prepared high-concentration
xylitol under microaerobic conditions by simulating and controlling with
Candida
magnolia
. The final xylitol concentration was 356 g
L
1
with batch fermentation
and strictly controlling the oxygen concentration. The theoretical yield was 82 %.
11.3.7
Research Methods for Lignocellulose Degradation
11.3.7.1
Investigation of the Lignin-Degrading Process with Model
Compound [
152
,
153
]
The structure of natural lignin is complex, which makes its degradation process
difficult to study directly. Many researchers have attempted to study the conversion
of substances with a structure similar to lignin to analyze the lignin degradation
