COMPOSITION OF MILK AND CREAM 43 



The solids make up the remainder of milk, amounting to about 

 13 per cent, and comprising the substances we have just enumer- 

 ated, proteids, fat, sugar and mineral matters. The ash or min- 

 eral matters consists of salts of lime, potash, magnesium, sodium 

 and iron in combination with chlorine, sulphuric and phosphoric 

 acids. Omitting the mineral matters or salts, we may, in a general 

 Avay, remember the proportion of proteids, fat and sugar as four 

 per cent, of each, the percentage of proteids being slightly below 

 and that of sugar slightly above these figures. The fat is the only 

 one of these constituents which varies greatly and this indeed varies 

 tremendously (from 1.5 to 13. per cent.) and owing to a great 

 variety of circumstances which will be noted. 



If the fat is all removed from milk — which can practically be 

 clone with the separator- — we have left the skim milk, which is 

 composed of the proteids, sugar and water. The sugar is of a 

 kind peculiar to milk and therefore called milk sugar. It is found 

 in no other substance and is not nearly so sweet as ordinary or 

 cane sugar. The souring of milk is due to fermentation of milk 

 sugar which takes place through the action of certain germs (lactic 

 acid germs), which we have already mentioned and which are 

 always present in the cleanest market milk. Since lactic acid bacilli 

 enter the milk in dirt, dust, etc., outside of the cow, it is wholly pos- 

 sible to prevent them from getting into milk if milk is withdrawn 

 from the udder through a tube into- a clean, closed vessel and does 

 not come in contact with the air. In this case natural souring will 

 not occur. These germs lead to the breaking up (fermentation) 

 of the sugar in milk into lactic acid (or milk acid). Milk sugar, 

 sold in the shops, is made from sweet whey — a bye-product in 

 cheese making. We have accounted for the sugar in the skim milk ; 

 we have left for consideration the proteids and mineral matters. 

 The proteids are of three kinds: Casein (or caseinogen), Albumin 

 and Globulin. Casein forms nearly four-fifths of the proteids (3 

 per cent, of milk) and is that part of milk which makes the curd 



