FREEZING INJURY TO POTATOES WHEN UNDERCOOLED. 3 



It was shown that potatoes could be undercooled several degrees 

 below the true freezing point without actual freezing taking place. 

 In addition, it was shown that when once cooled below the true freez- 

 ing point the process of undercooling can be terminated at any point 

 by inoculating the tuber. The term " inoculation " 3 is used in this 

 connection to indicate any disturbance which will start crystalliza- 

 tion or ice formation. Following inoculation the temperature of the 

 potato rises quickly to the true freezing point, accompanied by the 

 formation of ice within the tissue. Inoculation is caused by some me- 

 chanical disturbance, such as a jolt or jar sufficient to cause freezing 

 to commence. It is a well-known fact that water, for example, can 

 be undercooled below its freezing point. After a certain amount of 

 undercooling actual freezing will result from any slight disturbance 

 or by the addition of a crystal of ice. 



In the potato, inoculation is readily accomplished by striking or 

 jarring the undercooled tuber. The degree of force necessary to 

 accomplish this is determined by the extent of undercooling. It 

 seems that a potato can be undercooled to such a point that the 

 slightest perceptible jar will instantly inoculate it. Water on the 

 surface or sap from a bruise will prevent a potato from reaching the 

 degree of undercooling it would attain if dry, since this free moisture 

 will freeze at a higher temperature than the living tissue and will in 

 this way inoculate the entire potato. Muller-Thurgau (5) and Jones, 

 Miller, and Bailey (4) found that a freshly cut potato will freeze 

 more quickly ; that is, it will not undercool as far as a dry potato. 



Muller-Thurgau was of the opinion that the undercooling point 

 varies with the air temperature to which potatoes are exposed. He 

 attempted to justify this view by data. In one instance freezing oc- 

 curred in two hours at an exposure of 23.9° F. and in another in five 

 hours at an exposure varying from 15.8° to 10.7° F. Jones and his 

 colleagues (4), commenting on these results, state that possibly the 

 earlier freezing at the higher undercooling temperature was due to a 

 more rapid fall in temperature in this case. Data submitted by them 

 and also in Department Bulletin 895, heretofore cited (9), indicate 

 that the degree of undercooling is dependent for one thing upon the 

 rate of fall of temperature. It is also entirely possible that a sudden 

 unnoticed jar might have caused earlier freezing where the under- 

 cooling temperature was higher. In Department Bulletin 895 (9, 

 p. 6) a diagram is presented in which results are illustrated when two 

 potatoes were exposed to different temperatures. One potato ex- 



3 It is considered that the term " inoculation " is rightly used here, as it is directly 

 comparable to the process in physical chemistry, known by the same term, in which 

 crystallization is started in a concentrated solution by adding a crystal of the solute or 

 when freezing is started by adding a crystal of ice to pure water that has been 

 undercooled. 



