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 
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 
