2009_Book_FoodChemistry

4.4 Polysaccharides 331
4.4.4.19 Xanthan Gum
4.4.4.19.1 Occurrence, Isolation
Xanthan gum, the extracellular polysaccharide
from Xanthomonas campestris and 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-Manp as 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
>10 6 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.
Fig. 4.41. Viscosity of aqueous xanthan gum solution
as affected by shear rate (according to Whistler, 1973).
Viscometer: Brookfield model LVF
suspending abilities (turbidity problems, essential
oil emulsions in beverages). Due to its high thermal
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
Sclerotium species, e. g., S. glucanicum, produce
scleroglucan on a nutritive medium of glucose,
nitrate as N-source and minerals. The polysaccharide
is recovered from the nutritive medium by alcoholic
precipitation.
4.4.4.19.3 Utilization
The practical importance of xanthan gum is
based on its emulsion-stabilizing and particle-
4.4.4.20.2 Structure, Properties
The “backbone” of scleroglucan is a β-1,3-glucan
chain that, on an average, has an attached glucose