8 CITY MILK SUPPLY 



known; some believe that they are secreted from the mammary gland 

 while others suggest that they may be derived from the bacteria of the 

 udder. 



The enzymes are destroyed by heat; most of them withstand 140 to 

 149F. for some time without material injury; most have their activity 

 weakened between 149 and 158F. and all of them are destroyed on 

 relatively short exposures to temperatures between 158 and 176F. 



The function of the enzymes in milk is not known. It is the theory 

 of some physicians that they assist in digesting milk, and they attribute 

 the supposed difficulty of digestion of boiled as compared with raw milk 

 to the destruction of enzymes by heat; so partly for this reason much 

 attention has been paid to the temperature at which the pasteurization 

 of milk is conducted. 



With regard to the several chemical constituents of milk, certain 

 facts are important. 



Water in Milk. Of them all, water is present in the largest propor- 

 tion, for it makes up about 87 per cent, of the milk. 



Butterfat. Butterfat is present in milk in the form of transparent 

 globules of which it has been estimated there are 100 million in a single 

 drop. In freshly drawn milk the globules are suspended in the serum 

 but on setting the milk they gradually rise to the surface and form cream. 



Butterfat is not a single chemical compound but a mixture of several 

 ethereal salts of glycerol called glycerides. It is not known how the 

 fatty acids are combined but probably three acid radicals are united 

 with each glycerol residue, thus: 



( C 4 H 7 2 



CaEU | Ci8H 33 02 Glyceryl butyro-oleo-stearate. 

 I CisHssC^ 



For convenience the composition of butterfat is often stated as though 

 each glyceride existed separately; that is as though the butterfat contained 

 so much butyrin, caproin, etc. There are about 10 of these glycerides 

 and butterfat contains about 12.5 per cent, of glycerides in combination 

 with acids which are divided into two groups. 



Group 1. Non-volatile and insoluble in water: palmitic, oleic, 

 myristic, stearic and lauric. 



Group 2. Volatile and soluble in water: butyric and caproic. 



Butterfat on an average contains about 40 per cent, of palmytin, 

 34 per cent, of olein, 10 per cent, of myristin, 6 per cent, of butyrin and 

 from 1 to 3 per cent, of other glycerides. The glycerides composing 

 butterfat have different melting points; consequently the melting point 

 of butterfat varies with the proportion of the several glycerides present 

 and ranges from 85.1 to 91.4F. The character of butterfat, also, is 

 determined by the proportions in which the glycerides exist. Palmytin 



