CONTROL OF THE MILK SUPPLY 231 



Sterilisation and Pasteurisation 



Sterilisation means the use of heat at or above boiling-point, or 

 boiling under pressure. This may be applied in one application 

 of one to two hours at 212°-250° F., or it may be applied at stated 

 intervals at a lower temperature. The milk is sterilised — that is to 

 say, contains no living germs — is altered in chemical composition, 

 and is also boiled or " cooked," and hence possesses a flavour which 

 to many people is unpalatable. 



Now such a radical alteration is not necessary in order to secure 

 non-infectious mOk. The bacteria causing the diseases conveyable 

 by milk succumb at much lower temperatures than the boiling- 

 point. Advantage is taken of this in the process known as 

 pasteurisation. By this method the milk is heated to 167-185° F. 

 (75-85° C). Such a temperature kills harmful microbes, because 

 75° C. is decidedly above their average thermal death-point, and 

 yet the physical changes in the milk are practically nil, because 

 85° C. does not relatively approach the boiling-point. There is no 

 fixed standard for pasteurisation, except that it must be above the 

 thermal death-point of pathogenic bacteria, and yet below the 

 boiling-point. As a matter of fact, 158° F. (70° C.) will kill lactic 

 acid bacteria as well as most disease-producing organisms found in 

 milk. If the milk is kept at that temperature for ten or fifteen 

 minutes, we say it has been "pasteurised." If it has-been boiled, 

 with or without pressure, for half an hour, we say it has been 

 "sterilised." The only practical difference in the result is that 

 sterihsed milks have a better keeping quality than pasteurised, for 

 the simple reason that in the latter some living germs have been 

 unaffected. 



Sterilisation may, of course, be carried out in a variety of 

 modifications of the two chief ways above named. When the 

 process is to be completed in one event an autoclave is used, in 

 order to obtain increased pressure and a higher temperature. Milk 

 so treated is physically changed in greater degree than in the slower 

 process. The slow or intermittent method is, of course, based on 

 Tyndall's discovery that actively growing bacteria are more easily 

 killed than their spores. The first sterilisation kills the bacteria, 

 but leaves their spores. By the time of the second apphcation the 

 spores have developed into bacteria, which in turn are killed before 

 they can sporulate. 



The application of sterilisation to milk is now very widely 



adopted by corporations, dairy companies, etc. Eecently the writer 



has had the opportunity of studying an excellent system in vogue in 



Essex,* and which may be mentioned because it seems to emphasise 



* J. Carson, Crystalbrook Farm, Theydon Bois, Essex. 



