BACTERIA IN MILK AND FOOD 253 



Multiply by 5000. This equals the number in y^- c.c. 

 Count several fields and average the result. 



The Plate Method. Microscopic examination, while not 

 to be relied upon wholly, gives valuable and quick informa- 

 tion as to the general character of bacteria, their apparent 

 number, the presence or absence of barn-dirt and chain 

 bacteria. The microscopic count differs greatly from plate 

 count, because dead cells as well as living are shown. 



Certified milk should have less than 10,000 bacteria to the 

 cubic centimeter. According to the average taken from a 

 count of four specimens, a rating is given to the milk, and 

 this rating is to be interpreted only as other conditions are 

 considered, such as cleanliness of the cattle and stalls, and 

 chemic composition and method of handling the product. 



Temperature. Milk kept at 10 F. or lower will not allow 

 ordinary bacteria to develop to any considerable extent; 

 kept at a higher temperature, bacteria develop rapidly. 



Separating or centrifuging permits the bacteria to be con- 

 centrated, and top-milk and cream contain more bacteria per 

 cubic centimeter than whole milk. 



Time, an element. 



Milk freshly drawn, under proper precautions, may con- 

 tain but few bacteria, but in forty-eight to seventy-two hours 

 on ice bacteria will increase enormously. Market milk as 

 ordinarily found in cities may contain millions of bacteria 

 per cubic centimeter. 



Pasteurization. Milk heated to 60 C. for twenty minutes is 

 called pasteurized. This increases the keeping quality and 

 tends to destroy the vegetative forms of pathogenic bacteria. 



To kill lactic acid, the instantaneous method, higher tem- 

 perature, a few seconds only for pathogenic organisms is 

 required. Pasteurization is beneficial only when there are 

 supervision and inspection of original supply. 



Milk as Source of Contagion. Harmless Varieties. 

 Sour milk contains the Bacterium lactis acidi and is not 

 dangerous, and is even considered beneficial, as, for instance, 

 buttermilk. 



