COMPOSITION OF MILK 195 



how filthily many cows are kept, it is not to be wondered that many 

 intestinal organisms find their way to milk. 



3. Bacteria concerned in fermentations in milk cannot well be 

 understood without some appreciation of the different elements of 

 milk which are most affected by the changes of fermentation. It is 

 therefore necessary, before proceeding, to consider shortly what are 

 the constituents of milk upon which living ferments of various kinds 

 exert their action, for without these facts the action of fermentation 

 bacteria is not evident. A tabulation of the chief constituents of 

 milk may be stated as follows : 



(1) Water 



Ordinary (2) Milk-sugar 



fresh milk = \ (3) Fat 



100 per cent. | (4) Proteids (casein, etc.) 



(5) Mineral matter . 



8 7 '5 per cent. 

 4'9 

 3-6 

 3'3 

 0-7 



lOO'O 



Or the average milk constitution may be expressed thus : 



Fat ...... 4-1 per cent. 



Solids not fat . . . . . 8*8 



Total solids . . . . 12-9 ,, 



Water ...... S7'l 



It is not necessary to remark that milks vary in standard, and the above figures 

 can only be taken as fair averages. 



Milk-sugar, or Lactose (C 12 H 24 O 12 ), is an important and constant constituent of 

 milk. It forms the chief substance in solution in whey or serum, and is a member 

 of the cane-sugar group. Milk-sugar is found in varying quantities in the milk of 

 mammals. About 5 per cent, is present in human milk, and somewhat less in that 

 of the cow. It is very resistant to fermentation by yeast, and therefore undergoes 

 alcoholic fermentation very slowly. It is not acted upon by rennet, pepsin, or 

 trypsin. But of all the sugars it is most readily acted upon by micro-organisms. 



Fat occurs in milk as suspended globules of varying size. It forms the cream, 

 and by churning is, of course, made into butter, though both cream and butter con- 

 tain other constituents besides fat. Lloyd has shown that it is the large globules 

 that form the cream, and he has also made observations-upon the size of fat globules 

 in relation to breed of cattle. The decomposition and breaking down of milk-fat by 

 fermentation is the chief cause of gross abnormalities of cream and the rancidity of 

 butter. 



The Proteids of Milk include casein, lactalbumin, and lactoglobulin. Casein is 

 by far the most abundant and the most important. When milk separates naturally 

 into its constituent parts the fat rises and the casein falls, leaving a clear fluid, the 

 milk plasma or serum, between the two substances. The changes set up in casein 

 by bacteria are various, and furnish a means of diagnosis. 



Mineral Matter. The ash of milk, obtained by careful ignition of the solids, 

 contains calcium, magnesium, potassium, sodium, phosphoric acid, sulphuric 

 acid, chlorine, and iron phosphoric acid and lime being present in the largest 

 amounts. 



We may now consider the fermentations of milk and the pathogenic 

 organisms associated with milk. 



