CHEMICAL TESTS OF MILK 213 



the original milk was highly virulent, the milk after condens- 

 ing had lost its power to infect guinea-pigs on inoculation, all 

 the tubercle bacilli being killed. 



Dried Milk. The nature and preparation of dried milk 

 is described on pages 393-394. To examine bacteriologically, 

 the dried milk would have to be dissolved in sterile water and 

 then examined by methods similar to those employed for milk. 



Cream,. The bacteriological examination of cream is of 

 considerable importance, since it is usually richer in bacteria 

 than either whole milk or separated milk. This is shown by 

 direct bacterial examinations, while in guinea-pig inoculation 

 experiments it is not uncommon to find that a considerably 

 higher percentage succumb to acute infections when inoculated 

 with cream than when inoculated with the sediment of whole 

 milk. In examinations for pathogenic bacteria, such as B. 

 diplitheriae, B. tuberculosis, etc., the cream of the milk samples 

 should never be neglected, while it follows that market-cream 

 itself should be more often examined than is the case. 



Anderson * gives the following figures as to the number of 

 bacteria in cream. In 2 6 samples of milk the average number 

 of bacteria in gravity and centrifugally raised cream in the 

 sediment and in the mixed milk was 



d f cream layer . . 68,690,000 bacteria. 

 ) sediment layer . . 4,840,000 



r. , . ,. , ( cream layer 96,840,000 



Centnfugahsed 



Whole milk ..... 14,388,000 



Cream is bacteriologically examined by methods similar to 

 those used for milk, the cream being diluted with sterile 

 water. For examination for tubercle bacilli the cream itself 

 may be injected direct into guinea-pigs. 



A chemical examination of cream for preservatives should 

 always be carried out before the bacteriological examinations 

 are undertaken. 



1 Bulletin No. 56, Washington Treasury Department, 1909, p. 739. 



