Part D | 22.4
560 Part D Marine Algal Biotechnology
supports extensive conformation of the carrageenan
chain. The presence of the 3,6-anhydro-link in -carrageenan allows the ˛-D-Galp residues to revert to
their
1 C 4 conformation, which is necessary for the initial double helix formation required for gelling. Alkali
treatment of carrageenans or their precursors during
processing increases the gelling properties of the final
polysaccharide and the strength of gels due to conversion of ˛-D-Galp6S to the 3,6-anhydride. As a result,
Ä-carrageenans and furcellaran (Ä/ˇ-carrageenan) with
less sulfates but more 3,6-anhydro groups form a firm
gel with K
C . By contrast, more sulfated but less or
no anhydrated Ã- and -carrageenans are slightly affected by these cations; Ã-carrageenan needs Ca
2C to
form soft elastic gels, and -carrageenan cannot form
gels at all. Hot solutions of Ä and Ã-carrageenans form
gels after cooling of their hot aqueous solutions to
4060
ı C in the presence of the appropriate metal
cations. These gels are stable at room temperature,
can be re-melted by heating to 520
ı C above the
gelling temperature, and re-gel after repeated cooling. Highly flexible carrageenan molecules are able
to form helical structures when their concentration increases or temperature decreases. The mechanism of
Ä-carrageenan gelation in the presence of K
C and
other alkali metal cations (Na
C , Rb
C ) is thus based
on the cation supported aggregation of helical dimers,
which consist of two single [22.225] or double [22.210]
helices.
In any case, a two-step gelation mechanism has
been proposed for both à and Ä-carrageenans [22.212,
214]. Thermally reversible carrageenan gels exhibit
hysteresis or a difference between setting and melting
temperatures. The gelation of Ä-carrageenan solutions
is independent of liquid-liquid phase separation due to
the rapid formation of cross-linking points [22.226].
Furcellaran (Ä/ˇ-carrageenan) and Ä-carrageenan gels
are relatively rigid and undergo syneresis, but Ãcarrageenan yields soft and elastic gels with very little
tendency to undergo syneresis. In this case, the 2O-sulfate groups on the 3,6-anhydro-˛-D-Galp units
prevent the tightly-packed aggregation responsible for
the rigidity of Ä-carrageenan gels.
Agar is commonly defined as the mixture of neutral
agarose and highly sulfated agaropectin. The ratio between these parts depends on the algal source of agar.
Agarose demonstrates a much higher gelling ability
than amylopectin. Agarose is well soluble in hot water
(up to 80
ı C) because it has chains at stiff random coil
conformation [22.227–229]. Upon cooling below the
gelation temperature (40
ı C), the coils reorder to form
helices that subsequently laterally aggregate into a network of thick bundles to form a gel. It is unclear, however, if the proposed high-ordered structure is made of
a single helix [22.228–230] or a double helix [22.218].
Agarose gels ( 4 m=v %) are typically rigid and turbid, and undergo hysteresis between the melting and
setting temperatures. The length of helices suitable for
stable association increases with heating. Slow cooling
leads to longer helices than rapid cooling, and longer
helices promote better aggregation. As a result, the gels
formed by slow cooling are stronger and more turbid than those obtained by rapid cooling [22.231]. The
gelation process is followed by liquid–liquid phase separation because the network formation based on aggregated bundles leads to the dilution of solute agarose in
the network interspaces [22.226, 232]. The agar chains
form bundles of aggregates, generating a network at the
temperature of sol–gel transition. Longer agar chains
preferably aggregate, while shorter agar chains are retained in the solute and form isolated aggregates upon
further cooling. Reheating leads to easy dissociation of
the loose aggregates, whereas bundles of aggregates are
thermostable like the whole gel [22.233].
22.4.3 Gelling/Film Forming
by Fucoidans and Ulvans
In contrast to other phycocolloids, there is a little evidence about the gelling and film forming properties
of fucoidans. There are, however, several reports that
confirm that this polysaccharide by itself or in a mixture with other polymers is able to form gels and
films [22.234]. Varying amounts of fucoidan in a liquid
medium may provide specific rheological properties of
the resulting gel.
Green seaweed polysaccharide ulvans are well soluble in water. However, despite their high negative
charge, they demonstrate condensed conformation and
low intrinsic viscosity in aqueous saline or acidic solutions, which is probably due to the hydrophobic contribution of CH 3 in Rhap [22.156]. This polysaccharide
is probably not fully dissolved but forms free condensed spherical nanostructures (1018 nm) dispersed
in the solvent. At strong alkali conditions (pH 13) these
structures collapse into a dense homogeneous network,
which is the basis for strong films and gels. Gelation
and film formation at lower pH levels needs divalent
cations and/or boric acid. Ulvan forms thermoreversible
gel in the presence of boric acid, Ca
2C , or other divalent metal cations (but not Mg
2C ), and narrow region of
pH around 7:5 [22.11]. The mechanism of gel formation
560 Part D Marine Algal Biotechnology
supports extensive conformation of the carrageenan
chain. The presence of the 3,6-anhydro-link in -carrageenan allows the ˛-D-Galp residues to revert to
their
1 C 4 conformation, which is necessary for the initial double helix formation required for gelling. Alkali
treatment of carrageenans or their precursors during
processing increases the gelling properties of the final
polysaccharide and the strength of gels due to conversion of ˛-D-Galp6S to the 3,6-anhydride. As a result,
Ä-carrageenans and furcellaran (Ä/ˇ-carrageenan) with
less sulfates but more 3,6-anhydro groups form a firm
gel with K
C . By contrast, more sulfated but less or
no anhydrated Ã- and -carrageenans are slightly affected by these cations; Ã-carrageenan needs Ca
2C to
form soft elastic gels, and -carrageenan cannot form
gels at all. Hot solutions of Ä and Ã-carrageenans form
gels after cooling of their hot aqueous solutions to
4060
ı C in the presence of the appropriate metal
cations. These gels are stable at room temperature,
can be re-melted by heating to 520
ı C above the
gelling temperature, and re-gel after repeated cooling. Highly flexible carrageenan molecules are able
to form helical structures when their concentration increases or temperature decreases. The mechanism of
Ä-carrageenan gelation in the presence of K
C and
other alkali metal cations (Na
C , Rb
C ) is thus based
on the cation supported aggregation of helical dimers,
which consist of two single [22.225] or double [22.210]
helices.
In any case, a two-step gelation mechanism has
been proposed for both à and Ä-carrageenans [22.212,
214]. Thermally reversible carrageenan gels exhibit
hysteresis or a difference between setting and melting
temperatures. The gelation of Ä-carrageenan solutions
is independent of liquid-liquid phase separation due to
the rapid formation of cross-linking points [22.226].
Furcellaran (Ä/ˇ-carrageenan) and Ä-carrageenan gels
are relatively rigid and undergo syneresis, but Ãcarrageenan yields soft and elastic gels with very little
tendency to undergo syneresis. In this case, the 2O-sulfate groups on the 3,6-anhydro-˛-D-Galp units
prevent the tightly-packed aggregation responsible for
the rigidity of Ä-carrageenan gels.
Agar is commonly defined as the mixture of neutral
agarose and highly sulfated agaropectin. The ratio between these parts depends on the algal source of agar.
Agarose demonstrates a much higher gelling ability
than amylopectin. Agarose is well soluble in hot water
(up to 80
ı C) because it has chains at stiff random coil
conformation [22.227–229]. Upon cooling below the
gelation temperature (40
ı C), the coils reorder to form
helices that subsequently laterally aggregate into a network of thick bundles to form a gel. It is unclear, however, if the proposed high-ordered structure is made of
a single helix [22.228–230] or a double helix [22.218].
Agarose gels ( 4 m=v %) are typically rigid and turbid, and undergo hysteresis between the melting and
setting temperatures. The length of helices suitable for
stable association increases with heating. Slow cooling
leads to longer helices than rapid cooling, and longer
helices promote better aggregation. As a result, the gels
formed by slow cooling are stronger and more turbid than those obtained by rapid cooling [22.231]. The
gelation process is followed by liquid–liquid phase separation because the network formation based on aggregated bundles leads to the dilution of solute agarose in
the network interspaces [22.226, 232]. The agar chains
form bundles of aggregates, generating a network at the
temperature of sol–gel transition. Longer agar chains
preferably aggregate, while shorter agar chains are retained in the solute and form isolated aggregates upon
further cooling. Reheating leads to easy dissociation of
the loose aggregates, whereas bundles of aggregates are
thermostable like the whole gel [22.233].
22.4.3 Gelling/Film Forming
by Fucoidans and Ulvans
In contrast to other phycocolloids, there is a little evidence about the gelling and film forming properties
of fucoidans. There are, however, several reports that
confirm that this polysaccharide by itself or in a mixture with other polymers is able to form gels and
films [22.234]. Varying amounts of fucoidan in a liquid
medium may provide specific rheological properties of
the resulting gel.
Green seaweed polysaccharide ulvans are well soluble in water. However, despite their high negative
charge, they demonstrate condensed conformation and
low intrinsic viscosity in aqueous saline or acidic solutions, which is probably due to the hydrophobic contribution of CH 3 in Rhap [22.156]. This polysaccharide
is probably not fully dissolved but forms free condensed spherical nanostructures (1018 nm) dispersed
in the solvent. At strong alkali conditions (pH 13) these
structures collapse into a dense homogeneous network,
which is the basis for strong films and gels. Gelation
and film formation at lower pH levels needs divalent
cations and/or boric acid. Ulvan forms thermoreversible
gel in the presence of boric acid, Ca
2C , or other divalent metal cations (but not Mg
2C ), and narrow region of
pH around 7:5 [22.11]. The mechanism of gel formation