NUTRITIVE VALUE OF POTATOES 5 



Table 3. — The Carbohydrate Content of Cooked Potatoes as Served (32) 



New Old 



Carbohydrate Fraction Potatoes Potatoes 



Percent Percent 



Total reducing sugars 16.1 19.9 



Pentose sugars 0.2 0.37 



Non-fermentable sugars — 1.2 



Available Carbohydrate 15.6 19.2 



Pectin 



The pectin present in the potato differs in its physical and chemical properties 

 from that found in most fruits. The methoxyl content is low and the viscosity 

 of aqueous solutions is less than that of fruit pectins. Potato pectin is useless 

 in jelly making. 



Some investigators have suggested that mealiness in potatoes is caused by 

 the separation of cells brought about by the solution of pectic materials. In 

 studies conducted at this station, Freeman and Ritchie (15) determined the pectin 

 content of four varieties of potatoes and observed its change during storage and 

 as a result of baking. They found that raw potatoes contained from 20.0 to 26.8 

 milligrams of calcium pectate per gram of dry sample. No loss in pectin occurred 

 during six months' storage at 1.7° to 4.4° C. (35° to 40° F.). In most cases there 

 was a small loss of pectin as a result of baking. These authors suggest that potato 

 pectin can be adequately defined by two fractions: 



1. A fraction that is soluble in ammonium citrate (or oxalate) and 

 insoluble in hot water. This fraction may be pectic acid or insoluble 

 salts of the acid. 



2. A fraction that is easily dispersed by hot water but not by cold 

 water. Since this fraction is so easily removed, it is possible that it is 

 not the postulated cellulose-pectin complex of protopectin but rather a 

 lyophilic gel that is readily peptized by warm water. 



They conclude: 



Analyses of water-soluble pectin fractions of raw and cooked potatoes 

 offer additional evidence that the solution or degradation of pectic material 

 does not determine mealiness in potatoes. 



Protein 



The protein of the potato is chiefly a plant globulin known as tuberin (24). 

 The potato is not an important source of protein because it contains only a small 

 amount and even that is of relatively low biological value. Potato protein is 

 78 percent digestible and has a biological value of 67 and a protein value as a 

 food of 0.8 (36). 



The White House Conference of 1932 (54) suggested that sources of protein 

 be grouped according to the percentage of the total calories that presumably 

 should be provided by them if these proteins are to be the only ones in the diet. 

 On this basis potatoes rate as a fair source, providing 12-20 percent of the total 

 calories. However, the proteins are adequate for maintenance but not for growth. 



I 



