2009_Book_FoodChemistry

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
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 Naalginate
and calcium sulfate.
Depending on the concentration of calcium ions,
the gels are either thermoreversible (low concentration)
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 economic
importance. This ester is obtained by the
reaction of propylene oxide with partially neutralized
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 improves
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 applied
to stabilize fresh fruit juice and beer foam.
Fig. 4.17. Viscosity of aqueous alginate solutions. Alginate
with (a) high, (b) medium, and (c) low viscosity
4.4.4.3 Carrageenans
4.4.4.3.1 Occurrence, Isolation
Fig. 4.18. Schematic representation of a calcium alginate
gel (cross-linkage by egg box formation, cf. Formula
4.120; according to Franz, 1991)
Red sea weeds (Rhodophyceae) produce two
types of galactans: agar and agar-like polysaccharides,
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 alternatively
linked by 1 → 3and1→ 4 linkages.
Carrageenans are isolated from Chondrus
(Chondrus crispus, the Irish moss), Eucheuma,
Gigartina, Gloiopeltis and Iridaea species by hot