4 BULLETIN 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° IF. 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 1s 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 rest 
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. When 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 
3’ See Department of Agriculture Bulletins 895 and 916 for a discussion of undercooling 
in potatoes (5 and 6). 
* Actual injury is not necessarily caused by the low temperature, but it directly follows 
the formation of ice crystals within the cells or intercellular spaces. 
