The Preservation of Food in the Home. — Part III 1327 



of the bacteria on the length of time that the milk will keep is shown 

 b}'' the following figures: 



Bacteria per cc. in fresh milk 



Bacteria at 



the end of 



12 hours 



Hours to time 

 of curdUng 



187,000 



3,000 



325 



432 , 000 



14,000 



1,712 



45 



99 



121 



All of the samples used were held at the same temperature under uniform 

 conditions. The difference in the keeping time is the result of the differ- 

 ence in the number of bacteria m the fresh milk. Under ordinary 

 conditions, the smaller the number of bacteria in the milk, the longer it 

 will keep. 



Some of the ordinary bacteria in milk do not cause any particular 

 changes in it, but others produce bad flavors and odors that injure and 

 sometimes entirely spoil the milk. Souring is caused by the group of 

 organisms known as lactic acid bacteria. These act on the milk sugar 

 and produce acid. When enough of the milk sugar has been broken 

 down and has formed sufficient acid, the milk tastes sour and later curdles. 

 If these special kinds of bacteria could be kept out of the milk it would 

 not sour at all, but could be kept at ordinary room temperature for a 

 number of days. 



Many persons think that it is necessary to aerate milk in order to get 

 rid of the so-called animal odor, but this is unnecessary if the milk has 

 been handled properly. There is no disagreeable taste or odor to fresh 

 milk unless it has absorbed it from some outside source. Under some 

 conditions, aerating in a clean, pure atmosphere may help to remove 

 bad odors; but if the milk has been produced under clean conditions, 

 there are no bad odors to remove. The so-called animal odors are caused 

 either by the milk absorbing some outside odor or by the growth of 

 bacteria in the milk. 



the keeping of milk 



Even when milk is produced and handled under the best conditions, 

 there will be a certain number of bacteria in it. If these are allowed to 

 grow rapidly they will produce decomposition changes in the milk, in- 

 juring its food value and causing disagreeable flavors and sometimes 

 poisonous substances. It is therefore desirable to prevent the growth of 

 these organisms in the milk as much as possible. Like all other plants, 

 the bacteria do not grow in cold temperatures, and the milk should be 

 kept cold enough to prevent their rapid increase. It has been found by 



