CARBOHYDRATES 



65 



investigators to be a member of the group of hemicelluloses. It is very 

 widely distributed in nature, being found in varying quantities in most 

 fruits, vegetables', and roots. Ripening of the fruit, or action of acid 

 and heat, converts the insoluble protopectin into soluble pectin. This 

 change is well illustrated in jelly making, where boiling the fruit is 

 necessary to get the maximum amount of pectin. However, prolonged 

 boiling converts the pectin into hydrolysis products that do not have 

 the property of jelling. In the ripening of fruits, enzymes bring about 

 the hydrolysis of pectin, and, hence, overripe fruits are not suitable for 

 making jelly. 



Structurally, pectin is a polysaccharide consisting of a long chain of 

 D-galacturonic acid units (pp. 38, 39) in which some of the carboxyl groups 

 are united with metliyl alcohol through an ester linkage (— COOCH3). 

 The galacturonic acid units are joined through carbons 1 and 4, as are 

 the glucose units in starch, glycogen, and cellulose. Opinions differ, but 

 it appears that any arabinose or galactose obtained by hydrolysis of 

 pectin preparations comes from associated polysaccharides rather than 

 from the pectin itself. 



The manufacture of commercial pectin to aid the housewife in com- 

 pelling unwilling jellies to jell, or in making jellies from fruits that 

 contain little or no pectin, has become an industry of considerable pro- 

 portions. A well-known product of this kind is "Certo," which is made 

 from apple pomace. Dry pectin has recently been developed from 

 apples and lemons. On the basis of dry matter, apple pomace and 

 lemon pulp contain about 20 and 35 per cent, respectively, of pectin. 

 Rinds of "cull" lemons are used for this purpose and furnish a much 

 larger supply of raw material than can be utilized at the present time. 

 Sugar beet pulp contains on the dry basis about 25 per cent of pectin and 

 offers an almost unlimited supply of raw material for tho manufacture 

 of pectin. 



Sugar, acid, and pectin are necessary to form a gel. These three in- 

 gredients may be varied within rather wide limits, but a jelly of good 

 texture contains about 60-70 per cent sugar, 1-2 per cent acid (ex- 

 pressed as tartaric and equivalent to pH 3.2-3.5) and 0.5-1.0 per cent 

 pectin. 



Closely related to pectin is the acidic polysaccharide, alginic acid, which 

 is obtained from marine algae. Like pectin, it has the property of holding 

 large amounts of water in a colloidal gel. For this reason it is used, 

 in the form of its sodium salt, as a stabilizer in ice cream and other foods, 

 and in cosmetics. Because it is capable of forming hard, resistant, surface 

 films, it is also used in making special grades of paper, cloth, and printer's 

 ink. Chemically, alginic acid is composed of D-mannuronic acid residues 

 attached by /3-1,4-linkages in an unbranched chain structure. It is 

 remarkably resistant to hydrolysis, even when exposed to,. strong acid 



