AGAR AND RELATED PHYCOCOLLOIDS 59 



quantities. Buchanan, Percival, and Percival (1943) found that the galactose 

 residues are joined by 1,3-hnkages. The sulfuric ester group is attached to C*, 

 and not to Ca as in agar. 



Haas and Hill (1921) first pointed out that carrageenin contains at least two 

 different colloids, one soluble in cold water and the other soluble only in hot 

 water. Buchanan, Percival, and Percival believe that the essential difference be- 

 tween the types is that the hot water extract is chiefly a calcium salt and the cold 

 extract chiefly a mixture of sodium and potassium salts of carrageenin. They found 

 glucose, however, only in the hot water extract. Russell-Wells (1922) offered evi- 

 dence that the cold water extract consists of calcium and ammonium sulfates. She 

 prepared ammonium salts of both extracts by treatment of the solutions with 

 ammonium phosphate or ammonium oxalate, which causes a replacement of the 

 calcium with ammonium due to the formation of insoluble calcium phosphate or 

 calcium oxalate. The ash content of the resultant ammonium carrageenate was 

 only 5.87 per cent, as compared with 21.26 per cent in the original, purified extract. 

 Even after extensive purification a small quantity of nitrogen (about 1.0 per 

 cent) remains in carrageenin, although it is not a part of the carrageenin molecule. 

 In what form it occurs and why it is difficult to remove are not known. 



Properties of Carrageenin. The principal properties of carrageenin are pro- 

 foundly affected by the nature and relative amounts of other solutes contained in 

 a solution of carrageenin in water. Theoretically, a pure carrageenin solution will 

 not form a gel even if it is cooled to the freezing point. It is quite possible to pre- 

 pare solutions of sufficient purity that apparently do not form a gel when cooled 

 although the viscosity behavior at lower temperatures may be in part a manifesta- 

 tion of a form of incipient gelation. 



Since agar and carrageenin do not have the same properties, they have dif- 

 ferent uses. Agar is best suited to uses in which the tendency to form a firm gel 

 is required; indeed this is the property for which it is best known. Carrageenin 

 is superior for uses which require high viscosity and the concomitant thickening, 

 emulsifying, and suspending properties. In some cases, however, carrageenin is 

 used for its gel-forming property. 



Solutes present in a colloidal solution of carrageenin compete with the colloidal 

 micelles for water, with the result that the properties of the colloid are altered. In 

 general the greater the quantity of solute, the higher the temperature of gelation 

 and melting, and the greater the gel strength, the lower the viscosity of the melted 

 solution. Not only are these properties affected by electrolytes (various inorganic 

 salts), but also by organic solutes, such as sugars and alcohols, including glycerin 

 and various glycols. Salts of various organic acids, such as potassium acetate or 

 calcium lactate, are very effective. Among the simple inorganic salts potassium 

 chloride is one of the most effective and, at the same time, not objectionable in 

 small quantities in foods. Fig. 5-6 shows the effect of various concentrations of 

 KCl on the temperature of gelation of "Krim-Ko" gel, a commercially prepared 

 Irish moss extractive. The temperature of melting is only 7or9°F (4or5°C) 

 higher than the temperature of gelation throughout the range. In Fig. 5-7 the 

 gel strength of various concentrations of the same extractive containing 0.5 per 

 cent KCl are indicated. Fig. 5-8 shows the gel strength of a 2.0 per cent solution 

 of "Krim-Ko" gel with various concentrations of KCl. 



Ordinarily, solutions of carrageenin are prepared by heating either dry Irish 



