BACTERIA IN SEPARATED MILK 145 



ment were made in flat- bottomed flasks as illustrated on Plate 18.) 

 Without entering into all the minutiae of the experiments, the 

 general results may be stated as follows : — Set cream contains 

 more bacteria than set milk from the first day till after the twenty- 

 first day of setting. The two chief factors contributing to the pre- 

 dominance of organisms in the cream are {a) that as the fat cells 

 rise through the milk to form the cream layer, they carry up with 

 them the organisms contained in the whole milk, as on the principle 

 of " the raft theory " of Tyndall ; and {b) that the cream is exposed 

 to the air, acquiring from it a large number of organisms and 

 acting as a protective film over the milk. After the twenty-first 

 day the organisms in both cream and milk became equalised in 

 number. This change is, of course, mainly due to the fact that 

 both cream and milk came under similar conditions on account 

 of the milk being exposed to the air by the removal of the layer of 

 cream from its surface in the process of creaming. 



There is also a decline in the organisms of the set cream after 

 the twenty-first day, which is probably due to competition and 

 acidity, both of which conditions are present in much greater 

 degree in the cream than the milk. The increase of organisms in 

 both set cream and set milk continues to the twenty-first day or 

 thereabouts, after which date, or between the twentieth and 

 thirtieth days of the setting, organisms decline in number. This 

 decline, however, is not general, and does not affect all organisms 

 present. The chromogenic organisms and most of the bacteria 

 derived from the air disappear from both the cream and milk 

 between the third and eighth days, and do not reappear. The 

 lactic acid organisms, on the other hand, present in the set cream 

 at the beginning, increase and become predominant till between 

 the third and fourth week. In the set milk the lactic acid 

 organisms do not appear frequently until the third or fourth or 

 sixth day, being kept under by the common air saprophytes, 

 but after this period they become predominant, as in the cream. 

 The set cream contains many more colonies than the set milk, 

 but in both, the organisms present are of the lactic acid species, 

 all the common air and chromogenic organisms having long since 

 succumbed. 



A comparative series of experiments were also carried out in 

 respect of separated milk and separated cream. It is unnecessary to 

 explain that the principle of the separator is that of centrifugalisa- 

 tion. The whole milk being placed in the separator, the machinery 

 IS put in motion, and the milk by centrifugal action becomes 



