PRESERVATION OF MILK AND ITS TREATMENT 79 



of cleanliness, a fact which is undoubtedly to be attributed to the 

 delay in the cooling of the former. Mid-day milk can only be 

 satisfactorily preserved until the following morning by the use 

 of ice ; as this further adds to the cost of production incurred by 

 milking three times a day instead of twice, it is a question whether 

 the extra 6 to 7 per cent, of milk and milk fat which, according to 

 Fleischmann 1 , is gained in this way, is not too dearly paid for, 

 considering the present cost of labour. There is no doubt that 

 the bad state in which the mid-day milk arrives at the dairies can 

 only influence the dairy products for the worse. 



Heating. As already explained, cooling only serves to preserve 

 milk for a comparatively short time, for it would entail too great 

 an expense to maintain the low temperatures necessary to prevent 

 all bacterial development. Milk cannot be kept for any length of 

 time without the aid of heat. The temperature and time of 

 heating necessary to sterilise milk depend very largely on its 

 quality, and on the size of the bottles or tins which are used. 

 With tins, water filled autoclaves are best as the heat is transferred 

 more rapidly and uniformly by this method. By heating for one 

 hour at 105 C., or for a quarter of an hour at 115 to 120 C., milk 

 will usually be rendered sterile, and may then be kept for years, 

 especially if care be taken to close the bottle immediately after the 

 autoclave is opened, and while the milk is still boiling. The 

 bottles will be practically free from air, and as the most resistant 

 spores require an abundant supply of oxygen in order that they 

 may germinate, the keeping properties of the milk will be greatly 

 enhanced. In order to prevent the fat globules from collecting 

 as a solid lump in the neck of the bottle, the milk should first be 

 homogenised. As sterilised milk is brown in colour and consider- 

 ably altered in chemical composition, it cannot be recommended 

 for children. The steriliser invented by Jonas Nielsen is less 

 destructive in its action, the milk being momentarily heated to 

 130 to 135 C. by being pumped through a system of small thin 

 pipes heated by steam. Burning and the change in taste which 

 this causes, are completely avoided if only the milk is kept during 

 the time of heating at a pressure sufficiently high to prevent it 

 from boiling. According to the author's investigations, even milk 

 to which have been added very resistant bacterial spores, already 

 becomes sterile in forty seconds at 135 C., and in fifty seconds at 

 130, but only after ninety seconds at 125 C. It naturally follows 

 that the milk must be cooled very rapidly after this drastic heating 

 in order that it may not become brown ; this is accomplished oy 

 means of a system of piping similar to that which is used for the 



1 " Das Molkereiwesen," Brunswick, 1875, p. 81. 



