56 FERMENTS IN MILK 



enzymes that dissolve them. Nitrogen, carbon, oxygen, hydro- 

 gen and mineral matter are essentials for bacteria. These sub- 

 stances are furnished in abundance in milk from casein, albumen, 

 milk-sugar, and the mineral salts. Butter-fat in milk is said to be 

 of little value as a food for bacteria. 



Some organisms, including yeasts and molds, tolerate con- 

 siderable amounts of acid, while others do not. Most bacteria, 

 however, prefer a neutral or slightly alkaline substance. Dark- 

 ness is essential to some bacteria, and is preferred by the majority 

 of the different species, Bright sunlight is a very effective 



FIG. 7. a, single bacterium; 6, progeny resulting from the growth of a bacterium 

 during 24 hours in milk at 50 F.; c, progeny of a bacterium during 24 hours 

 growth in milk at 70 F. At 50 F. multiplication v/as 5-fold. At 70 F. 

 the multiplication was 750-fold. (Bui. 26, Storrs, Conn.) 



germicide; it is fatal to all species, so far as known. Some germs 

 require air for their growth. These are called aerobic. Others 

 again grow only in the absence of air. These are called anaerobic. 

 Some grow under either or both conditions, and are called 

 facultative. 



Temperature. Favorable temperature is essential to bac- 

 terial growth. Temperature is, indeed, the most important 

 means by which the growth and development of bacteria can be 

 controlled. The range of temperature at which bacterial 

 growth is likely to occur may be placed between freezing-point and 

 a little above 110 F. There are, however, exceptions to this 



