The structure of kappa and iota carrageenan allows segments of the two molecules to form so called double helices which bind the chain molecules in the three dimensional network, a gel. Lambda carrageenan has a structure that does not allow such double helix formation.

Gel Formation of Kappa and Iota Carrageenans

When dissolved by heating, followed by cooling below certain temperatures, kappa and iota carrageenans form thermoreversible water gels in a concentration as low as 0.5%, provided gelling cations are present. A gel has some properties of a solid and some of a liquid. Thus, it keeps its shape when tipped out of a container and yet retains the vapor pressure and conductivity of the liquid from which it is made.

Kappa carrageenan gels in the presence of potassium ions, the rigidity of the gel increasing with increasing potassium ion concentration.

Potassium ions also have the effect of increasing the melting and gelling temperature.

Effect of Potassium Chloride on Gelling and Melting Temperature of Carrageenan

Calcium ions and barium ions increase the rigidity of a carrageenan gel, the effect being most pronounced when potassium ions are added as well.

Effect of Calcium Chloride on
Gel Strength of
Kappa
Carrageenan Gel
1.50% Kappa Carrageenan

Effect of Barium Chloride on
Gel Strength of
Kappa
Carrageenan Gel
1.50% Kappa Carrageenan

It is believed that calcium and barium ions form bridges between adjacent double helices through an electrostatic binding to two adjacent sulfate groups, thus stabilizing and strengthening the network.

When removing cations which cause gelation of carrageenan from the medium as well as from the carrageenan, a solution of carrageenan is obtained which does not form a gel irrespective of the temperature. As soon as gelling cations are present the carrageenan solution will gel at a specific temperature, the gelling temperature. Thus, the gelling temperature of a carrageenan solution is a function of the concentration of gelling cations present in the system.

Tg = f (C_{gelling cations} )

The use in practice of the effect of potassium chloride on the gel strength and gelling temperature is limited by organoleptic considerations, due to the bitter taste of potassium chloride. The upper limit for delicately flavored foods is 0.1-0.2% chloride. However, in salty foods such as meat products up to 0.5% of sodium chloride may be replaced by potassium chloride without detection.

Potassium chloride has the highest effect on gel strength per potassium unit but other potassium salts may be used for taste considerations. Potassium phosphates are neutral in taste whereas potassium citrate yields a more acidulous taste in acid systems at pH-values below 4.

A secondary function of the phosphates and citrates is to buffer the system to a relatively high pH, where the carrageenan is more stable.

The strongest kappa carrageenan gel is produced in the presence of potassium and calcium ions. However, the presence of calcium ions makes the kappa carrageenan gel brittle, whereas a pure potassium carrageenan gel is elastic, cohesive and transparent. Addition of sodium ions to a carrageenan gel makes the gel short and brittle. Large amounts of sodium ions disturb the gelation of carrageenan and reduce the gel strength. This is of special interest in gelled meat products in which sodium chloride is added as a spice.

Strength, texture and gelling temperature are influenced by other solutes than salts. Sucrose increases the gelling and melting temperature. High solids means that a higher temperature is needed in order to dissolve kappa carrageenan, and low pH increases the hydrolytic degradation of kappa carrageenan. Therefore, working with high solids one should add the acid as late in the process as possible.

In practice it is not possible to use kappa carrageenan in foods with a sugar content higher than 60%.

Effect of Sucrose on Gelling and Melting Temperature of
a Kappa Carrageenan Gel
0.65% Kappa Carrageenan
0.13% Potassium Chloride

Contrary to kappa carrageenan, iota carrageenan gels most strongly with calcium ions to form a very elastic and very coherent gel which is completely syneresis-free. Iota-carrageenan is an excellent water binder in concentrations as low as 0.2%. In combinations with kappa carrageenan it adds elasticity and prevents syneresis.

Iota carrageenan is the only carrageenan type which is freeze/thaw stable. Thus, a frozen iota carrageenan gel will be completely syneresis-free when thawed, unlike a kappa carrageenan gel.

Interaction with Other Gums
Kappa carrageenan forms strong and brittle gels which exude water (syneresis) and in many applications such textural properties are disadvantageous.

Where gelatin has traditionally been the preferred gelling agent - particularly in dessert gels - combinations of kappa carrageenan and other gums have been used in an attempt to duplicate the texture produced by gelatin.

When locust bean gum is added to kappa carrageenan, the gel breaking strength increases reaching a maximum at a 1 : 1 ratio of the two hydrocolloids. Rigidity also increases, reaching a maximum at approx. 0.25% locust bean gum, while cohesiveness increases steadily with increasing locust bean gum concentration. From a sensory standpoint, locust bean gum makes the kappa carrageenan gel less brittle and more elastic, thus approaching the texture of a gelatin gel. At high locust bean gum levels, the gel becomes gummy and very difficult to break in the mouth.

Gels containing a 1 : 1 ratio of the two hydrocolloids are considered the most palatable. However, the breaking strength is too high. The breaking strength is decreased by reducing the concentration of hydrocolloids but this has an adverse effect on gel syneresis.

Contrary to locust bean gum, iota carrageenan significantly decreases the breaking strength and rigidity of kappa carrageenan gels, the effect being related to the proportion of iota carrageenan in the system. Cohesiveness stays constant, and this combined with lower breaking strength indicate that iota carrageenan makes the gel less brittle.

Iota carrageenan also increases water holding and sensory elasticity of the gels, thus making the gels more „gelatin-like" than locust bean gum does.

Contrary to locust bean gum and iota carrageenan, amidated low methoxyl pectin does not contribute significantly to kappa carrageenan gel formation, the textural parameters of cohesiveness and elasticity being essentially the same as for pure kappa carrageenan. However, due to its excellent water holding properties, amidated low methoxyl pectin allows a decrease in the concentration of kappa carrageenan and produces softer, more palatable gels. The low cohesiveness of such gels may be increased by incorporating locust bean gum.

Another advantage of using amidated low methoxyl pectin is its excellent flavor release. On the negative side, pectin gels are always somewhat cloudy and dessert gels made with amidated low methoxyl pectin are not as transparent as those made from pure carrageenan.

Addition of xanthan gum makes a kappa carrageenan gel softer, more cohesive, and more elastic. In addition, xanthan gum reduces syneresis as much as iota carrageenan does. A disadvantage of using xanthan gum in combination with kappa carrageenan is that such gels contain air bubbles which detract from their appearance. This is apparently due to the high „working yield value" of xanthan gum, which makes the air bubbles difficult to remove even at higher temperatures.

Contrary to locust bean gum, guar gum does not exhibit synergistic effect with kappa carrageenan. Like locust bean gum guar gum is a galactomannan, but two factors make these gums different. Firstly, guar gum contains approximately twice the amount of galactose. Secondly, the length of unsubstituted regions in guar gum is significantly shorter than those in locust bean gum.

This difference in arrangement of galactose explains why guar gum is cold soluble whereas locust bean gum must be heated to temperatures above 80 - 90°C in order to dissolve.

Furthermore it explains why carrageenan may interact with locust bean gum and not with guar gum.

Schematic representation of the
structure of locust bean gum and
guar gum.
G = a(1^® 6)-D-galactose.
The arrows indicate possible
interaction sites of kappa and
iota carrageenan.

Milk Reactivity
Carrageenan reacts with the fraction of milk protein called kappa casein, resulting in the formation of a three dimensional network (a gel) within which water, salts, and particles are trapped.

Being a protein, kappa casein possesses a positively and a negatively charged terminal, and the overall charge is dependent upon the pH of the medium. At the isoelectric pH, the overall charge of the protein is zero.

For kappa casein the isoelectric pH is 4.4, and below pH = 4.4 the protein is positively charged while the overall charge is negative at pH-values above 4.4.

Being a sulfated galactan, carrageenan is negatively charged independent of the pH of the medium. At pH-values below 4.4 kappa casein and carrageenan are oppositely charged, and a carrageenan-kappa casein complex will precipitate. At pH-values above 4.4 carrageenan and kappa casein bear the same overall charge, but the two molecules do not repel one another.

Milk contains large amounts of calcium ions, and one way of explaining the reaction between carrageenan and kappa casein is the formation of a calcium bridge between the two molecules.

This theory, however, is not able to explain the interaction between carrageenan and kappa casein completely, as carrageenan and kappa casein react without calcium ions present.

Kappa casein is composed of a large number of amino acids (approx. 170), some of which are more electropositive than others. In fact, an extensive positively charged region exists between the residues 20 and 115 of kappa casein.

Such a distribution of negative and positive amino acids is not found in beta casein nor in alpha S1 casein. The positively charged region in kappa casein is large enough to make the electrostatic interaction with carrageenan possible.

The interaction between kappa casein and carrageenan is, however, not responsible for the gelation of milk products.

On cooling below the gelling temperature of carrageenan, sectors of carrageenan molecules form double helices as in water systems, but the kappa casein-carrageenan interaction reinforces the network, and as a result the necessary amount of carrageenan to gel a milk system is much less (approx. 1/5) of the amount necessary to gel a water system. Similarly lambda carrageenan produces viscosity in milk in concentrations of 0.05-0.1% where a similar effect in a water system would require a concentration of 0.5-1.0%.

Rheology
Carrageenan solutions show pseudoplastic flow behavior as do most hydrocolloids: With increasing shear rate the viscosity decreases whereas the viscosity instantly increases as the shear rate is decreased.

Flow Curve of Kappa Carrageenan
Water Solution
1.50% Kappa Carrageenan

Viscosity of Kappa Carrageenan
Water Solution
1.50% Kappa Carrageenan

Solutions of carrageenan have low viscosity and are thus easy to handle. A kappa carrageenan water gel is irreversibly destroyed when subjected to shear. Kappa carrageenan water gels are thus not thixotropic.

When used in low concentrations in milk, kappa carrageenan shows rheological properties similar to those of iota carrageenan in water. Hence, the weak gel produced by the kappa carrageenan-milk protein complex breaks when shear is applied and the flow becomes pseudoplastic. When shear is decreased the gel reforms thus showing a thixotropic nature.

Flow Curve of Iota Carrageenan
Water Gel
0.30% Iota Carrageenan
2.00% Sodium Chloride

Flow Curve of Kappa Carrageenan
Milk Gel
0.025% Kappa Carrageenan

The weak gel - the small yield value () - is sufficient to prevent solid particles, for instance cocoa particles, from settling.

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