on Milk and Milk-Products, 
395 
prevented from entering the milk, the latter can be completely 
sterilized at this comparatively low temperature. 
At the temperature of boiling water (212' Fahr.), and keeping 
the receptacles containing the milk tightly closed, the process of 
sterilization may be completed in one operation, and is therefore 
less troublesome. A greater security still is obtained by heating 
under pressure to 230' Fahr. 
The complete sterilization of milk by heating it repeatedly 
to 165' Fahr., under proper precautions, has been accomplished, 
but with small quantities only, and in an experimental way. 
The carrying of this out on a large scale would be beset with 
manv obstacles and great difficulties. The experiments occupied 
a period of five days, during which time the small samples of 
milk were heated for one hour daily. The reason why the 
repeated heating at intervals to a lower temperature has the 
same effect as once raising the temperature to a higher degree 
(^always assuming that the introduction of germs is excluded) is 
casilv explained. The temperature of 165' Fahr. is, generally 
speaking, sufficient to kill developed micro-organisms, but not 
their spores. If, therefore, the milk after heating is allowed to 
cool down again, there is the chance given to the intact spores 
to germinate and to be killed by the following heating. 
Thus, repeated heating at intervals will free the milk gradu- 
ally from all organic life. The spores which will resist lower 
temperature succumb to higher degrees of heat, and if exposed 
sufficiently long are killed outright. This is the case, at least, 
with the organisms of lactic fermentation, so that it is a com- 
paratively easy task to prevent milk getting sour for a long time. 
Xot infrequently, however, a preserved milk which shows no 
sign of turning sour possesses a bitter taste, proving that the 
organisms which are able to transform proteids into peptones 
have survived. 
Butter. 
If with regard to the action of micro-organisms on milk much 
remains to be cleared up, we must admit that with reference to 
butter very little has, so far, been done. What we know for 
certain is that butter is subjected to certain decompositions, as 
shown by a more or less marked change in its taste ; but how 
these decompositions are brought about and in what they actually 
consist we are, with our present knowledge on the subject, for 
the most part unable to say. With the exception of very slight 
traces of colouring matter (annatto) in the case of coloured, 
and a quantity of common salt in the case of salt butter, 
genuine butter contains nothing but compounds which are also 
present in milk. The percentage of fat is of course vastly 
