22 CIRCULAR 713, U. S. DEPARTMENT OF AGRICULTURE 



Another, the net type, is marked by more or less blackening of the 

 vascular ring and the finer strands which extend from it into ad- 

 jacent tissue. Both types are frequently restricted to the stem 

 end. Finally, there is a blotch type, marked by irregular patches 

 ranging in color from an opaque gray or blue to sooty black. These 

 patches occur everywhere in the tuber, though most generally in 

 the vascular ring and the cortex. When these blotches are in the 

 cortex they may be apparent externally in clean tubers with white 

 skins. This is the only type of freezing necrosis that may be vis- 

 ible externally. If tubers affected with any or all of these types of 

 freezing necrosis are held in storage, they generally shrivel or wilt 

 more than unaffected tubers. Excessive shriveling alone, how- 

 ever, cannot be relied upon as a sign of freezing necrosis. 



It is popularly believed that the symptoms here described as 

 freezing necrosis are not due to freezing but to chilling, since 

 freezing, it is contended, is always followed by a complete break- 

 down after thawing. This is not the case. Freezing necrosis is as 

 truly a freezing injury as the complete type, and it does not occur 

 unless ice is formed in the tissues. The difference lies in the fact 

 that not all cells of the tuber, but only those that become discolored, 

 have been killed. 



Rutabagas 



experimental observations 



Rutabagas, with starting temperatures of about 36° F. and 64°, 

 in bushel bags were kept in a 25° room for 29 and 31 hours, re- 

 spectively. A third bag with a starting temperature of about 57° 

 was held for 24 hours in an 18° room. After thawing at 45° for 

 5 days neither the lightly frozen nor the severely frozen lots 

 showed injury of any kind. When the roots were cut while still 

 frozen there was a water-soaked ring about one-quarter to one- 

 half inch deep around the outside, but this disappeared after the 

 roots had thawed. A second freezing at 18° for about 24 hours 

 caused no detectable injury. 



For a discussion of the rate of cooling of two other lots of ruta- 

 bagas see pages 26 and 27. The rate of cooling of one of these is 

 shown in figure 3. 



MARKET OBSERVATIONS 



Severely frozen roots remain fairly firm but are water-soaked 

 and light brown. Free water runs from cut surfaces. A pro- 

 nounced mustard odor is given off. 



There are considerable variation among different lots of ruta- 

 bagas and considerable difference of opinion among market 

 handlers as to the importance of freezing injury in this crop. The 

 situation can be summed up as follows : Most lots show no damage 

 after thawing, but some become flabby and are predisposed to 

 decay. Some dealers believe that one or two freezes do not cause 

 injury but that more do; others believe that the extent of the in- 

 jury depends on the temperature and on whether the roots are 

 handled while frozen and soon after thawing. 



