30 



from the dust of the air. Under the most cleanly conditions large num- 

 bers of bacteria are found in milk, while when the conditions are re- 

 verse, the number is marvellously increased It may be said that one 

 twenty-eighth of an ounce of milk has been found to contain from 6,000,- 

 000 to 170,000,000 bacteria. Bacteria develop most rapidly at a temper- 

 ature of 100° F. and their development is cheeked the nearer the temper- 

 ature is kept to the freezing point. 



Changes in Pure Milk. 



Impure milk is simply the result of^acterial action. 



Sour Milk. — One class of bacteria feeds upon the milk sugar of the 

 milk, and as a result lactic acid is formed. This acid gives the milk 

 its sour taste, and causes at the same time the casein or curd to separate 

 out. The dirtier the animals, stables, milker and milk vessels, the 

 larger the number of bacteria that will find their way into the milk, and 

 the more quickly will the milk become sour. Milk sours more rapidly 

 in hot weather, because the temperature is better suited to the rapid 

 development of the bacteria. 



Bitter rni/.k is sometimes due to improper food, or condition of the ani- 

 mal, and also to certain bacteria, which, having gained access to the 

 milk, attack the casein and decompose it, producing butyric acid, pep- 

 tones or other substances. 



Ropy or stringy milk is also caused by bacteria, which render the milk 

 more viscous, or cause it to adhere to anything that touches it, drawing 

 out into threads of considerable length. 



Rtd nnlk is sometimes caused by the actual presence of blood in the 

 milk, due to a wound in the udder or to the effects of certain feeding 

 stuffs. An excess of ensilage has been claimed to produce a bleeding 

 of the udder. Most frequently red milk is due to the presence of bac- 

 teria {Bacillus 2)rodigiosus). Its growth in the milk is accompanied by 

 the production of a coloring matter, especially near the upper surface of 

 the milk. 



Blue iiiilk is the result of the production of a blue pigment of bacillus 

 cyanogenus Blue jjatches are sometimes noticed on the surface, or the 

 whole surface may become covered with a blue coating 



Soapy milk is occasionally observed, also occasioned by a specific 

 bacteria. 



There are forms of bacteria which produce various gases in milk, as 

 well as alcohol and similar substances. Milk being a favorable medium 

 for the growth of all kinds of bacteria, it occasionally happens that the 

 bacteria producing typhoid fever, diphtheria and similar contagious 

 diseases gain access to milk, and result in the milk becoming a trans- 

 mitter of these most serious diseases. 



The bad odors that arise in the stable, and not infrequently in the dairy 

 house, are caused by bacteria which are at work decomposing various 

 kinds of filth found there. 



The lactic acid producing bacteria are by far the most numerous in 

 milk, and hence the most ordinary change in milk consists in the process 



