46 



BUTTER-MAKING. 



absence of air. These are called anaerobic. Some grow under 

 either or both conditions, and are called facultative aerobic or 

 facultative anaerobic. 



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 can occur may be placed between freezing-point and 



<r?T>'-.;i.'\'' ^} ' ' ' 'l,' ''r''|\M 



i^'r^>' 



'r,/,^' ^' V i' 



Fig. 7. — a, single bacterium; b, progeny resulting from the growth of a bac- 

 terium during 24 hours in milk at 50° F.; r, progeny of a bacterium 

 during 24 hours growth in milk at 70° F. At .50° F. multiplication was 

 5-fold. At 70° F. the multiplication was 750-fold. (Bui. 26, Storrs, Conn.) 



a Uttle above 110° F. The growth of bacteria at these ex- 

 treme temperatures is very slight. Even at 50° F. the rate 

 of growth is very slow. According to experiments conducted 

 by Dr. Conn, the multiphcation of bacteria at 50° F. was 5- 

 fold. while at 70° F. the multiplication was 750-fold. The 

 following table shows the number of bacteria per cubic centi- 

 meter in milk kept at different temperatures : * 



* Bull. 26 Storr's Stn., Cona 



