204 DOMESTIC ANIMALS, DAIRYING, ETC. 



any noticeable change. In ordinary milk, however, the conditions 

 are different, in that many kinds of bacteria are growing together 

 and the milk is usually consumed before there is any marked change 

 in the flavor. 



When a number of different kinds of bacteria grow together, 

 as they usually do in milk, the development is not equal. One 

 variety finds the conditions of food or temperature or acidity more 

 suited to its peculiar habits of life and develops more rapidly than 

 other kinds. In a short time this rapidly growing form may so 

 change the milk, that, while the conditions are more favorable to 

 its own growth, they become less and less adapted to the needs of the 

 others. In the course of time this form crowds out all others and 

 an examination would show large numbers of this kind, while the 

 others originally present would have entirely disappeared, or would 

 occur only occasionally. It is in this way that the special fermenta- 

 tions develop. 



If milk shows a peculiar flavor when it is first drawn, it is safe 

 to say that the flavor is not produced by bacteria but by the cow, 

 usually through something in the feed. If, on the other hand, the 

 fresh milk is normal and the flavor develops as the milk stands, 

 it is usually due to bacteria. 



Sweet Curdling and Digestion. It sometimes happens that 

 milk curdles without showing the usual acid taste. This is followed 

 by the separation of a straw-colored whey, which slowly increases 

 until the curd has nearly all disappeared. This condition is the 

 result of a series of complicated changes brought about by bacteria. 



When milk is taken into the stomach, especially the stomachs 

 of young animals whose diet is largely or wholly milk, a certain 

 constituent of the digestive juice precipitates the casein, forming a 

 firm curd. This curd resembles the acid curd in appearance, but 

 differs from it chemically and has no sour taste. The part of the 

 digestive juice which produces this change is the rennet, with which 

 we are familiar in cheese making and in the junket tablets of the 

 kitchen. It is what is technically known as an enzyme, and while 

 it has some of the properties which we ordinarily attribute to living 

 beings, its action is purely chemical. 



In this precipitated condition the casein is not in a form to be 

 used by the animal. It must be so changed that it will go into a 

 solution and pass through the membranes lining the digestive tract. 

 This change is brought about by another enzyme, pepsin. Pepsin 

 changes the curd rapidly and completely into compounds soluble in 

 water. 



Bitter Milk. The distinct bitter taste which sometimes ap- 

 pears in milk may be caused by (1) certain weeds that the cow has 

 eaten, (2) an abnormal condition of the udder, (3) an advanced 

 period of lactation, or (4) the action of certain bacteria. It is prob- 

 able that the bacteria causing bitterness are not at all uncommon and 

 that they could be found in many lots of milk showing no bitter- 

 ness. Some of these bacteria form acid and sour the milk; the 



