2 BULLETIN 916, U. S. DEPARTMENT OF AGRICULTURE. 



PHYSIOLOGY OF THE FREEZING PROCESS. 



The frost necrosis of potato tubers is described by Jones, Miller, 

 and Bailey (4) 2 as being characterized by three types, viz, ring, net, 

 and blotch. In the ring type only the vascular ring, an area near the 

 surface, is discolored. In the net type threadlike areas radiating 

 from the center are discolored. The third type is marked by irregular 

 blotched areas. Ordinarily no further change takes place after the 

 injured potato thaws, although in the most severe cases the tubers 

 soon begin to break down if kept warm. 



Frost injury seems to be the result of actual ice formation within 

 the potato tissue. Abbe (1) , in his investigations of the effect of freez- 

 ing upon plant tissue, considers that as the tissue cools water exudes 

 from the cells into the intercellular spaces. After sufficient under- 

 cooling this water freezes. The concentrated sap remaining in the 

 cells will not freeze until cooled below the freezing point of water. 

 On thawing, this intercellular water escapes by transpiration and the 

 plant wilts. Goppert (3) and Sachs (6) both observed the presence 

 of ice within the cells and the intercellular spaces of plant tissue. 

 However, Sachs found crystals of ice usually present in the inter- 

 cellular spaces. Miiller-Thurgau (5) was one of the first to publish 

 (1880) upon frost injury of potatoes in his investigations upon the 

 freezing point of plant tissue. The phenomena of undercooling were 

 investigated by him to a considerable extent. He showed that plant 

 tissue required undercooling below the true freezing point before 

 actual freezing commences. He also showed that the expressed sap of 

 the potato tuber freezes at a higher temperature than the living tis- 

 sue. The ultimate freezing point of potatoes is placed at 30.21° F. by 

 Miiller-Thurgau and between 28° and 26° F. by Appleman (2). 

 Vaughan and Miller (7) state that "actual freezing does not begin 

 in potatoes until the temperature drops below 28° F." Jones, Miller, 

 and Bailey (4) place the freezing point at 28.4° F. 



The author of an unsigned article in the Potato Magazine (8) states 

 that exposure at 30° F. for 9 or 10 hours or at 16° F. for 1 hour 

 will develop signs of frost necrosis. In results published by Wright 

 and Harvey (9) the freezing point varied from 29.67° to 28.13° F. 

 according to variety and season. In this investigation the freezing 

 points of 18 different varieties were determined. The freezing 

 points varied in different varieties and according to family groups. 

 The early and medium-early potatoes froze at a higher point than 

 the purely late varieties. The subject of undercooling was discussed. 

 It was stated that potatoes, since their sap consists of salts, sugar, and 

 other soluble material, freeze at a lower temperature than pure water. 



2 The serial numbers in parentheses refer to " Literature cited " at the end of this 

 bulletin. 



