4 MASS. EXPERIMENT STATION BULLETIN 390 



Potatoes from one locality may differ significantly in starch, dry matter, 

 protein and ash from those of another locality. Varieties may differ 

 significantly in starch, dry matter, protein and ash. A significant differ- 

 ence in dry matter, protein and ash between years has also been obtained. 

 Irrigated potatoes are higher in starch and dry matter, but lower in pro- 

 tein than dry-land potatoes. 



It has been reported by Hayne (17), McClendon, Barrett, and Canniff (33), 

 and Freas (14) that there is a marked variation in the iodine content of potatoes, 

 which appears to vary with the iodine present in the soil in which they are grown. 

 The iodine content of potatoes and other vegetables in relation to its presence 

 in the soil is of particular interest in considering the incidence of goiter. 



Storage 



The temperature at which potatoes are stored has a marked effect on their 

 relative starch and sugar content. Cool storage temperatures favor the con- 

 version of starch to sugar in the potato tuber, and warm storage temperatures 

 favor the reverse reaction. In a recent study Wright et al. (57) found that the 

 sugar and starch content of potatoes stored at 50°-60° F. remained practically 

 unchanged, whereas the sugar in potatoes stored at lower temperatures progres- 

 sively increased and the starch decreased. Denny and Thornton (10) reported 

 large differences in the amount of reducing sugar present in different varieties of 

 potatoes both before and after storage at different temperatures. Low storage 

 temperatures caused an increase in the reducing sugar. 



In a discussion of the storage of potatoes Barmore (2) believes it probable 

 that there are only two reactions which are responsible for their sugar content: 

 respiration, or the oxidation of sugar to carbon dioxide and water; and second, 

 the transformation of starch to sugar and vice versa. When the temperature of 

 storage is reduced, respiration is retarded and this effects a shift in the starch- 

 sugar equilibrium so that the sugar concentration is increased. At higher storage 

 temperatures these reactions are reversed and the sugar is slowly decreased with 

 an increase in starch. 



The question of sugar content is particularly important from the standpoint 

 of making potato chips from stored potatoes. The presence of reducing sugar is 

 responsible for objectionable brown colors in pota'-o chips (10), and when potatoes 

 are to be used for this purpose, storage conditions should be controlled so as to 

 keep the sugar content low. 



THE CARBOHYDRATE, PROTEIN, AND MINERALS OF THE POTATO 



Carbohydrate 



As mentioned in the introduction, potatoes are generally thought of as an 

 important source of energy. The caloric or energy value of potatoes is due almost 

 entirely to their carbohydrate content. McCance and Lawrence (32) have 

 reported a comprehensive investigation to distinguish between the available and 

 unavailable carbohydrate of cooked foods. Their information is important in 

 that it actually evaluates the available carbohydrate content of foodstuff's as 

 they are served. The data for potatoes are summarized in Table 3. "Total 

 Reducing Sugars" includes starch, sucrose, and other carbohydrates which may 

 be hydrolyzed with acid to reducing sugars, as well as reducing sugars them- 

 selves. Practically all of the carbohydrate of the potato is available as a source 

 of energy. 



