' 43 2 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



The immense numbers of bacteria found in milk in the large cities 

 are usually the result of the rapid growth of the Bad. lactis acidi group 

 resulting from the age of the milk and the temperature at which it has 

 been kept. Such milk may also contain large numbers of those sapro- 

 phytic organisms which occur freely in nature and which may be 

 abundant about the stables and milk-house. The number of this group 

 depends largely upon the sanitary conditions of production and the 

 initial contamination. In ordinary milk organisms of the Bact. lactis 

 acidi type will constitute a very large percentage of those present when 

 the milk reaches the city even before it shows any perceptible signs of 

 souring. During the past few years great progress has been made in the 

 production of clean milk and at present quite an important part of the 

 general raw milk supply of our cities has a very much lower germ con- 

 tent than it had a few years ago. 



SPECIAL MILKS. In this class may be considered those milks known 

 as Selected, Inspected, or Guaranteed. As commonly used these terms 

 mean milk which has been produced and handled with considerably 

 more care than ordinary market milk but not with the extreme care 

 required for certified milk. While these and similar terms do not always 

 mean milk of the same grade in different places, they usually mean milk 

 produced by herds which have been shown by the tuberculin test to be 

 free from tuberculosis. Considerable care is exercised in all the opera- 

 tions of handling the milk. The result is that these milks usually have 

 a much lower germ content than the ordinary milk supply of the same 

 city. Sometimes the germ content of such milk compares favorably 

 with that of certified milk. These milks may contain various types of 

 normal milk organisms but they should not contain any tubercle 

 bacteria. 



CERTIFIED MILK. Certified milk means milk which has been pro- 

 duced according to the regulations of and under the supervision of a 

 medical milk commission. The stables and cows are kept extremely 

 clean. No dust is allowed in the stable at milking time. The cow's 

 flanks and udder are washed just before milking, the milkers wear white 

 suits and wash their hands before milking each cow. Small-top pails 

 are used and the milk is cooled as soon as drawn from the cow. The 

 extreme care exercised in the production and handling of this milk has 

 a very marked effect on the number of bacteria found in it. The follow- 

 ing counts are typical of certified milk. 



