MILK AND ITS PRODUCTS 171 



nisl.es a source of heat and energy for the body. When acted 

 upon by certain bacteria, a portion of it changes into lactic 

 acic and makes the milk sour. The lactic acid unites with 

 the iime in the casein. This results in precipitating the casein 

 as 1 [ie curd of sour milk. 



When milk is used for butter making, the greater part of 

 the sugar goes with the skim-milk, another part with the 

 but ermilk, and only a very minute quantity into the but- 

 ter. In cheese making a very small amount goes into the 

 che< se and the remainder into the whey. 



158. Mineral Matter or Ash. This is the portion remain- 

 ing if milk solids are burned. It varies little in quantity 

 or composition. It seldom falls below 0.6 or exceeds 0.85 per 

 cent. It is composed largely of potassium, calcium, and 

 phosphorus, with smaller quantities of several other ele- 

 meri ts, including iron and sodium. When milk is used as food, 

 the ash serves to furnish material for the bones and to supply 

 other necessary demands for mineral matter in the body. 



159. Color of Milk. The white color is due to some 

 extent to the fat, but mostly to the casein. The yellowish 

 color observed to some extent in milk is associated with the 

 fat. This yellow coloring matter is carotin. Its source is 

 the plants used by the cow for food. 1 This pigment is 

 found along with the green pigment in growing plants. It 

 passes from the stomach through the circulation of the cow 

 and into the milk-fat in an unchanged condition. When the 

 feed is low in coloring matter, as for example dry hay and 

 grain, the color of the milk-fat is reduced, and the butter 

 may appear almost white as it often does in winter. The 

 yellow coloring matter has no food value, neither does it 

 give any taste to the milk or fat. The preference for yellow 



1 Missouri Agricultural Experiment Station,' Research Bulletins 9, 10, 11, 12. 



