Food Chemistry

4.4 Polysaccharides 331

4.4.4.19 XanthanGum

4.4.4.19.1 Occurrence,Isolation

Xanthan gum, the extracellular polysaccharide
fromXanthomonas campestrisand some related
microorganisms, is produced on a nutritive
medium containing glucose, NH 4 Cl, a mixture of
amino acids, and minerals. The polysaccharide
is recovered from the medium by isopropanol
precipitation in the presence of KCl.

4.4.4.19.2 Structure,Properties.....................................

Xanthan gum can be regarded as a cellulose
derivative. The main chain consists of 1,4 linked
β-glucopyranose residues. On an average, every
second glucose residue bears in the 3-position
a trisaccharide of the structure β-D-Manp-
(1→4)-β-D-GlcpA(1→2)-α-D-Manpas the
side chain. The mannose bound to the main
chain is acetylated in position 6 and ca. 50% of
the terminal mannose residues occur ketalized
with pyruvate as 4,6-O-(1-carboxyethylidene)-
D-mannopyranose (cf. Formula 4.162; GlcpA:
glucuronic acid).

(4.162)

The molecular weight of xanthan gum is

106 dal. In spite of this weight, it is quite
soluble in water. The highly viscous solution
exhibits a pseudoplastic behavior (Fig. 4.41).
The viscosity is to a great extent, independent
of temperature. Solutions, emulsions and gels,
in the presence of xanthan gums, acquire a high
freeze–thaw stability.

4.4.4.19.3 Utilization

The practical importance of xanthan gum is
based on its emulsion-stabilizing and particle-

Fig. 4.41.Viscosity of aqueous xanthan gum solution

as affected by shear rate (according toWhistler, 1973).

Viscometer: Brookfield model LVF

suspending abilities (turbidity problems, essential

oil emulsions in beverages). Due to its high ther-

mal stability, it is useful as a thickening agent in

food canning. Xanthan gum addition to starch

gels substantially improves their freeze-thaw

stability.

Xanthan gum properties might also be utilized

in instant puddings: a mixture of locust bean

flour, Na-pyrophosphate and milk powder with

xanthan gum as an additive provides instant jelly

after reconstitution in water. The pseudoplastic

thixotropic properties, due to intermolecular

association of single-stranded xanthan gum

molecules, are of interest in the production

of salad dressings, i. e. a high viscosity in the

absence of a shear force and a drop in viscosity

to a fluid state under a shear force.

4.4.4.20 Scleroglucan

4.4.4.20.1 Occurrence, Isolation

Sclerotiumspecies, e. g.,S. glucanicum, produce

scleroglucan on a nutritive medium of glucose,

nitrate as N-source and minerals. The polysaccha-

ride is recovered from the nutritive medium by al-

coholic precipitation.

4.4.4.20.2 Structure,Properties.....................................

The “backbone” of scleroglucan is aβ-1,3-glucan

chain that, on an average, has an attached glucose