306 4 Carbohydrates
as the carrageenan sulfate content increases and
as the content of anhydrosugar residue decreases.
The viscosity of the solution depends on the
carrageenan type, molecular weight, temperature,
ions present and carrageenan concentration.
As observed in all linear macromolecules with
charges along the chain, the viscosity increases
exponentially with the concentration (Fig. 4.20).
Aqueousκ-carrageenan solutions, in the pres-
ence of ammonium, potassium, rubidium or cae-
sium ions, form thermally reversibly gels. This
does not occur with lithium and sodium ions.
This strongly suggests that gel-setting ability is
highly dependent on the radius of the hydrated
counter ion. The latter is about 0.23 nm for
the former group of cations, while hydrated
lithium (0.34 nm) and sodium ions (0.28 nm)
exceed the limit. The action of cations is visu-
alized as a zipper arrangement between aligned
segments of linear polymer sulfates, with low
ionic radius cations locked between alternating
sulfate residues. Gel-setting ability is probably
also due to a mechanism based on formation of
partial double helix structures between various
chains. The extent of intermolecular double helix
formation, and thus the gel strength, is greater,
the more uniform the chain sequences are. Each
substitution of a 3,6-anhydrogalactose residue by
another residue, e. g., galactose-6-sulfate, results
in a kink within the helix and, thereby, a decrease
in gelling strength. The helical conformation is
also affected by the position of sulfate groups.
The effect is more pronounced with sulfate in
Fig. 4.20.Viscosity curves of carrageenan aqueous so-
lutions. A:Eucheuma spinosum, C: Chondrus crispus,
B: A and C in a ratio of 2:1, 40◦C, 20 rpm (according
toWhistler, 1973)
the 6-position, than in 2- or 4-positions. Hence,
the gel strength ofκ-carrageenan is dependent
primarily on the content of esterified sulfate
groups in the 6-position.
The addition of carubin, which is itself non-
gelling, toκ-carrageenan produces more rigid,
more elastic gels that have a lower tendency
towards synaeresis. Carubin apparently prevents
the aggregation ofκ-carrageenan helices.
The 6-sulfate group can be removed by
heating carrageenans with alkali, yielding 3,6-
anhydrogalactose residues. This elimination
results in a significantly increased gel strength.
Carrageenans and other acidic polysaccharides
coagulate proteins when the pH of the solution
is lower than the proteins’ isoelectric points. This
can be utilized for separating protein mixtures.4.4.4.3.3 Utilization
Carrageenan utilization in food processing is
based on the ability of the polymer to gel,
to increase solution viscosity and to stabilize
emulsions and various dispersions. A level as low
as 0.03% in chocolate milk prevents fat droplet
separation and stabilizes the suspension of cocoa
particles. Carrageenans prevent syneresis in fresh
cheese and improve dough properties and enable
a higher amount of milk powder incorporation
in baking. The gelling property in the presence
of K+ salt is utilized in desserts and canned
meat. Protein fiber texture is also improved.
Protein sedimentation in condensed milk is
prevented by carrageenans which, likeκ-casein,
prevent milk protein coagulation by calcium ions.
Carrageenans are also used to stabilize ice cream
and clarify beverages.4.4.4.4 Furcellaran.............................................
4.4.4.4.1 Occurrence,Isolation
Furcellaran (Danish agar) is produced from red
sea weed (algaeFurcellaria fastigiata). Produc-
tion began in 1943 when Europe was cut off from
its agar suppliers. After alkali pretreatment of al-
gae, the polysaccharide is isolated using hot wa-
ter. The extract is then concentrated under vac-
uum and seeded with 1–1.5% KCl solution. The