128 MILK 



9 and 10. Differences in composition of milk due to the health 

 of the animal and to dilution of milk are discussed in other 

 connections. 



Changes in Composition of Milk 



After milk has left the udder rapid physical changes occur and 

 decomposition sets in due to the action of micro-organisms. While 

 in the udder, the decomposition of milk proceeds slowly, as there 

 are forces at work which restrain growth of bacteria. Further- 

 more, gases do not escape, and the rising of the fat is slow, since 

 the milk is more or less in constant motion. As soon as the milk 

 has left the udder conditions are materially changed. Micro- 

 organisms multiply rapidly, causing profound changes in com- 

 position; the gas escapes in large quantity and the fat rises. Be- 

 sides these conditions milk may be exposed to freezing tempera- 

 ture, heating, centrifugation, fete. For convenience these causes 

 may be grouped as follows: 



1. Spontaneous changes. 



2. Freezing. 



3. Heating. 



4. Centrifugation. 



5. Dialysis. 



6. Electricity. 



7. Micro-organisms and enzyms. 



Spontaneous Changes. — When milk stands in contact with the 

 air an exchange of gases takes place, as has been stated before. 

 By absorption of oxygen and nitrogen and release of carbon dioxid 

 an equihbrium with the air is estabUshed. The loss of gases in- 

 creases the specific gravity, while the loss of carbon dioxid reduces 

 the acidity. This loss amounts to about 0.5 N.NaOH per liter. 

 Further, some soluble phosphates are rendered insoluble by the 

 loss of carbon dioxid and are precipitated, but remain suspended. 

 Milking machines depending upon suction remove the gases rapidly 

 and more completely than mere intercourse with the air. 



The rising of the fat globules is also a spontaneous change, but 

 since this has been fully discussed previously it is only necessary 

 to mention this phenomenon here for the sake of completeness. 



Freezing of Milk.— Milk may freeze accidentally or may be 

 frozen artificially to preserve it for transportation. Milk as it 

 freezes undergoes profound physical changes which are more pro- 

 nounced the longer it remains frozen. Under no conditions is 

 thawed milk exactly the same in every respect as unfrozen milk. 



The freezing-point of milk is slightly below that of water, as 

 has been stated before. This is due to the presence of sohds in 

 solution. Milk freezes about 0.55° C. below the freezing-point of 



