614 A MANUAL OF BACTERIOLOGY 



and which also by the products they form tend to set up 

 gastro-enteritis. The percentage of samples infected with 

 tubercle bacilli varies much : Barton and Hewlett found 

 only one out of 26 samples taken at London railway 

 termini. The supply of the large dairy firms is also 

 comparatively free from tuberculous infection, as con- 

 siderable precautions are taken to exclude tuberculous 

 animals. For the quarter ending March 31, 1911, of 

 760 samples examined for the London County Council, 

 106, or 13-9 per cent., were found to be tuberculous, and 

 since 1907 of 5698 samples, 640, or 11-2 per cent., proved 

 tuberculous (see also p. 321). A poisonous body, tyro- 

 toxicon (p. 38) has been isolated from milk and milk 

 products. Sources of contamination and infection are 

 derived from the insanitary conditions of many farms and 

 dairies and the dirty methods of those handling the milk. 

 In order to render milk wholesome for infants and free 

 from infective organisms under the present conditions of 

 supply, two methods may be adopted sterilisation and 

 pasteurisation. To ensure sterilisation it is necessary to 

 heat the milk to boiling-point for six hours, or to expose 

 it for a shorter period to steam under pressure. Such 

 treatment, . however, markedly alters the flavour of the 

 milk, and is said to diminish its nutritive value. If the 

 milk be heated to a temperature not exceeding 70 C., 

 the flavour and nutritive qualities are far less altered, while 

 the pathogenic species are all destroyed. This method is 

 termed " pasteurisation," and consists in heating the milk 

 to about 60-68 C. for twenty to thirty minutes. Pas- 

 teurisation destroys 92-99 per cent, of the total organisms 

 present. The objections to pasteurised milk are that the 

 natural enzymes present in fresh milk are destroyed and 

 such heated milk is stated to induce scurvy rickets, 1 the 



1 Dr. Lane-Claypon denies this, and considers that the enzymes in milk 

 are derived from the bacteria in it (Rep. to theLoc. Gov. Board, 1913). 



