20 CLEAN MILK 



they tend to break up complex, natural constituents in milk and its. 

 products into simpler bodies. The commonest germs in milk — as- 

 we have noted — are those causing souring of milk; they are in- 

 variably present and are about the only kind existing in very clearL 

 milk. They act to ferment or change the natural sugar of milk 

 into an acid (lactic acid), and if they occur in large numbers a few- 

 hours after milking it is a sign that the milk has not been properly 

 cooled and will sour early. Lactic acid germs, or those producing- 

 souring of milk, besides being the most common, are of most im- 

 portance in their influence on milk and its products. They exist in 

 very small numbers in milk soon after leaving the cow, but as 

 they grow more readily than all other germs in milk at favorable, 

 temperatures (above 50 and better over 70 ° F.), they often con- 

 stitute almost 50% of all the germs in twenty-four hours. While, 

 after this time, they gradually crowd out the different varieties- 

 of competing germs until they produce so much acid that the 

 milk or cream sours and curdles, and they have multiplied so rapidly 

 and have made the milk so unfavorable for other germs that they 

 form from 90 to 99% of all the germs present. This is a 

 most favorable occurrence, because the flavor of most butter and 

 cheese is chiefly dependent on the action of the lactic acid germs, and 

 in their growth they protect the milk from the action of miscellan- 

 eous germs which would spoil these products. 



Even to man the growth of the lactic acid germs is a favorable 

 happening, as they are not harmful to adults in themselves and 

 tend to check the development of other harmful germs in the di- 

 gestive canal. Indeed Metchnikoff, perhaps the most celebrated 

 living authority on the action of germs on the body, believes that 

 lactic acid germs in sour milk constitute one of the best agencies for 

 prolonging life. The acidity of sour milk is, however, harmful to 

 children and may cause vomiting, etc. As we have pointed out, 

 heating milk to 155° or 165" F. readily kills the lactic acid germs. 

 Therefore such milk does not sour, but is changed by the action of.' 



