AGAR AND RELATED PIIYCOCOLLOIDS 



57 



ties of the extractive in each batch are carefully determined by the laboratory 

 stalf, proper mixing of various batches is done in packaging in order to obtain a 

 finished product of precise specifications. A final laboratory check is made of the 

 blended material before it is distributed to the user. Many refinements have been 

 added to the process, some of which are trade secrets, in order to obtain a higher 

 yield, lighter color, controlled viscosity and gel strength, and other favorable 



{Courtesy McGraw-Hill Publishing Co.) 



Fig. 5-3. A large basket-type centrifuge separates the 

 carrageenin solution from the seaweed residue after cooking 

 is completed. The seaweed residue is saved and cooked again. 



properties. A yield of from 60 to 80 per cent is obtained from the clean, thoroughly- 

 dry raw material. Most of the ultimate residue is cellulose. 



Although a white and highly purified finished product can be obtained by pre- 

 cipitation with alcohol (following filtration and concentration by evaporation), 

 the cost of this procedure is prohibitive. Freezing out Irish moss gel, as is done 

 in the manufacture of agar, is also unsatisfactory because of the tendency of 

 carrageenin to reabsorb large quantities of water at low temperature. 



The yield and properties of carrageenin vary with the season of collection and 

 with the locality. The highest yield is obtained from moss collected in mid-summer, 

 at the height of the harvesting season, while the lowest gelose content occurs in 

 winter (Butler, 1936). 



Chemical Nature of Carrageenin. The extractive from Irish moss is funda- 

 mentally similar to agar in that both are sulfuric acid esters of galactan. One 

 of the principal differences is the greater complexity of the carbohydrate portion 

 of the agar micelle, with the result that carrageenin has a much higher ash content 

 and a lower gel strength. Carrageenin also contains a small percentage of Z-keto- 

 gluconic acid. 



The ethereal sulfate nature of carrageenin is well-established, but there is 

 disagreement among investigators as to its exact nature. Although its carbohydrate 

 portion is principally galactose, glucose and pentose have been found in small 



