SOURCES OF COMMON MILK BACTERIA 7 1 



is sometimes the udder, sometimes the food, sometimes the 

 faeces, sometimes the water, sometimes infected cans from a 

 creamery or cheese factory, etc. The dairyman must simply use 

 his own ingenuity in detecting the source of the trouble in each 

 individual case. 



SIGNIFICANCE OF DIRT IN MILK 



From the facts above outlined it is evident that the dirt 

 which gets into milk is a very important source of organisms 

 which are likely to produce trouble, since they may injure the 

 milk or butter, and may produce even more trouble in cheese- 

 making. The presence of dirt means infection with bacteria 

 and, in many cases, with types of bacteria that are emphatically 

 mischievous in the dairy; it always means filthy dairy condi- 

 tions. The recognition of this fact has led to attempts to 

 estimate the amount of dirt present in milk, as one of the means 

 of determining its quality. Milk which contains a large amount 

 of dirt must be far inferior in quality to a clean milk, since it 

 will contain more bacteria. Clean milk will keep better, and it 

 will be less liable to contain disease germs. Several methods 

 of estimating the amount of dirt in milk have been devised, 

 none of which are very accurate or very satisfactory. These 

 are described in experiment 46. 



The methods there given will give the dry weight of the 

 solid, insoluble particles of dirt which get into the milk, and 

 which may be filtered out. But they do not give any estimation 

 of such particles as are too small to separate out by sedimenting 

 or centrifugalizing, and they give no indication of the portion 

 of the dirt that may have been dissolved in the milk. The ab- 

 solute amount of dirt found is of no special significance, but if 

 numerous estimations are made by the same individual on a 

 series of milk samples, the relative amounts will indicate the 

 relative cleanliness of the different samples. Estimates made 



