GENERAL CHEMISTRY OF MILK 71 



on the proteins and carbohydrates. Possibly amino-acids are also 

 bacterial decomposition products. Milk direct from the glands 

 — that is to say, milk which has not come in contact with micro- 

 organisms — ^is difficult to obtain in sufficient quantity for satis- 

 factory chemical analyses. Consequently, such analyses have 

 rarely been made, and it may never be known exactly just what 

 the original mammary secretion is composed of. 



Sometimes the mammary glands permit soluble chemicals or 

 volatile substances, derived from odoriferous food, to pass- into 

 the milk. Chemicals given for therapeutic purposes have been 

 recovered in the milk and aromatic substances derived from the 

 food have imparted their flavor and odor to the milk. The taste 

 of silage is said to be communicated to milk when given shortly 

 before milking. However, observations on these points are not 

 always above criticism. Milk, especially fresh warm milk, rap- 

 idly absorbs odors from the air, and the presence of odoriferous 

 food in the stable maj^ impart its peculiar flavor to the milk. 

 Furthermore, it has been shown that there are bacteria able to 

 produce odors closely resembling those of aromatic foods. There- 

 fore caution should be used in accepting statements in regard to 

 the transmission of aromatic substances through the mammary 

 glands. 



Milk-fat, as has been stated, is in the form of an emulsion; 

 some of the proteins — namely, casein, globuhn, and mucin — are in 

 colloidal solution; the other constituents are in true solution. 

 When the fat is removed there remains a fluid containing the pro- 

 teins, sugar, salts, and water. This fluid is known as milk plasma. 

 When both fat and casein are removed a straw-colored fluid re- 

 mains which contains all the soluble constituents. This is the 

 milk-serum or whey. 



The most important proteins are casein and lactalbumin. 

 Besides these there is a globulin and traces of other proteins, 

 which, however, may be decomposition products of the original 

 milk proteins, or the results of reactions caused by reagents used 

 for analysis. There is but one carbohydrate in mflk, namely, 

 milk-sugar or lactose. The statement is sometimes made that 

 traces of dextrose are also present, but this is not reliable, as will 

 be shown later. The fat is composed of a large number of non- 

 volatile and volatile fatty acids combined with glycerin. The 

 mineral matter contains the elements of which the salts in milk 

 are composed. The gases are CO2, N, and 0. The foflowing 

 analyses of cow's milk are given by Kastle in BuUetin 56 of the 

 Hygienic Laboratory: 



