MILK AND ITS PRODUCTS 195 



starter. Usually three or four pounds of the mother 

 starter added to one hundred pounds of pasteurized skim- 

 milk will sour it in twenty-four hours at a temperature 

 of 65 F. Under certain conditions of weather this 

 amount may possibly have to be modified a little, for it 

 is well known that on hot sultry days milk will sour 

 more quickly at a given temperature than on cooler days. 

 The best rule to follow is to use enough of the mother 

 starter to sour the milk in twenty-four hours at a tem- 

 perature of 65 F. 



In the heating process all of the active bacteria in the 

 skim-milk have been destroyed, thus leaving a clean field 

 for the development of the lactic acid bacteria added to 

 it from the bottle. 



From what has been said it will be seen that the 

 method of using the lactic acid bacteria is similar to 

 the use of yeast germs in bread making. The original 

 germs obtained in the way above described, or from the 

 manufacturer, may be propagated for weeks by daily 

 transferring a small amount of the thickened skim-milk 

 to newly pasteurized skim-milk. 



COMMERCIAL STARTERS. 



Commercial starters usually consist of a single species 

 of lactic acid organisms. These starters are prepared in 

 laboratories where the utmost precautions are taken to 

 keep them free from undesirable germs. The methods 

 by which the good bacteria are separated from the bad 

 are quite complicated and of too little practical value to 

 permit a discussion of them here. Suffice it to say that 

 such separation is possible only with the skilled bac- 

 teriologist. 



