Bacteria and Cheese Making. 165 



fragments by means of a curd knife, and as the mass is 

 warmed, the acid develops, causing the curd particles to 

 shrink, thus expressing the whey. Within a short time, 

 the volume of the curd is not more than one-eighth that 

 of the milk, but in the curd are held over 75 per cent 

 of the bacteria of the milk. To secure rapid curdling in 

 the vat, the milk is warmed to 85 to 90 F., a tem- 

 perature that is most favorable for the growth of the 

 lactic bacteria. Since there is a large number of bac- 

 teria concentrated in a small volume, and the tempera- 

 ture, as well as all other -conditions, is favorable to 

 growth, multiplication of the bacteria goes on rapidly, 

 and as a consequence, acid is formed in large amounts, 

 as is shown by the following figures given by Publow for 

 the manufacture of the export type of cheddar cheese: 



Acidity of milk before adding rennet .2 to .21 per cent 

 Acidity of whey before heating curd .14 to .145 " 

 Acidity of whey before removing 



from curd 16 to .18 



Acidity of whey coming from the 



curd after removal of whey and 



curd is packed. .24 to .30 



Acidity of whey coming from curd 



before milling 65 to .75 



Acidity of whey coming from curd 



before salting 90 to 1.10 



If the milk had been kept at the same temperature as 

 the curd, the acidity would have increased much more 

 slowly since the acid would have been distributed 

 through a larger volume. In the cheese curd the same 

 amount of acid is probably formed, as would have been 

 produced in the total amount of milk during the same 

 interval. 



The acid produced by this bacterial activity has a most 

 marked effect on the curd. At first the curd masses are 



