34 



FIRST LESSONS IN DAIRYING 



unless quickly cooled, as it is at a temperature for 

 their rapid growth ; they are prevented from grow- 

 ing by cold ; are killed by moist heat, most of them 

 by a temperature of boiling water ; they remain 

 inactive in the spore form for a long time, then 

 grow rapidly when conditions again become favor- 

 able ; they grow or multiply usually by division, 

 which may happen every twenty minutes, or may re- 

 quire several hours ; in the proc- 

 ess of their growth they cause 

 changes in the material in 

 which they are growing. These 

 are many, some desirable, as in 

 the souring of milk, when the 

 milk sugar turns to lactic acid ; 

 in vinegar making, when the 

 fruit sugar turns to acetic acid ; 

 and in wine, when the grape 

 sugar is converted into alcohol. 

 In Swiss cheese the eyes are 

 formed by the imprisonment of 

 gases developed by bacteria. 



Among the undesirable prod- 

 ucts are the gases by bacteria 

 associated with stable filth, a common illustra- 

 tion being the pinholy or "spongy" cheese ; bitter 

 flavors, which are most common when milk is 

 held for some time at a low temperature, espe- 

 cially in winter; ropy or slimy milk that is appar- 

 ently not so when drawn from the cow, but be- 

 comes so after standing. Bacteria causing this 

 trouble come from stagnant water. Cows wading 



SLIMY OR ROPY MILK 



