SANITARY MILK PRODUCTION 161 



of the air of the milking room of the open stable, was less than that of 

 the closed barn; that fecal contamination of the milk from cows kept 

 in the open stable, where the cows receive little attention, was no greater 

 than in closed stables, where they receive a good deal, and that there is 

 advantage in the open stable as a place to produce milk of low germ 

 content. 



Milk is polluted by bacteria derived from dust and from many 

 sources in the immediate environment of the stable and other places 

 where it is handled most. Among these sources of pollution are the air, 

 the feed, the bedding, utensils of various sorts and the milkers. 



Air -borne Contamination. The belief in air-borne diseases has been 

 held for many centuries and from the early days of bacteriology until 

 very recently air was believed to be an important factor in transmitting 

 contagion, but more complete understanding of microbial life has shown 

 that while germs do float in the air, air-borne infection is not to be greatly 

 dreaded. There is of course a great difference in air as to the germ con- 

 tent; some is very dusty and carries many bacteria while other air is 

 very clean and nearly germ-free. It may be advisable to filter or wash 

 the air that comes into a milk plant located on a dusty city street, but 

 wholly unnecessary to, that of one in the country. Also, some dust 

 may be rather heavily polluted with animal excretions while other may 

 be only slightly so. In any case it is the visible and invisible dirt that 

 floats in the air which is the cause of bacterial contamination and not 

 the air itself, for the germs are attached to dust particles, though not to 

 all of them for some, as for instance those of soot, are sterile and all the 

 motes that float in the air tend to become so by drying and the germicidal 

 action of sunlight. 



At the New York Experiment Station Rhuele and Kulp have made a 

 careful quantitative study of how the germ content of the stable air 

 affects milk. As a whole their results show that the microbial con- 

 tamination that milk derives from standing in the air of a well-kept barn 

 is unimportant. Out of 54 tests in the station barn there were but 17 

 that showed the number of precipitating bacteria to be greater than 100 

 per square centimeter in a 5-min. interval, and the highest of these was 

 only 269 per square centimeter. Many of these tests were made when 

 the barn air was disturbed by the feeding of silage, the brushing of cows, 

 etc. Even during the worst of these conditions, an open pail of milk 

 could have stood 15 min. in the stable without a detectable increase 

 in the bacterial content of the milk occurring. 



A brief exposure of the milk to the very dusty atmosphere that 

 prevailed for short intervals, during certain barn operations in com- 

 mercial dairy barns was sufficient to appreciably raise the bacterial 

 content of the milk and such conditions should be guarded against. 



The experiments indicate that bacterial contamination of milk from 

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