Carrageenan exhibits the solubility characteristics normally shown by hydrophilic colloids. It is water soluble and insoluble in most organic solvents. Water miscible alcohols and ketones, while themselves non-solvents for carrageenan, are tolerated in admixture with carrageenan solutions at levels up to 40%. More highly polar solvents, such as formamide and N,N-dimethylformamide, are tolerated in still higher proportion and alone cause a marked swelling of the polymer.

The solubility characteristics of carrageenan in water are influenced by a number of factors most important of which are

a. the type of carrageenan
b. counter ions present
c. other solutes
d. temperature
e. pH

Type of Carrageenan

The many forms of carrageenan possible through variation in structural detail, provide much variability in regard to solubility properties. For practical purposes, however, it is convenient to speak in terms of several general structural types and to equate solubility with the overall balance of hydrophilicity as provided by the hydrophilic sulfate and hydroxyl groups on one hand and the more hydrophobic 3.6-anhydro-D-galactose residues on the other.

Thus, lambda carrageenan, by definition void of 3.6-anhydro-D-galactose units and being highly sulfated is easily soluble under most conditions. Kappa carrageenan containing 3.6-anhydro-D-galactose as part of the repeating unit and fewer sulfate groups is less hydrophilic and less soluble. Intermediate is iota carrageenan, more hydrophilic by virtue of its 2-sulfate which in addition to its position counteracts the less hydrophilic character of the 3.6-anhydro-D-galactose residue.

Counter Ions

Solubility characteristics are also affected by the salt form of the sulfated ester groups, particularly in the case of less soluble kappa carrageenan. The sodium forms are generally more easily soluble, while forms in which the cation is potassium dissolve with more difficulty.

Thus, kappa carrageenan in the potassium form may for practical purposes be considered insoluble in cold water, heat being required to bring it into solution, whereas in the sodium form it dissolves readily.

The potassium salt of iota carrageenan is also insoluble in cold water although it swells markedly. Lambda carrageenan is soluble in all its salt forms.

Other Solutes

Both the dissolving rate and solubility of carrageenan are affected by the presence of other solutes, the effect mainly being to compete for available water and thus to alter the state of hydration of the polysaccharide. Sensitivity to other solutes parallels solubility with kappa carrageenan being the most sensitive to the presence of solutes.

Inorganic salts are most effective in altering the hydration of carrageenan, particularly when the cation is potassium. 1.5 to 2% potassium chloride is sufficient to prevent the dissolution of kappa carrageenan at normal temperatures while sodium chloride solutions of 4 to 4.6% and above are also non-dissolving. Considerably higher concentrations of nonionics are required before similar effects are noted.

Sucrose shows little effect on hydration of kappa carrageenan until present in concentrations of 50% and above while glycerol must be present in very high amounts before appreciable effects are seen. In cases where more than one solute is present, their combined effect on hydration is generally additive and may be predicted from a knowledge of individual effects. For example, in the presence of high amounts of glycerol, solubility is markedly influenced by traces of potassium ions.

Of practical importance is the fact that iota carrageenan will dissolve with heating in solutions containing relatively high concentrations of salts and thus is able to provide gelation in certain applications where an excessive amount of salts would preclude the use of kappa carrageenan.

Dispersion

Being a water soluble polysaccharide, carrageenan is difficult to disperse in water due to the formation of a film layer around each carrageenan particle. This leads to the formation of large agglomerates (lumps) which, due to the protective film layer, are very difficult for the water molecules to penetrate.

The less soluble the carrageenan the easier the dispersion, for example, a potassium kappa carrageenan being insoluble in cold water is much easier to disperse in cold water than a sodium kappa carrageenan. Both, however, are soluble in hot water and therefore equally difficult to disperse in hot water. Other factors which decrease the solubility of carrageenan will improve the dispersibility.

In most applications carrageenan may be preblended with other ingredients such as sugar, and in order to achieve complete dispersion 1 part of carrageenan should be blended with 10 parts of sugar.

In applications where carrageenan cannot be preblended with other ingredients, a high speed mixer is necessary in order to break up the lumps formed by adding the carrageenan to the water.

Although potassium or calcium carrageenans are not or only slightly soluble, they swell in cold water producing viscous dispersion. Thus, dispersing carrageenan in cold water using a high speed mixer limits the strength of the dispersion to approx. 3% depending on the type of mixer.

In hot (60 - 75EC) water carrageenan can be dissolved to make 7 - 8% solution. This leads immediately to the formation of lumps, but high speed mixers are available which easily break these lumps. In hot water the carrageenan goes into true solution, and in this state the viscosity is much lower than in the state of hydration.

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