2S^ SCIENCE AND PRACTICE OF CHEESE-MAKING 



higher plants. Spores are not so easily killed by heat 

 as are bacteria. Under favorable conditions, the 

 rapidity of growth of bacteria is remarkable. Thus, 

 in some cases, one cell divides into two cells in 20 

 minutes; if this rate were kept up for 24 hours, the 

 one cell would multiply into several millions. 



(3) Food requirements of bacteria. — Bacteria re- 

 quire as food for satisfactory growth compounds con- 

 taining nitrogen, carbon, hydrogen and, in addition, 



FIG. 41 — ROD-SHAPED BAC- 

 TERIA (bacillus). CLEAR 

 AREAS IN SOME ARE 



SPORES. (Rogers) 



FIG. 42 — B A C T E R I A WITH 

 HAIR-LIKE ORGANS, WHICH 

 THEY USE IN MOVING 

 THEMSELVES ABOUT IN 



LIQUIDS (Rogers) 



small amounts of inorganic or mineral matter. The 

 sugar, casein, albumin and salts in milk and its 

 products furnish a supply of food very readily 

 utilized by bacteria. 



(4) Temperature. — The bacteria commonly pres- 

 ent in milk grow between the limits of 40° and 110° 

 F., the miost favorable limits being between 80° and 

 95° F. Many bacteria are killed between 130° and 



