Biomedical Engineering Reference
In-Depth Information
Scheme 7.13
Chemical structure of xanthan.
accumulates, the broth viscosity increases dramatically and the broth rheology
presents serious problems to mixing, heat transfer, and oxygen input. This limit-
ing factor of the process effi ciency can be overcome by water-in-oil cultivation
technologies, where the xanthan produced stays within the aqueous droplets dis-
persed in an immiscible organic phase (e.g., vegetable oil or
n
- hexadecane) and
the effective viscosity of the overall system is kept low. Xanthan concentrations of
more than 200 g/L culture medium can be reached by this approach [194].
Xanthan is known as a highly stable polysaccharide, which is not easily degraded
by microorganisms. Still some microorganisms such as
Bacillus
sp. 13 - 4,
Paeni-
bacillus alginolyticus
XL - 1,
Cellulomonas
sp. LX,
Microbacterium
sp. XT11, and
Paenibacillus
sp. XD were reported to participate in depolymerization of xanthan
[195] . The enzymatic basis of xanthan degradation was studied by Nankai
et al.
who analyzed the structures of xanthan depolymerization product by electrospray
mass spectrometry and identifi ed the enzymes involved in the process. In
Bacillus
sp., GL1 Xanthan is depolymerized to constituent monosaccharides by two extra-
cellular and three intracellular enzymes. In the initial step, the terminal pyruvate-
substituted mannose is cleaved by a xanthan lyase and the polymeric backbone is
subsequently depolymerized by a
- d - glucanase (endoxanthanase) [196] . Recently,
a novel endoxanthanase catalyzing the hydrolysis of the main chain of xanthan
with an intact side chain was isolated from
Microbacterium
sp. strain XT11 and
characterized. This enzyme may be used in the treatment of
Xanthomonas
infec-
tious disease or for the biodegradation of xanthan injected into underground
oil [197] .
Xanthan is easily soluble in cold and hot water. The viscosity of the xanthan
solution is nearly independent of temperatures up to 90 °C and of the pH value.
Also higher salt concentrations have no big infl uence on the viscosity. The polymer
has relatively good stability during sterilization and the solutions are very stable
toward chemical, enzymatic, or bacterial degradation allowing a wide range of
applications. Also xanthan can be crosslinked, gelled, and associated with other
β
