22 STORRS AGRICULTURAL EXPERIMENT STATION. 
from within the milk gland, but only from the milk ducts. 
They do not, therefore, come from within the animal, but 
really from the exterior. ‘They are bacteria from external 
sources, which have made their way into the ducts, and not 
bacteria from within the animal making their way out. This 
difference in the bacterial flora of milk from cows in the same 
barn is certainly a somewhat surprising and interesting fact. 
It gives us a suggestion as to the complex mixture of bacteria 
in cream of an ordinary creamery coming from hundreds of 
cows. It shows further how impossible it must be to obtain a 
uniform quality of cream (so far as bacteria are concerned) 
from many contributing sources. 
METHOD OF EXPERIMENT. 
The method of experiment has been to separate a lot of 
cream from the milk by a centrifugal machine and then divide 
it into four equal parts. In more recent experiments a larger 
amount of cream was taken and eight experiments were carried 
parallel with each other. All of the cream was heated toa 
temperature of 69°—70° C. (156°-158° F.) for fifteen minutes 
and then allowed to cool. ‘This heating (pasteurizing) de- 
stroyed most of the bacteria which chanced to be present in 
the cream, only such bacteria as produce spores remaining 
alive. Hxperience has shown that such heating will kill all 
the lactic bacteria. ) 
The species of bacterium to be tested was grown in sterilized 
milk. Two days before the experiment began a sufficient 
number of vessels of sterilized milk were inoculated, each with 
a different species of bacterium. These were then allowed to 
grow for two days. When the lots of cream were pasteurized 
and cooled, as above described, one of these milk starters was 
poured into each. Each of the lot received a starter made from 
a different species of bacterium, and one lot was always left for 
a control experiment without any starter. 
The four samples were then placed under similar conditions 
as to temperature and allowed to ripen for the same length of 
time. After considerable experience it was found that the 
most satisfactory method of procedure was to use a ripening of 
forty-eight hours at a somewhat high temperature (about 
21° C., 70° F.). After the ripening the cream was cooled and 
