POTATOES AND OTHER STARCHY ROOTS AS FOOD. 7 



ment that potatoes alone make a very incomplete diet; only a very 

 small amount of nitrogenous material would be obtained from 

 a quantity of potatoes sufficient to supply the body with all the 

 energy-yielding material required. 



The protein has been found to consist in part of a form of globulin, 

 for which the name tuberin has been suggested. Other nitrogenous 

 compounds present are a proteose, asparagin, and amino acids. 



The most important mineral matters found in potatoes are potas- 

 sium and phosphorus compounds. There are several organic acids 

 (such as citric, tartaric, and succinic acid 1 ), which vary in tubers of 

 different ages and account in some measure for the flavor of potatoes. 



The potato, like many other foods, also contains minute amounts 

 of physiologically active substance or substances valuable in nutrition 

 and commonly called vitamins. 2 



The question is often asked, why the outer surface of freshly pared 

 potatoes turns brown. According to experimental evidence this 

 change is due to the action of enzyms, or ferments, naturally present 

 in the plants. In the presence of the oxygen of the air they work 

 upon tannin-like bodies in the tuber in such a way that the latter 

 change color. This browning may be prevented by putting the pared 

 tubers into plain cold water or, better, into salted water. 



COOKING. 



When boiled, the temperature of the interior of the potato does 

 not exceed 212° F. (the temperature of boiling water). When baked, 

 the temperature of the interior of the potato reaches 212° F., but 

 does not exceed it, if cooked only until it is done. If overcooked, 

 the temperature may be considerably higher, which may account for 

 changes which cause the peculiar taste of overbaked potatoes. The 

 heat affects the various constituents of the potato in different ways. 

 The water expands into steam, part of which evaporates from the 

 surface. Within the minute cells making up the tuber it presses so 

 hard against the walls that the tough cellulose is ruptured, just as 

 any air-tight vessel may be broken by the pressure of expanding 

 steam. The starch grains inside the cells are thus released, some of 

 them being also disintegrated, while part are changed into the soluble 



1 In proximate analyses such as those given on page 5 these acids would be included 

 in the " carbohydrates," a matter of analytical convenience, not of chemical accuracy. 

 Proximate analyses, it should be remembered, are designed to show group characteristics, 

 and not the individual compounds which more detailed analyses would deal with. 



2 The vitamins are apparently in part dissolved in the juice and in part stored with 

 the starch of the cortical layer of cells. Exactly what is their nature and the part they 

 play in human nutrition is not yet fully understood, but recent investigations suggest 

 that they do important work in regulating cell metabolism and are possibly concerned in 

 the process of growth. It is not unlikely due to some of these compounds that potatoes 

 prevent scurvy on shipboard or in other circumstances where fresh fruits and vegetables 

 are not to be had. 



