222 CREAMERY BUTTER MAKING 



of this time a mass of curd will be found at the bottom 

 of the can in which there has been a vigorous develop- 

 ment of bacteria throughout the test. 



If the sample of milk is free from taint, this curd when 

 cut with a knife will be perfectly smooth and close. If, 

 on the other hand, the sample contains gas germs, these 

 in course of eight hours' development will have produced 

 enough gas to give the curd an open spongy appearance 

 when cut. The openings are usually small and round, 

 hence the name ''pin holes" has been applied to them in- 

 dicating holes the size of a pin's head. 



Whenever, therefore, milk produces a curd that an- 

 swers this description it may be taken for granted that it 

 contains undesirable bacteria. 



Sometimes the milk may be tainted and yet produce a 

 close textured curd, but in such cases the taint can be 

 detected by carefully smelling of the curd. 



Precautions. In making a test as above outlined two 

 things must constantly be kept in mind : first, that to se- 

 cure the desired bacterial development, the temperature of 

 the samples must be maintained as nearly as possible at 

 98° F., which is accomplished by surrounding them with 

 water at a temperature of 103" ; second, that to avoid con- 

 taminating one sample with another, the knife used for 

 mixing the rennet with the milk and cutting the curd 

 must be sterilized for each can. The thermometer used 

 must also be sterile. 



The temperature of the samples can easily be main- 

 tained by using a well insulated box like that shown in 

 Fig. 66. When a common tin box is used it becomes 

 necessary to change the water in it about once every half 

 hour. 



