394 
Micro-organisms and their Action 
has not the effect of destroying the fermentative organisms, but 
simply of diminishing or retarding their development, and their 
action will be seen after a longer or shorter period. 
The system most recommended — and a more reliable one than 
the addition of chemical preservatives, which have been shown to 
alter the composition of the milk — is the process of subjecting 
the latter to temperatures which are unsuitable to organic life. 
Although the temperatures suitable for the development of 
micro-organisms are for the different varieties not exactly the 
same, they do not vary very much, so far as they are known 
at present, and are very close to the temperature at which milk 
leaves the cow. It is, then, of the greatest importance that the 
period at which freshly-drawn milk is kept at this most dan- 
gerous temperature should be shortened as much as possible. 
The easiest way to accomplish this in practice will be found 
in cooling down the milk by running it over a refrigerator. 
The lower the temperature which can be reached and main- 
tained the better, and the longer the milk will keep. At the 
temperature of or near freezing point the development of micro- 
organisms appears to cease altogether, so that milk at this 
temperature may be kept almost an indefinite length of time. 
This is a point of considerable importance, and has been 
taken advantage of by some of our large steamship companies. 
Instead of being compelled, as they were formerly, to take on 
board cows in milk in order that they might be in a position to 
supply infants, invalids, and others desirous of having fresh 
milk, they now provide themselves before leaving port with a 
supply of fresh milk large enough to last the whole voyage, a 
quantity sufficient for a day's consumption being placed in 
separate cans, and these cans stored in a room kept at a tem- 
perature considerably below freezing point. The milk being 
transformed to ice keeps perfectly fresh for months. 
If the object in view be to preserve fresh milk for a few days 
only, a temperature not higher than 45° Fahr. serves the purpose 
admirably. The last mentioned, and most probably even much 
lower temperatures, do not, however, destroy micro-organisms 
and still less their spores, their development only being more or 
less impaired by the action of the cold. 
The other extreme of temperature, viz., heat, produces a similar 
efFect. Milk can be preserved for days by simply boiling it for a 
few minutes repeatedly, and at proper intervals, say once a day. 
But even raising its temperature for a moment to 165° Fahr. will 
have a marked effect with regard to its keeping qualities. If 
heating to this temperature is repeated at intervals, the effect is 
better still, and if in addition proper precautions are taken to 
carry out the heating process in a way by which fresh germs are 
