304 4 Carbohydrates
be adjusted as desired. Freezing and thawing of
a Na-alginate solution containing Ca^2 +ions can
result in a further rise in viscosity. The curves
in Fig. 4.17 show the effect on viscosity of the
concentrations of three alginate preparations:
low, moderate and high viscosity types. These
data reveal that a 1% solution, depending on
the type of alginate, can have a viscosity range
of 20–2000 cps. The viscosity is unaffected in
a pH range of 4.5–10. It rises at a pH below 4.5,
reaching a maximum at pH 3–3.5.
Gels, fibers or films are formed by adding Ca^2 +or
acids to Na-alginate solutions. A slow reaction is
Fig. 4.17.Viscosity of aqueous alginate solutions. Al-
ginate with (a) high, (b) medium, and (c) low viscosity
Fig. 4.18.Schematic representation of a calcium algi-
nate gel (cross-linkage by egg box formation, cf. For-
mula 4.120; according toFranz, 1991)
needed for uniform gel formation. It is achieved
by a mixture of Na-alginate, calcium phosphate
and glucono-δ-lactone, or by a mixture of Na-
alginate and calcium sulfate.
Depending on the concentration of calcium ions,
the gels are either thermoreversible (low concen-
tration) or not (high concentration). Figure 4.18
shows a schematic section of a calcium alginate
gel.4.4.4.2.3 Derivatives.............................................
Propylene glycol alginate is a derivative of eco-
nomic importance. This ester is obtained by the
reaction of propylene oxide with partially neutral-
ized alginic acid. It is soluble down to pH 2 and,
in the presence of Ca^2 +ions, forms soft, elastic,
less brittle and syneresisfree gels.4.4.4.2.4 Utilization
Alginate is a powerful thickening, stabilizing and
gel-forming agent. At a level of 0.25–0.5% it im-
proves and stabilizes the consistency of fillings
for baked products (cakes, pies), salad dressings
and milk chocolates, and prevents formation of
larger ice crystals in ice creams during storage.
Furthermore, alginates are used in a variety of gel
products (cold instant puddings, fruit gels, dessert
gels, onion rings, imitation caviar) and are ap-
plied to stabilize fresh fruit juice and beer foam.4.4.4.3 Carrageenans...........................................
4.4.4.3.1 Occurrence,Isolation
Red sea weeds (Rhodophyceae) produce two
types of galactans: agar and agar-like polysac-
charides, composed of D-galactose and 3,6-
anhydro-L-galactose residues, and carrageenans
and related polysaccharides, composed of
D-galactose and 3,6-anhydro-D-galactose which
are partially sulfated as 2-, 4- and 6-sulfates
and 2,6-disulfates. Galactose residues are alter-
natively linked by 1→3and1→4 linkages.
Carrageenans are isolated from Chondrus
(Chondrus crispus, the Irish moss),Eucheuma,
Gigartina, GloiopeltisandIridaeaspecies by hot