CHANGES IN MILK 53 



been formed, coagulation of the casein occurs — at room 

 temperature — which either is precipitated as flakes or 

 thickens to a firm, jelly-like substance (curdling of 

 milk) and frequently contains small gas bubbles. At a 

 higher temperature, coagulation occurs in the presence 

 of a smaller percentage of acid. If coagulation is caused 

 by acid, alone, the process unquestionably is a simple 

 separation of the casein-lime compound by which the 

 casein, insoluble in water, is precipitated unchanged. 

 AVhen tlie casein is filtered out and treated with lime 

 water, it dissolves readily. In heating sour milk to 70° 

 C. and above, a real coagulation will occur, however, and 

 the precipitated casein cannot be dissolved in lime water. 

 After the formation of lactic acid has reached a cer- 

 tain degree (about 0.8 per cent., which corresponds to 

 an amount of acid in 100 c.c. of milk that requires Jor its 

 neutralization about 100 c.c. of a one-tenth normal solu- 

 tion of Na^O), the activity of the l)acteiia ceases, for 

 they cannot thrive in the strongly acid liquid, and the 

 greater number of bacteria rather (juickly perish. The 

 different bacteria and yeast fungi which may be the 

 cause of lactic acid fermentation, are not sensitive to the 

 same degree of acidity, and under the influence of these 

 various forms, tlie milk becomes sour irregularly. These 

 various organisms are influenced very largely by exter- 

 nal conditions, in respect to their ability to produce 

 lactic acid, so that sometimes a longer and sometimes a 

 shorter time is required to sour milk, while, in other 

 cases, their ability to produce this change is almost lost. 

 Of the micro-organisms which can produce a lactic acid 

 fermentation, are to be considered the lactic acid bacte- 

 ria used in creameries (a group, the various species of 

 which have not been definitely differentiated), certain 

 yeast-like fungi {Saccharo)iiyces lactis and Saccharo- 

 unjcrs acidi lactici) the Bacillus coli communis, strep- 



