18 
BULLETIN 1041, U. S. DEPARTMENT OF AGRICULTURE. 
four varieties were opened and physical tests made to determine their 
relative plasticity. 
Not having available a suitable plastometer, the resistance of the 
material to pressure was measured by a simple device constructed and 
employed as follows: To the upper end of a plunger made from a 
piece of polished brass rod half a square centimeter in cross section 
a small weight pan of sheet tin was attached by means of a drop of 
solder. At just 2 centimeters from the other end of the plunger, 
which was carefully squared off perpendicularly to the axis, a file 
mark was made. This plunger, which was held in a vertical position 
by a glass sleeve supported in a ring stand clamp, was so arranged 
that the test can of material could be placed under it and the end of 
the plunger lowered to the surface of the material. 
To carry out the test the entire end of the can was removed, the 
plunger lowered to the surface of the test material, and weights added 
to the weight pan until the plunger penetrated the material. The 
sum of all the weights (weight of the plunger plus the added weights) , 
expressed in grams, required to push the plunger into the test ma- 
terial up to the file mark in just 1 minute was taken as the factor 
expressing the relative plasticity of the samples. Table 2 gives the 
figures thus obtained. They represent averages of many tests made 
upon both ends of the cans of material tested. 
Table 2. 
-Relative plasticity of siveet potatoes canned in the form of pie stock 
during the various stages of handling. 
Plasticity factor (grams). 
' 
Freshly 
dug. 
After curing for 10 days at 35°F. 
Variety. 
No stor- 
age 
period. 
Subsequent period 
of storage at 55° 
to 65° F. 
10 days. 
20 days. 
Big-Stem Jersey 
230 
92 
104 
216 
94 
71 
30 
20 
87 
34 
22 
25 
85 
Southern Queen 
20 
Nancy Hall . . 
17 
Porto Rico ". 
31 
From the above it will be seen that at the time of digging all four 
varieties gave a relatively firm product. The Big-Stem Jersey and 
Porto Rico were especially firm. The Nancy Hall was somewhat 
softer, but still quite firm, while the Southern Queen ranked slightly 
lower. The figures obtained at the other stages of handling are very 
interesting. After curing, the Big-Stem Jersey lost much of its 
firmness but was still quite resistant to penetration. During the 
storage period there was a slight gradual decline, but this was not 
