84
1
General Principles
structure of the polysaccharide portion has a main chain of D-galactopyranose residues linked
β-(1→3) and D-glucopyranosyl uronic acid linked β-(1→6). The main chains have branch
chains of two to five residues, consisting of α-L-arabinofuranosyl, α-L-rhamnopyranosyl,
β-D-glucopyranosyl uronic acid, and 4-O-methyl-β-D-glucopyranosyl uronic acid [101]. The
latter two uronic acids occur most frequently at terminal ends of the branched chains.
Another plant exudate is gum ghatti or Indian gum that can be obtained from a large tree grown
in the deciduous forests of India and Sri Lanka. Gum ghatti is composed of L-arabinofuranose,
D-galactopyranose, D-mannopyranose, D-glucopyranose uronic acid, and D-xylopyranose in
approximately the molar ratios of 10:6:2:2:1 [102]. A third exudate gum is gum tragacanth
that is primarily obtained from trees growing in Iran, Syria, and Turkey. It is a highly branched
arabinogalactan with α-D-xylopyranose and α-L-fucopyranose branch residues [103]. These
gums are primarily used to increase viscosity, provide body, stabilize emulsions, and suspend
other materials and have been used for thousands of years in confectioneries, cosmetics, textiles, coatings, paints, pastes, and polishes.
10 Occurrence of Carbohydrates in Bacterial Polysaccharides
A large number of bacterial polysaccharides are known [104]. The major structural component of the bacterial cell wall is a polysaccharide, known as murein and composed of a repeating unit of one N-acetyl-D-glucosamine and an O-lactyl substituted N-acetyl-D-glucosamine
(N-acetyl-D-muramic acid) see > Sect. 7.3.
10.1 Xanthan, a Water-Soluble Bacterial Polysaccharide
In the 1950s, the US Department of Agriculture’s Northern Regional Research Laboratories in Peoria, Illinois screened bacterial cultures to obtain a replacement for the plant exudates, which had become rare and expensive. They found that Xanthomonas campestris, when
grown on D-glucose in an aerobic submerged fermentation, produces xanthan, a water-soluble
polysaccharide gum [105,106]. It has a cellulose backbone of β-(1→4) linked D-glucopyranose residues with a trisaccharide of D-mannopyranose linked β-(1→4) to D-glucopyranosyl uronic acid linked β-(1→2) to a D-mannopyranosyl [β-D-Man p-(1→3)-β-D-Glc pUA(1→2)-α-D-Man p-(1→3)-] attached to every other D-glucose residue in the cellulose chain by
an α-(1→3) linkage [107,108]. Some of the nonreducing terminal D-mannopyranose residues
of the trisaccharide have a cyclic six-membered pyruvic acid ketal attached to C4 and C6, and
some of the inner D-mannopyranose units are acetylated at C6 [109].
The branching of the cellulose chain by the trisaccharide makes the otherwise insoluble cellulose molecule water-soluble. At low concentrations, xanthan produces high viscosities at low
temperatures. These properties provide a number of uses as a thickener and bulking agent for
prepared foods, such as salad dressings, syrups, toppings, relishes, ice cream, and baked goods.
It is also used as a carrier and emulsifying agent in cosmetics and pharmaceuticals [110].
There are other bacterial gel polysaccharides with different properties, composed of D-glucopyranose, L-rhamnopyranose, D-glucuronic acid, and L-mannopyranose that are obtained
from Pseudomonas elodea (syn. Sphingomonas elodea), which produces gellan, and also
species of Alcaligenes that produce welan and rhamsan [111].
1
General Principles
structure of the polysaccharide portion has a main chain of D-galactopyranose residues linked
β-(1→3) and D-glucopyranosyl uronic acid linked β-(1→6). The main chains have branch
chains of two to five residues, consisting of α-L-arabinofuranosyl, α-L-rhamnopyranosyl,
β-D-glucopyranosyl uronic acid, and 4-O-methyl-β-D-glucopyranosyl uronic acid [101]. The
latter two uronic acids occur most frequently at terminal ends of the branched chains.
Another plant exudate is gum ghatti or Indian gum that can be obtained from a large tree grown
in the deciduous forests of India and Sri Lanka. Gum ghatti is composed of L-arabinofuranose,
D-galactopyranose, D-mannopyranose, D-glucopyranose uronic acid, and D-xylopyranose in
approximately the molar ratios of 10:6:2:2:1 [102]. A third exudate gum is gum tragacanth
that is primarily obtained from trees growing in Iran, Syria, and Turkey. It is a highly branched
arabinogalactan with α-D-xylopyranose and α-L-fucopyranose branch residues [103]. These
gums are primarily used to increase viscosity, provide body, stabilize emulsions, and suspend
other materials and have been used for thousands of years in confectioneries, cosmetics, textiles, coatings, paints, pastes, and polishes.
10 Occurrence of Carbohydrates in Bacterial Polysaccharides
A large number of bacterial polysaccharides are known [104]. The major structural component of the bacterial cell wall is a polysaccharide, known as murein and composed of a repeating unit of one N-acetyl-D-glucosamine and an O-lactyl substituted N-acetyl-D-glucosamine
(N-acetyl-D-muramic acid) see > Sect. 7.3.
10.1 Xanthan, a Water-Soluble Bacterial Polysaccharide
In the 1950s, the US Department of Agriculture’s Northern Regional Research Laboratories in Peoria, Illinois screened bacterial cultures to obtain a replacement for the plant exudates, which had become rare and expensive. They found that Xanthomonas campestris, when
grown on D-glucose in an aerobic submerged fermentation, produces xanthan, a water-soluble
polysaccharide gum [105,106]. It has a cellulose backbone of β-(1→4) linked D-glucopyranose residues with a trisaccharide of D-mannopyranose linked β-(1→4) to D-glucopyranosyl uronic acid linked β-(1→2) to a D-mannopyranosyl [β-D-Man p-(1→3)-β-D-Glc pUA(1→2)-α-D-Man p-(1→3)-] attached to every other D-glucose residue in the cellulose chain by
an α-(1→3) linkage [107,108]. Some of the nonreducing terminal D-mannopyranose residues
of the trisaccharide have a cyclic six-membered pyruvic acid ketal attached to C4 and C6, and
some of the inner D-mannopyranose units are acetylated at C6 [109].
The branching of the cellulose chain by the trisaccharide makes the otherwise insoluble cellulose molecule water-soluble. At low concentrations, xanthan produces high viscosities at low
temperatures. These properties provide a number of uses as a thickener and bulking agent for
prepared foods, such as salad dressings, syrups, toppings, relishes, ice cream, and baked goods.
It is also used as a carrier and emulsifying agent in cosmetics and pharmaceuticals [110].
There are other bacterial gel polysaccharides with different properties, composed of D-glucopyranose, L-rhamnopyranose, D-glucuronic acid, and L-mannopyranose that are obtained
from Pseudomonas elodea (syn. Sphingomonas elodea), which produces gellan, and also
species of Alcaligenes that produce welan and rhamsan [111].