June. 1940] Experiments \\'itii Potatoes 27 



tested and were immersed in one of the solutions, preferably starting with 

 the solution in which the bulk of the tubers would barely float. Floaters 

 were then transferred to the solution of lower specitic gravity and sinkers 

 to solutions of higher specific gravity. Tubers are classified as having the 

 same specific gravity as the solution in which they barelv float." 



The Relation Between Specific Weight and Starch Content of Potato* 



In view of the relationship between the specific gravity and potato 

 quality, it seemed desirable to review the literature dealing with specific 

 gravity and starch content to see whether it would be possible to speak 

 of specific gravity, quahty, and starch content in one breath. 



German starch factories have used the specific gravity of potatoes for 

 many years to calculate the amount of starch in the wagon loads brought 

 in to the factory, and payment for the potatoes has been based upon such 

 determinations. Von Scheele, Svensson & Rasmusson (28) (1937), cite 

 the work of seven European workers who, as a result of chemical analyses, 

 published tables showing the relationship between starch content and spe- 

 cific gravity to be a straight line. Von Scheele and co-workers recalcu- 

 lated the relationship using a more accurate method for the determination 

 of starch, and they have compared their results with those of the other 

 workers. Von Scheele and co-workers used 540 samples of two kilos 

 each, covering several varieties and several soil types over a period of 

 four years. They found a correlation coefficient of -{-0.947 in the rela- 

 tionship between specific gravity and per cent of starch, and showed that 

 with the help of a calculated straight line, the specific weight could be used 

 as an accurate index for the percentage of starch present. The few values 

 that fell off the line were off by not more tlian one or two per cent. 



The following equation was given as the regression line expressing 

 the relationship between starch content and specific gravity. 



Y = 17.564+199.07 (X - 1.09879) 

 where Y=Starch content X = Specific gravity 



An admirable review of the literature of starch chemistry up to 1928 

 has been made by Walton (30) (1928). The references which he cites in- 

 dicate that European workers have known of the close correlation between 

 starch content and specific gravity for years. The most common method 

 for the determination of the specific gravity of potatoes appeared to be 

 based upon the well-known Archimedes' principle whereby the tubers are 

 first weighed in water and then in air. This same method evidently is 

 now used in the German starch factories (c.f. \"on Scheele). 



However, several investigators suggested the use of several salt solu- 

 tions with different specific gravity. The tubers either float or sink in the 

 solutions, and the specific gravity of the solution in which the tuber neither 

 floats nor sinks is also the specific gravity of the tuber. Schultze (20) 

 (1871) placed a number of potatoes in a concentrated solution of sodium 

 cliloride and diluted the solution until one-half of the potatoes sank to the 



*The authors are indebted to W. H. Lyford, Jr., of the New Hampshire Experiment Station 

 for the review of the literature of relationships between potato starch and specific gravity and for 

 the starch determination presented in Table XXVII. 



