GENERAL CHEMISTRY OF MILK 129 



water, but it does not freeze completely, according to Hastings, 

 since the water freezes first and the solids form a more highly con- 

 centrated solution with depression of the freezing-point. The 

 water freezes at first at the outside on the wall of the vessel; the 

 solids are forced toward the center; a more concentrated solution 

 is formed, and this freezes at lower temperature. The fat rises 

 and is partially churned when the milk freezes. The natural 

 emulsion of fat is never completely restored after thawing and the 

 casein appears in flakes rather than in the original colloidal condi- 

 tion. The fat content of the upper layers may be three times as 

 high as the original amount and much higher in the central por- 

 tion of the milk than at the periphery. The emulsion of fat is 

 destroyed more rapidly than the colloidal condition of the casein. 

 Milk which has been frozen and then thawed is said to decompose 

 more rapidly than normal milk. 



Analyses of frozen milk are given by Mai, and the following 

 table gives some of his figures. The degree of acidity is expressed 

 in degrees or that amount of one-fourth normal alkali required 

 to neutralize 100 c.c. of milk. For convenience the writer has 

 calculated the acidity in lactic acid. 



INFLUENCE OF FREEZING ON THE COMPOSITION OF MILK 



Specific 

 gravity. 

 Sample 1: 



Original milk 1.0317 



Upper loose ice 1 .0233 



Solid ice wall 1.0165 



Liquid part 1 . 0534 



Reunited 1.0321 



Sample 2: 



Original milk 1.0312 



Crystals from sieve 



Liquid part 1.0352 



Solid ice 1.0172 



Reunited 1.0320 



Sample 3: 



Original milk 1.0318 



Upper loose ice 1 .0256 



Liquid part ... 1 . 0534 



Hard ice 1.0201 



Reunited 1.0320 



The table shows that the upper layers have lower specific 

 gravity than the original milk, but considerably more fat, while 

 the solids have not changed materially. The central hquid 

 portion has a much higher specific gravity than the original 

 milk, much higher sohds, and somewhat less fat. The thawed 

 reunited milk differs from the original milk chiefly in having 

 higher acidity. Since the mineral constituents of the milk go 

 to the central part which remains liquid longer than the other 

 parts of the milk, the acidity of this part is somewhat higher than 

 that of the original milk. 



The changes indicated by chemical analysis do not show the 

 above-mentioned physical changes in the fat. Milk which has 



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