Food Chemistry

4.4 Polysaccharides 305

water extraction under mild alkaline conditions,
followed by drying or isolate precipitation.

4.4.4.3.2 Structure,Properties.....................................

Carrageenans are a complex mixture of various
polysaccharides. They can be separated by
fractional precipitation with potassium ions.
Table 4.20 compiles data on these fractions
and their monosaccharide constituents. Two
major fractions areκ(gelling and K+-insoluble
fraction) andλ(nongelling, K+-soluble).

Table 4.20.Building blocks of carrageenans

Carrageenan Monosaccharide building block

κ-Carrageenan D-Galactose-4-sulfate,

3,6-anhydro-D-galactose-

2-sulfate

κ-Carrageenan D-Galactose-4-sulfate,

3,6-anhydro-D-galactose

λ-Carrageenan D-Galactose-2-sulfate,

D-galactose-2,6-disulfate

μ-Carrageenan D-Galactose-4-sulfate,

D-galactose-6-sulfate,

3,6-anhydro-D-galactose

ν-Carrageenan D-Galactose-4-sulfate,

D-galactose-2,6-disulfate,

3,6-anhydro-D-galactose

Furcellaran D-Galactose-D-galactose-

2-sulfate,D-galactose-4-sulfate,

D-galactose-6-sulfate,

3,6-anhydro-D-galactose

κ-Carrageenan is composed ofD-galactose, 3,6-
anhydro-D-galactose and ester-bound sulfate in
a molar ratio of 6:5:7. The galactose residues are
essentially fully sulfated in position 4, whereas
the anhydrogalactose residues can be sulfated
in position 2 or substituted byα-D-galactose-6-
sulfate or -2,6-disulfate. A typical sequence of
κ-(orι-)carrgeenan is:

(4.138)

The sequence favors the formation of a dou-

blestranded helix (Fig. 4.19). λ-Carrageenan

contains as the basic building block β-D-

Galp-( 1 → 4 )-α-D-Galp (cf. Formula 4.139),

which is joined through a 1,3-glycosidic linkage

to the polymer. Position 6 of the second galactose

residue is esterified with sulfuric acid as is ca.

70% of position 2 of both residues. The high

sulfate content favors the formation of a zigzag

ribbon-shaped conformation.

(4.139)

The molecular weights ofκ-andλ-carrageenans

are 200–800 kdal. The watersolubility increases

Fig. 4.19.ι-Carrageenan conformation.aDouble helix,

bsingle coil is presented to clarify the conformation

(according toRees, 1977)