COW'S MILK. 645 



Perfectly fresh amphoteric milk does not coagulate on boiling, but 

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

 rapidly re-forms after being removed. After a sufficiently strong spon- 

 taneous formation of acid it coagulates on 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, espe- 

 cially 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 sometime, 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 very 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 whose diameter is 0.0024-0.0046 rnm. and 0.0037 mm. as an average 

 for different kinds of animals. It is unquestionable that the milk-globules 

 contain fat, and we consider it as positive that all the milk-fat exists in 

 them. Another disputed question is whether the milk-globules consist 

 entirely of fat or whether they also contain protein. 



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

 Milk, Wisconsin Exp. Station, 6, 1892. 



