150 BACTERIA IN MILK. 



ordinary sour milk organisms produce from 1.2 per cent, to 1.5 per 

 cent, of lactic acid, and then cease to grow; but this Bulgarian type 

 produces as much as 3.0 per cent, of acid, double the amount pro- 

 duced by the common type. This type is very vigorous and when 

 growing in milk will soon destroy other bacteria. Quite a number 

 of commercial products containing this organism are now on the 

 market, and are used somewhat widely in making a fermented milk. 

 Though originally found in Bulgaria, bacteria that 

 agree with it in all essential respects have been 

 / %T found elsewhere. It has been found in this country 

 as well as in Europe, but thus far on grain rather 

 than in milk. Several of the fermented milks found 



in different countries appear to contain representa- 

 tions of this type of lactic acid organism. 



Peptonizing and Rennet-forming Bacteria. Occasionally 

 a dairyman is puzzled by a somewhat unusual phenomenon: his 

 milk curdles, but remains sweet. This is apt to occur in the fall or 

 spring when the food of the cattle is being changed, and is due to a 

 class of bacteria that secrete enzymes. The bacteria in question 

 really secrete two enzymes, one of which is similar to rennet, secreted 

 by the stomach of a calf, and the other is similar to trypsin, se- 

 creted by the pancreatic gland of man and other animals. Hence 

 these bacteria secrete two enzymes that have actions essentially 

 like those of digestvie fluids. 



When this class of bacteria grow in milk, both of these enzymes 

 act .upon it. The rennet enzyme shows its effect first, and causes the 

 milk to curdle; but since no acid is produced by these organisms this 

 curd is not sour. The curd is also softer than that produced by 

 the lactic acid bacteria. The phenomenon is sometimes called 

 sweet curdling. After a short time, usually two or more days, the 

 second enzyme begins to show its effects. This, acting like a 

 digestive fluid, changes the nature of the casein from an insoluble 

 to a soluble condition, and as fast as this occurs the curd is dis- 

 solved in the liquid of the milk. The curd thus disappears, as the 

 casein is dissolved, and, finally, the whole curd may be dissolved so 

 that the milk becomes liquid again (Fig. 31, c, d). But it is a totally 



