3 i2 THE CHEMISTRY OF DAIRYING 



deleterious to health if it remained unchanged ; but when 

 added to the milk, it is rapidly decomposed into H 2 O and O. 

 This change is greatly accelerated by heat. 



The extremely small quantities of formalin and of hydro- 

 gen peroxide that are required to preserve the milk are 

 probably harmless to healthy adults, but they are liable to 

 prove injurious to invalids and young children. They are, 

 however, objectionable even for healthy adults. Antiseptics 

 which effectively prevent, or even retard, the fermentation of 

 the food must interfere to some extent with the processes of 

 digestion ; for these processes are essentially fermentative in 

 character (p. 86). 



Sterilisation. Milk can be effectively sterilised by heat- 

 ing to a temperature of 112 C. for half an hour. At this 

 temperature all forms of bacteria are rendered permanently 

 inactive, and if no others are allowed to enter, the milk may 

 be kept unchanged indefinitely. A similar result is produced 

 by prolonged boiling. In fact, all the common forms are 

 destroyed on boiling for a few minutes, but some of the spores 

 can withstand this treatment. 



These methods are of scientific interest only. They are 

 too expensive for commercial purposes. Also the process of 

 boiling or heating to higher temperatures renders the casein 

 less easily digestible, and imparts a disagreeable flavour to the 

 milk (p. 309), which would render it unsaleable. 



It was shown by Pasteur that over 95 per cent, of the 

 organisms commonly present are destroyed by heating the 

 milk to about 60 C. for 15 or 20 minutes. This process 

 scarcely affects the taste of the milk, it secures practical im- 

 munity from disease, and as it is much less objectionable on 

 all grounds than boiling, it has been extensively employed. 

 It is called Pasteurisation. 



Aseptic Treatment. To keep bacteria out of the milk 

 (aseptic) is much better than any method of destroying them 

 (antiseptic) after they have entered. It is impossible in 

 practice to keep milk absolutely sterile, but by observing the 

 following simple precautions the number may be kept down to 

 a minimum. The cows, cow-houses, and all utensils should 



