AGAR AND RELATED PHYCOCOLLOIDS 83 



1943). If cooled slowly, incipient gelation may begin as high as 63° C (145° F) 

 in some samples with the formation of a soft gel, followed by a sudden increase in 

 firmness as the temperature falls to 43° C (109° F), or lower. Apparently, the 

 fraction having the high temperature of gelation comprises less than half of the 

 total extractive and varies in its proportion with the season. Seaweed taken in 

 early or mid-summer may lack it entirely. The two fractions may differ with 

 respect to the metal ion involved in their chemical structure. Efforts to separate 

 them have not been successful so far. 



In Table 20 are data comparing temperature of gelation and gel strength of a 

 variety of agars and Irish moss gel. 



Swelling of Agar. Although insoluble in cold water, dry agar will absorb it in 

 large quantities, accompanied by swelling and the evolution of heat. The amount 

 of water absorbed at equilibrium is a variable that depends upon a variety of 

 factors, including the original moisture content of the dry agar sample. According 

 to the observations of Clark (1925), maximum absorption occurs if the original 

 moisture content is 313 mg of water per g of agar. The total absorption is pro- 

 gressively less with less original moistme down to zero, or with greater original 

 moisture, as shown in Fig. 5—16. Clark's work should be repeated using a variety 

 of agars of known source. His agar was apparently of Japanese origin and probably 

 a mixture of extractives of a dozen or more species of seaweeds. 



Solutes of all kinds also affect the swelling of agar, along with their effect upon 

 other properties as discussed in a preceding section. Dilute solutions of KCl and 

 NaCl at concentrations between O.OOIM and O.OIM cause an increase in the 

 amount of water absorbed by agar. At concentrations above 0.12V electrolytes 

 inhibit the swelling of agar in the order of the lyotropic series by competition of 

 ions with the agar micelles for water. Non-electrolytes produce the same efiFect 

 (Dokan, 1924). 



Flocculation of Agar. In order to effect precipitation of agar it is necessary to 

 remove two factors that stabilize lyophilic colloidal solutions— hydration and 

 electric charge. The presence of minute amounts of electrolytes, always present in 

 agar solutions with the exception of those prepared from agar purified by 

 electrodialysis, remove the electric charge from the agar particles. Under these 

 circumstances agar can be precipitated by pouring a melted solution into an equal 

 volume of 95 per cent alcohol or by adding 1.0 per cent tannin. Alcohol precipita- 

 tion is used to purify and dehydrate agar on a commercial scale although the 

 process is ordinarily prohibitory in cost. 



Agar can be precipitated from solution by high concentrations of electrolytes— 

 the "salting out" phenomenon. In sufficient concentration salts remove both the 

 electric charge and the water of hydration. Buchner and Kleijn (1927) studied 

 the relative efiFect of electrolytes as agar flocculating agents and arranged the 

 following sodium salts in the order of decreasing eflFectiveness : ferrocyanide, 

 citrate, dibasic phosphate, tartrate, thiosulfate, tribasic phosphate, molybdate, 

 formate, acetate, bromate, and chloride. They also found that certain sodium salts 

 (chlorate, bromide, iodide, and chromate) not only failed to cause flocculation 

 but inhibited it if they were present when the flocculating salts were added. 



Uses of Agar. In general the uses of the three most important phycocolloids, 

 agar, algin, and carrageenin, are alike in that they serve as stabilizers, emulsifiers, 

 thickeners, vehicles or body-producers, and gelling agents. There are, however. 



