Biomedical Engineering Reference
In-Depth Information
7.7.1
Production Strains and Their Transformation
Xanthomonas
spp. are used in xanthan gum production.
Xanthomonas campestr
is a natural bacteria originally isolated from turnip. This bacterium generates
polysaccharide with the same chemical composition in the medium of cabbage
extract and artificially made medium. Therefore,
X. campestr
is an excellent strain
for industrial production of xanthan gum. In addition,
X. phaseoli
,
X. malvacearum,
and
X. carotae
can be used as fermentation strains [
107
]. The xanthan gum
production process is shown in Fig.
7.4
.
The strain has critical effects on the yield and quality of products. It was
reported that gene recombinant technology was used in the sequencing, separation,
screening, and expression of gum-producing genes; however, recombinant strains
for industrialization have not yet been reported. If the gum-producing genes can be
isolated by genetic engineering and then imported into complete new strains to make
the strain show double or more production genes and be stably inherited, xanthan
gum production would be increased exponentially.
Xanthan gum can be produced in batch fermentation, semibatch fermentation,
or continuous fermentation [
109
-
111
]. In industrial production, aeration-agitation
fermentors by deep fermentation in batch culture are usually applied. The actual
processes include seed fermentation, seed expansion culture, and fermentation. The
type that should be selected for a specific fermentation process is determined by the
metabolic characteristics of the microorganism used and the optimum conditions.
In batch fermentation, the concentration of substrate will decrease gradually. In
continuous fermentation, the concentration of substrate throughout the fermentation
process remains substantially unchanged, and lack of substrate does not appear
under normal circumstances, which is beneficial to cell growth. On the other hand,
the substrate may lead to inhibition, leading to decreased final yield. In semibatch
fermentation, substances can be added according to the characteristics of different
growth stages of cell growth and to create an optimal physiological environment
for cell growth. Many studies have shown that semibatch fermentation is better for
xanthan gum fermentation [
111
].
Because of the viscosity of the xanthan gum solution and the large amount
of oxygen needed in the fermentation process, some reactors are not suitable for
production. An air-lift reactor can effectively solve the problem of heat caused
by agitation and aeration in xanthan gum fermentation and can reduce energy
consumption; thus, it is suitable for continuous fermentation. A tower reactor
equipped with a fluidized bed was applied in batch fermentation for its high bubble
transfer coefficient. A pump-type statically mixing and recycling reactor is able
to increase the transfer rate of the oxygen in the fermentation broth with high
viscosity, which can improve the production of xanthan gum. Because of the special
characteristics of the xanthan gum fermentation broth, it is difficult for ordinary
mixers for pulps such as a disk turbine agitator and a blade turbine-type impeller
to meet the requirements, although their energy consumption is smaller. In recent
years, to meet the demands of the high viscosity and high ventilation rate of the
