358 On Carrageen. Chondrus crispus 



the scale carrageen extracted with hot and with cold water differs in 

 certain physical characters, notably the rapidity of solution in water, 

 hygroscopic properties, viscosity of their aqueous solutions and the be- 

 haviour of such solutions towards Rochelle salt. 



Action of Rochelle salt. Rochelle salt (sodium potassium tartrate) has 

 a marked effect upon the gelatinising of solutions of carrageen too weak 

 to form a gel under ordinary conditions; thus 5 c.c. of a 1 per cent, 

 solution of a hot water extract of carrageen boiled with five drops of a 

 20 per cent, solution of Rochelle salt sets to a gel on cooling whereas 

 a 1 per cent, solution of a cold water extract is not gelatinised by boiling 

 with Rochelle salt. 



This indicates that carrageen contains more than one colloidal sub- 

 stance extractable by water, one of which is more soluble in cold water 

 than the other and this has an important bearing on the mode of pre- 

 paration of the extract, since a cold water extract is deficient in one of 

 the constituents which may be extracted by hot water. 



Gelatinising Power. 



The best method of gelatinising scale carrageen is to soak the material 

 first in water : a rapid absorption takes place accompanied by considerable 

 swelling; solution is effected quickly'if the vessel be placed in water 

 heated to about 60° C. As compared with gelatine and agar, carrageen 

 occupies an intermediate position. Both gelatine and carrageen dissolve 

 easily in water at relatively low temperatures; agar, on the other hand, 

 requires prolonged heating at about 90° C. The melting point of agar is 

 correspondingly high, while the melting points of gelatine and carrageen 

 are relatively low. There is a marked difference between these last two 

 substances with regard to the sharpness of their melting points: whilst 

 gelatine passes from the solid to the liquid state with some degree of 

 suddenness, carrageen on melting yields a viscous liquid which unlike 

 gelatine remains thick while hot, and this fact makes it hard to say 

 exactly at what temperature the jelly melts. For this reason Hatschek's 

 method, which works well for gelatine, is consequently not altogether 

 suitable for carrageen and the figures given below obtained by this 

 method are therefore only approximate. 



The accompanying table gives the approximate melting points 

 obtained for equivalent strengths of gelatine and carrageen (hot extract) : 



Gelatine Carrageen 



3 % 27-7° C. 27-30° C. 



5 % 29-5° C. 40-41° C. 



