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. Bergey 
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 
