612 MILK. 



Perfectly fresh amphoteric milk does not coagulate on boiling, but 

 forms a pellicle consisting of coagulated casein and lime-salts, which 

 rapidly reforms after being removed. Even after passing a current of 

 carbon dioxide through the fresh milk it does not coagulate on boiling. 

 In proportion as the formation of lactic acid advances this behavior 

 changes, and soon a stage is reached when the milk, which has previously 

 had carbon dioxide passed through it, coagulates on boiling. At a second 

 stage it coagulates alone on heating; then it coagulates by passing carbon 

 dioxide alone without boiling; and lastly, when the formation of lactic 

 acid is sufficient, it coagulates spontaneously at the ordinary temperature, 

 forming a solid mass. It may also happen, especially in the warmth, 

 that the casein-clot contracts and a yellowish or yellowish-green acid 

 liquid (acid whey) separates. 



Milk may undergo various fermentations. Lactic-acid fermentation, brought 

 about by HUPPE'S lactic-acid bacillus and also other varieties, takes first place. 

 In the spontaneous souring of milk we generally consider the formation of lactic 

 acid as the most essential product, but a formation of succinic acid may also take 

 place, and in certain bacterial decompositions of milk, succinic acid and no lactic 

 acid is formed. The materials from which these two acids are formed are lactose 

 and lactophosphocarnic acid. Besides the lactic acids, the optically inactive 

 as well as the dextro and levo acids, and succinic acid, volatile fatty acids, such 

 as acetic acid, butyric acid, and others, may be formed in the bacterial decompo- 

 sition of milk. 



Milk sometimes undergoes a peculiar kind of coagulation, being converted 

 into a thick, ropy, slimy mass (thick milk). This conversion depends upon a 

 peculiar change in which the milk-sugar is made to undergo a slimy transforma- 

 tion. This transformation, which requires further investigation, is caused by 

 special micro-organisms. 



If the milk is sterilized by heating, and contact with micro-organisms 

 prevented, the formation of lactic acid may be entirely stopped. The 

 production of acid may also be prevented, at least for some time, by many 

 antiseptics, such as salicylic acid, thymol, boric acid, and other bodies. 



If freshly drawn amphoteric milk is treated with rennet, it coagulates 

 quickly, especially at the temperature of the body, to a solid mass (curd) 

 from which a yellowish fluid (sweet whey) is gradually pressed out. This 

 coagulation occurs without any change in the reaction of the milk, and 

 therefore it is distinct from the acid coagulation. 



In cow's milk we find as form-elements a few colostrum corpuscles 

 (see Colostrum) and a few pale nucleated cells. The number of these 

 form-elements is veiy small compared with the immense amount of the 

 most essential form-constituents, the milk-globules. 



The Milk-globules. These consist of extremely small drops of fat 

 whose number is, according to WoLL, 1 1 .06-5.75 millions in 1 c.mm., and 



1 On the Conditions Influencing the Number and Size of Fat-globules in Cow's 

 Milk, Wisconsin Exp. Station, 6, 1892. 



