186 PLANT BIOCHEMISTRY 



Pectin is characterized by its water-binding capacity, particularly in 

 the presence of dehydrating agents such as sucrose. This property is 

 used in the preparation of jams and jellies. Commercial pectin is pre- 

 pared from apple pumice and from citrus peel. Since enzymic hy- 

 drolysis takes place and since pectic acid and the lower polymers 

 resulting from pectinase activity have little or no gelling power, it is 

 important that the quality of the starting material be controlled. 



Certain plants produce additional polysaccharide derivatives which 

 are noted for their viscosity in water solutions. Since a number of 

 these are important commercial items, they will be mentioned briefly. 

 They are not components of the cell wall, but rather serve as protec- 

 tive coatings of either seeds or wounded areas. 



Gum Arabic 



Gum arabic is obtained as an exudate from the bark of the Acacia 

 tree of Central Africa. It has a highly branched structure containing 

 D-galactose, L-arabinose, L-rhamnose, and glucuronic acid. Gum arabic 

 is used widely in the food industry to control viscosity and texture. 



Gum Tragacanth 



Gum tragacanth is produced on a semidesert shrub of the Astragalus 

 family in Asia Minor. The exudate is only partially soluble in water. 

 The soluble portion is extracted from the harvested gum and sold as 

 traganth for use in the drug, cosmetic, and printing industries. Again, 

 the function is the control of viscosity and texture. 



Mucilages 



Mucilages are obtained by water extraction of the seed coat of flax, 

 Plantago, mustard, and certain other seeds followed by precipitation 

 with alcohol. As the name implies, a water solution of mucilage 

 produces a sticky, viscous mass. In nature this property probably 

 serves in seed distribution and also as a means of retaining moisture 

 needed for seed germination. 



A number of gel-forming substances have been isolated from sea 

 algae. These include the commercially important agar, isolated from 

 red algae of the Pacific Ocean, and carrageen, recovered from another 

 red algae commonly known as Irish moss. Both these polysaccharides 

 are polygalactansulfate esters of varying molecular weight. 



Lignin 



The cell walls of woody parts of plants are strengthened by a com- 

 plex organic material called lignin. It may occur in a range from a 



