88 Dairy Bacteriology. 



also found in the barn dust and on cultivated plants. 

 Under ordinary farm conditions, the larger part of 

 those found in milk come directly from the utensils. 

 If the milk is drawn under extremely clean conditions 

 and care is taken to sterilize the utensils, but few acid- 

 forming bacteria of any kind will enter the milk ; under 

 such conditions most of the acid-forming bacteria will 

 belong to the group in question. They find, however, 

 such favorable conditions for growth in milk that they 

 develop more rapidly than most other types with which 

 milk becomes seeded ; consequently under normal con- 

 ditions, they gain the ascendency and so control the 

 type of fermentation. 



The desirable type of acid-forming bacteria do not 

 form spores; hence, are easily killed by heating the 

 milk. They can grow in the presence or in the absence 

 of free oxygen. In the bottom of a can of milk or in 

 the middle of a cheese, there is no air, yet these bacteria 

 grow as well under these conditions, as in milk exposed 

 to the air. The range of temperature for growth varies 

 from 50 to 100 F. but development is most rapid at 

 90 to 95 F. and about 1 per cent of acid is formed. 



Another group of bacteria which may be classed 

 among the desirable acid-forming organisms is con- 

 stantly found in milk. They have little to do with the 

 ordinary acid fermentation as they grow very slowly 

 at ordinary temperatures. If a sample of raw milk is 

 placed at the temperature of the animal body, the 

 acidity will reach 1 per cent in a few hours. There- 

 after the acidity will increase slowly and may reach 

 three per cent or above. The continued increase in acid 

 is due to the growth of long rods of the Bacillus Bui- 



