102 MILK 



porcelain filter. Iron is present in cow's milk, according to Edel- 

 stein and Csonka, in amounts of 0.4 to 0.7 mgr. per liter, or an 

 average of 0.5. 



The salts are essential for the food of the young. There is 

 a balance between the salts and the other constituents of milk, 

 and any cause which affects the one also affects the other. The 

 condition of the salts is changed by loss of CO2, heating, formation 

 of acid, diseases of the animals, food, lactation period, etc. Ren- 

 net action is influenced by the salts in milk. These changes will 

 be discussed later. The salts sometimes produce almost an al- 

 kaline reaction in milk derived from aged or diseased cows, and 

 rennet then fails to coagulate the casein. 



The salts are indirectly derived from food and drinking-water. 

 Some salts pass through the mammary gland and reappear in the 

 milk. Sodium sulphate, if given in large quantity with the food, 

 can be recovered from the milk, while sodium chlorid passes 

 through only in small quantity. 



Rich milk does not necessarily contain more mineral matter 

 than poor milk; in fact, sometimes the reverse is true. 



Gases in Milk 



The gases found normally in milk are oxygen, nitrogen, and 

 carbon dioxid. The total amount of gas in fresh cow's milk is 

 about 7 to 9 volume per cent.; in human milk much less — about 

 one-tenth this amount. Since exposure causes milk to readily 

 absorb gases from the air, it has long been the custom to remove 

 gases by aeration. Animal odors and taints can be largely re- 

 moved by aeration, but it is clear that milk obtained under cleanly 

 conditions will not need aeration, since the source of taints is 

 removed. Gases may originate from bacterial activity in milk, 

 and this also can be prevented in a measure by cleanly methods 

 of milking. It is obvious that presence of undesirable gases largely 

 influences the quality not only of the milk but also of milk prod- 

 ucts, such as butter and cheese. As a matter of fact, cheese fre- 

 quently suffers in quality through the action of undesirable gas- 

 forming bacteria. 



Whether the methods of aeration in vogue are effective is a 

 question which has been exhaustively investigated by Marshall, 

 who also gives a complete review of the Hterature on the subject. 

 He analyzed the gas formed by absorption of the carbon dioxid 

 with potassium hydrate, the oxygen by alkaline pyrogaflate, and 

 the remnant was estimated as residual gas. This residual gas is 

 usually considered to be nitrogen, but Marshall thinks that this 

 assumption is not justified, since analyses have not been made. 



