Bacteria in Cheese. 1611 



which acts upon the casein. This acidity is measured by> 

 the development of the lactic-acid bacteria that normally 

 abound in the milk; or, as the cheese-maker expresses it, 

 the milk is "ripened " to the proper point. The action of 

 the rennet, which is added to precipitate the casein of the 

 milk, is markedly affected by the amount of acid present, 

 as well as the temperature. Hence it is desirable to have 

 a standard amount of acidity as well as a standard tem- 

 perature for coagulation, so as to unify conditions. It 

 frequently happens that the milk is abnormal with refer- 

 ence to its bacterial content, on account of the absence of 

 the proper lactic bacteria, or the presence of forms capable 

 of producing fermentative changes of an undesirable char- 

 acter. In such cases the maker attempts to overcome the 

 effect of the unwelcome bacteria by adding a "starter; " or. 

 he must vary his method of manufacture to some extent 

 to meet these new conditions. 



Use of Starters. A starter maybe employed to hasten 

 the ripening of milk that is extremely sweet, so as to cur- 

 tail the time necessary to get the cheese to press; or it 

 may be used to overcome the effect of abnormal conditions. 



The starter that is employed is generally one of domestic 

 origin, and is usually taken from skim milk that has been 

 allowed to ferment and sour under carefully controlled con- 

 ditions. Of course much depends upon the quality of the 

 starter, and in a natural starter there is always the possibility 

 that it may not be perfectly pure. 



Within recent years the attempt has been made to con- 

 trol the effect of the starter more thoroughly by using pure 

 cultures of some desirable lactic-acid form. 1 This has ren- 



1 Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 112; Campbell, Trans. High. & 

 Agr. Soc. Scotland, 5 ser., 1898, 10:181. 

 11 



