26 MISC. PUBLICATION" 16 8, U.S. DEPT. OF AGEICULTURE 



This phenomenon — flowering the temperature of the tissue below its 

 freezing point without inducing the formation of ice crystals — ^is 

 known as " undercooling." If the temperature continues to fall, or if 

 the exposure to the low temperature is prolonged, this resistance to 

 freezing is overcome,^ the undercooling period is completed, ice 

 quickly forms in the tissues, and the temperature rises rapidly to the 

 approximate true freezing point regardless of the air temperature 

 around the apple. After ice is formed in the tissues a further 

 drop in the temperature of the surrounding air will be reflected in 

 the temperature of the fruit. Somewhat under-ripe apples are more 

 resistant to freezing than those fully ripe. 



SYMPTOMS OF FREEZING 



Many persons think that the presence of ice in an apple at the time 

 of examination is prima facie evidence of freezing injury. Theo- 

 reticall}^, they are right; practically, they may not be. The confu- 

 sion is due to a failure to distinguish between slight freezing and 

 " freezing to death ", in which enough ice is formed to cause perma- 

 nent and visible injury. Doubtless the least incipient ice formation 

 injures the apple flesh to some degree, but so far as present knowledge 

 goes there is no visible evidence of injury by such slight freezing 

 nor any effect upon the market value of the fruit. If, however, the 

 freezing process is carried somewhat further, a slight noticeable 

 injury results, even though the cells may appear practically normal; 

 if carried still further, the cells may be killed, in which event they 

 turn brown. This condition is described subsequently. But, regard- 

 less of how much ice appears in the fruit, unless a significant pro- 

 portion of the cells show this browning it is inaccurate to say that 

 the apple shows freezing injury. 



EKTEKNAL APPEIAKAIVTCB 



If freezing has been slight, there may be no marked external 

 symptoms of any sort; if it has been severe, the general outside 

 appearance of the apple is strikingly affected. The surface is dis- 

 colored in irregularly shaped areas — became so, in fact, very soon 

 after the apple thawed out — and appears considerably darkened. It 

 often assumes a water-soaked brown color closely resembling the 

 color of apple scald, or it may become much darker, in some cases 

 almost black. When apples are in a frozen condition the skin be- 

 comes slightly shriveled, but the shriveling usually occurs in the 

 form of a network of wrinkles rather than as parallel lines of shrink- 

 age such as are produced by normal evaporation. Careful measure- 

 ments have shown also that the fruit actually becomes smaller, 

 sometimes by as much as 10 percent of its original volume. On 

 thawing, it practically regains its original volume, unless the freez- 

 ing was very severe. 



When apples thaw after having been badly frozen the skin becomes 

 shriveled, particularly if the air in the storage place is very dry. 

 This form of shriveling seems to be due to rapid evaporation after 

 thawing of the water withdrawn from the cells and changed into 

 ice in the spaces between the cells during the freezing process. 



