190 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



isms are lactic acid and putrefactive bacteria. Even 

 when the vessels and utensils are thoroughly cleansed 

 in the manner described, they still contain bacteria. To 

 reduce contamination of milk from this source to the 

 minimum, sterilization is necessary in addition to thor- 

 ough washing. The effects of sterilization are illustrated 

 by the investigations of Prucha, Harding, and Weeter, 

 who found that the same milk which contained 515,203 

 bacteria per c.c. when the vessels and utensils were 

 washed but not sterilized contained only 3875 per c.c. 

 when the apparatus was sterilized after washing. Itergey 

 observed that when the apparatus was sterilized by steam, 

 the character of the organisms present in the milk after 

 it had come in contact with the milk pail, strainer, cooler, 

 etc., did not differ from those found in samples taken 

 directly from the udder. Milk bottles should be steril- 

 ized before refilling for the additional reason that it is 

 a protection against the infection of the milk supply by 

 bottles which may be returned from houses where infec- 

 tious disease exists. 



Exposure to live steam is the most certain method 

 of destroying bacteria in milk vessels and utensils. To 

 obtain complete sterilization, it is necessary to expose 

 the vessels and apparatus to steam under 5 pounds pres- 

 sure for 20 minutes. This is possible with the cast-iron 

 sterilizers which can be tightly closed. Bottles will stand 

 this amount of pressure. The galvanized-iron steam 

 chests frequently used in dairies are not sufficiently tight 

 to hold the steam under pressure ; in these, 99.2 per cent, 

 of the bacteria are destroyed, the spore formers surviv- 

 ing. Rinsing or scalding the vessels and utensils with 

 boiling water after washing does not destroy many bac- 

 teria, and while exposure to the sun has a germicidal 



