294 SCIENCE AND PRACTICE OF CHEESE-MAKING 



per cent of lactic acid, decomposing a correspond- 

 ing amount of milk-sugar. 



The range of temperature most favorable to lactic 

 acid organisms is 90 to 95 F. Below 80 F. their 

 activity gradually decreases and practically ceases at 

 50 F. At 105 F., they are fairly inactive; many 

 are killed at 135 to 140 F., and all at 150 to 

 160 F. 



While the lactic acid fermentation spoils milk for 

 the taste of most people, at least for ordinary uses, it 

 is a very essential factor in the manufacture of cheese. 

 Very few lactic acid bacteria are found in fresh milk, 

 but they increase so rapidly at ordinary temperature 

 (70 F.) that in 12 to 18 hours they generally exceed 

 in number all other bacteria in milk. In summer 

 weather, when the temperature is especially favorable 

 to their rapid growth, the lactic acid bacteria usually 

 constitute, at the time the milk sours, more than 95 

 per cent of all the micro-organisms in the milk. 



While the growth of lactic acid organisms in milk 

 is favored by the presence of a small amount of acid, 

 most other organisms do not thrive so well in an acid 

 environment. Therefore, as soon as enough milk- 

 sugar has been converted into lactic acid to produce 

 a slightly acid condition, other organisms decrease 

 in activity, while the lactic acid organisms vigorously 

 increase, unhindered. It is quite commonly thought 

 that milk is peculiarly liable to sour during thunder- 

 storms, as the result of some peculiar electrical con- 

 dition or other mysterious influence. The hot weather 

 preceding such storms favors the more rapid growth 

 of the lactic acid bacteria and this is a 'sufficient ex- 

 planation, and the proper one. Milk free from such 

 micro-organisms never sours during thunderstorms. 



