4 BULLETIX 1099, U. S. DEPARTMENT OF AGRICULTURE. 



UNDERCOOLING. 



Tomatoes, along with many other fruits and vegetables, may be 

 undercooled to a considerable extent.^ Briefly, undercooling is the 

 cooling of the tissue, without actual freezing, below the true freez- 

 ing point. The freezing point is the temperature at which ice 

 crystals begin to form in the tissue, with subsequent injury to the 

 surrounding tissue. Experiments have shown that tomatoes will 

 often remain in this undercooled state for several hours without 

 injury provided they are undisturbed.* Six whole Sunrise tomatoes 

 were placed in a temperature of 22° F. In seven hours after passing 

 32° F. the specimens reached an average maximum undercooled 

 temperature of 22.63° F. Shortly after this, without being dis- 

 turbed in any way, freezing commenced, and the temperature rose 

 to the actual freezing point of 30.64°, where it remained for several 

 hours. Experience has shown that at any time after the tempera- 

 ture of a tomato goes below its true freezing point, or, in other 

 words, at any time it is undercooled it can be made to freeze by a 

 sudden jar. This fact is of practical importance to growers, and 

 especially to shippers, because if a shipment is known to have been 

 subjected for any length of time to a temperature below the freezing 

 point of tomatoes part or all of the fruit may be undercooled but 

 not frozen provided they have not been disturbed. If the tempera- 

 ture is raised above the freezing point within a limited time, it is 

 possible that the tomatoes will not be frozen. Tomatoes, however, 

 that are subject to a freezing temperature while actually in transit 

 will probably freeze with little or no undercooling, owing to the con- 

 stant jarring received from the vibration of the car or truck. It 

 is reasonable to assume that in general tomatoes in storage or at r^t 

 will undergo a lower temperature without injury than they would in 

 transit. 



Other observations were made on the freezing points of whole 

 fruits in an air bath in which the temperature and rate of fall of the 

 temperature could be accurately controlled. Dry tomatoes were 

 placed in the compartment and the temperature lowered gradually 

 in three hours from 50° to 32° F. This allowed the fruits to come 

 to the same temperature irrespective of their size. Water was then 

 sprinkled over them. AATien the temperature reached 30.9° the 

 water on the surface froze, but the tomatoes did not show frozen 

 spots until an air temperature of 30.3° F. was reached. Usually 

 much greater undercooling occurred with tomatoes with an unbroken 

 skin. The temperature then was maintained at 29° F. Freezing 



2 Spc Department of Agriculture Bulletins 895 and 916 for a discussion of undercooling 

 in potatoes (5 and 6). 



* .\ctual injury is not necessarily caused by the low temperature, but it directly follows 

 the formation of ice crystals within the cells or intercellular spaces. 



