82 MARINE PRODUCTS OF COMMERCE 



pared from Gelidium latifoliwn. After repeated purification they obtained an 

 ash content of 2.59 per cent and a sulfur content of 0.364 per cent. This propor- 

 tion of sulfur corresponds to one sulfate group to each 53 galactose units. They 

 agreed with Percival and Thompson that agar does not contain as much as one 

 non-reducing end group for every 140 galactose units because of the lack of 

 detectable amounts of tetramethylgalactose in the hydrolysis products of methyl- 

 ated agar. 



One of the difficulties that confuses chemical work on agar is the fact that very 

 few investigators have used agar from a positively known source. Agar of com- 

 mercial manufacture is worthless (harmful, actually) as a material for chemical 

 study because it is composed of the extractives of a variety of seaweeds which are 

 impossible, at present, to separate. This is particularly true of Japanese agar as 

 it is made from raw material composed of intentionally mixed species of sea- 

 weeds. It should be assumed, until data indicate otherwise, that each species of 

 seaweed produces a phycocolloid that is different chemically from all other 

 phycocolloids. For this reason the results of Barry and Dillon (1944) may not 

 actually conflict with the work of Jones and Peat (1942) as the two groups of 

 investigators were using agar from different species of algae. 



Chemical studies of phycocolloids, to be reliable, must begin with the dry 

 seaweed, identified by a qualified taxonomist. Part of the material should be 

 deposited in an herbarium for future reference. The investigators should then 

 extract their own agar, after carefully removing all traces of other species of sea- 

 weeds that might contribute foreign phycocolloid substances. Other chemists will 

 then be able to repeat the work and verify the results; there will be no doubt about 

 the source of the agar under examination nor of its purity, at least with reference 

 to contaminating phycocolloids. 



More detailed information on the chemical constitution of agar and other 

 phycocolloids is given by Tseng (1946) and Hassid (1944). 



Physical Properties of Agar. Agar differs from all other phycocolloids in its 

 unique combination of physical properties, of which two are virtually exclusive 

 with agar: its high gel strength and its wide hysteresis range (i.e., difference 

 between the temperature of gelation and melting). Other properties are also im- 

 portant, but vary with the agar source. 



Agars from Gelidium cartilagineum of California and Mexico and from G. aman- 

 sii (plus the usual adjunctive species) of Japan are noted for their low viscos- 

 ity when in melted or liquid condition, for low syneresis (exudation of water 

 in the gel state), and for their uniformly low temperature of gelation (34 to 40° C, 

 93 to 104° F). It is these properties that make Gelidium agar the ideal gelhng 

 agent for bacteriological culture media. 



Agars from Gracilaria confervoides and G. foliifera are slightly lower in gel 

 strength (although there are exceptions) and more viscous in liquid form, ex- 

 hibit more syneresis in the gel state, and form a gel at a higher temperature than 

 Gelidium agar. Gracilaria confervoides from South Africa is said to be exceptional 

 because it gels at about 37° C (98° F). Hypnea agar is similar to that from 

 Gelidium in that it possesses a relatively low viscosity (in the presence of solutes); 

 but it resembles Gracilaria agar in its high degree of syneresis. 



Agar from G. confervoides from North Carolina appears to be composed of 

 two or more fractions that possess different temperatures of gelation (Stoloff, 



