310 4 Carbohydrates
4.4.4.7 Gum Tragacanth
4.4.4.7.1 Occurrence.............................................
Gum tragacanth is a plant exudate collected from
Astragalusspecies shrubs grown in the Middle
East (Iran, Syria, Turkey).
4.4.4.7.2 Structure,Properties.....................................
Gum tragacanth consists of a water-soluble
fraction, the so-called tragacanthic acid, and the
insoluble swelling component, bassorin. Tra-
gacanthic acid contains 43% ofD-galacturonic
acid, 40% ofD-xylose, 10% ofL-fucose, and
4% ofD-galactose. Like pectin, it is composed
of a main polygalacturonic acid chain which
bears side chains made of the remaining sugar
Fig. 4.22.The effect of shear rate on viscosity of aque-
ous tragacanth solutions.aFlake form tragacanth, 1%;
btragacanth, ribbon form, 0.5% (according toWhistler,
1973)
residues (Formula 4.142). Bassorin consists
of 75% of L-arabinose, 12% of D-galactose,
3% of D-galacturonic acid methyl ester, and
L-rhamnose.
Its molecular weight is about 840 kdal. The
molecules are highyl elongated (450× 1 .9nm)in
aqueous solution and are responsible for the high
viscosity of the solution (Table 4.21). As shown
in Fig. 4.22, the viscosity is highly dependent on
shear rate.4.4.4.7.3 Utilization
Gum tragacanth is used as a thickening agent and
a stabilizer in salad dressings (0.4–1.2%) and in
fillings and icings in baked goods. As an additive
in ice creams (0.5%), it provides a soft texture.4.4.4.8 KarayaGum
4.4.4.8.1 Occurrence.............................................
Karaya gum, also called Indian tragacanth, is an
exudate from an Indian tree of theSterculia ureus
and otherSterculiaspecies.4.4.4.8.2 Structure,Properties.....................................
The building blocks areD-galactose,L-rhamnose,
D-galacturonic acid, andL-glucuronic acid. The
sugars are partially acetylated (13% acetyl groups
based on dry weight). The molecule consists of
three main chains which are polymers of different
disaccharide units (cf. Formula 4.143). The main
chains carry side chains and are also covalently
linked via the side chains.(4.142)