AGAR AND RELATED PHYCOCOLLOIDS 79 



even at relatively low temperatures. If, however, a solute is added, gelation will 

 occur. If salt crystals are added, a ball of gel will form instantaneously around 

 each crystal. Complete dissolution of the salt can be accomplished only by heating 

 the solution to the melting temperature of the highest effective concentration of 

 the salt used. Once the salt is dissolved, the temperature of gelation and melting, 

 the gel strength, and other properties will depend upon the concentration and 

 nature of the solute. No differences have been detected in the behavior of ex- 

 tractives of Hypnea from North Carolina, Florida, and Brazil. 



The effect of various concentrations of Hypnea extractive with two different 

 concentrations of KCl may be seen in Table 19. 



Table 19. Effect of Various Concentrations of Hypnea Extractive and of Two 

 Concentrations of KCl on Gel Strength and Temperature of Gelation. 



The marked effect of solutes upon the viscosity of Hypnea extractive is shown 

 in Fig. 5-13, in which a comparison is made with Gelidium agar and distilled 

 water. Viscosity units are arbitrary numbers. It is obvious from these data that 

 filtration of a Hypnea extractive solution is more readily accomplished if solutes 

 are present; the same is true of Irish moss extractive, since it reacts to solutes in a 

 similar, if less dramatic, manner. 



The temperature of gelation and of melting, as well as the magnitude of 

 hysteresis, of 1.0 per cent concentration of Hypnea agar containing various con- 

 centrations of potassium chloride is shown graphically in Fig. 5—14. The differ- 

 ence between the temperature of gelation and the melting temperature (hysteresis 

 range), as determined by potassium chloride, is about 28.8° F (16° C) for all 

 concentrations of the salt and the extractive (except, possibly, at the extremes 

 of either variable). Other salts or solutes, however, produce different hysteresis 

 ranges. There appears to be a positive correlation between gel strength and mag- 

 nitude of hysteresis range produced by any given solute. Potassium chloride is 

 toward the upper limit; that is, it produces a relatively high gel strength and wide 

 range between gelation and melting. 



Hypnea extractive was manufactured and marketed by the Beaufort Chemical 

 Corporation in North Carolina during 1945 and 1946. Apparently, it has not been 

 manufactured on a commercial scale since then (as of 1950) although it would 

 seem to have excellent commercial possibilities by virtue of the great range of its 

 important properties, as affected by various solutes, and the fact that these proper- 

 ties can be controlled independently of each other to a considerable extent. Gel 

 strength is controlled by variation in the concentration of the extractive, with 



" Gel strength data are gm per sq. cm when the weight is added at the rate of 

 0.5 g per second. 



