PRESERVATION OF MILK AND ITS TREATMENT 81 



tion is not necessary and a less drastic treatment, i.e., pasteurisation, 

 will suffice. The easiest method consists in heating the milk for 

 a short time, two minutes at the most, in a continuous pasteuriser. 

 It has been shown * that the keeping powers of milk may be 

 improved by heating even to 70 to 75 C. in this way. According 

 to American researches 2 a far better result is obtained at 80 C., 

 and as this temperature is necessary to ensure the destruction of 

 the tubercle bacteria, it has been fixed as the minimum by the 

 Danish pasteurisation law. Very little is gained by increasing the 

 temperature to 85 C., and the milk or cream easily acquires a 

 " cooked " flavour if it cannot be cooled rapidly enough. On the 

 other hand, skim milk should be heated to 90 to 95 C. if, as is 

 the practice in Denmark, the milk is to be returned from the 

 co-operative dairy to the farms in the unrinsed cans ; in this 

 way the remains of milk in the cans will also be pasteurised. 



From the bacteriological point of view, approximately the same 

 results may be achieved by prolonged heating at lower tempera- 

 tures as by short heating at higher temperatures ; in America 

 milk is often pasteurised for half an hour at 60 to 70 C. The 

 author has found that this method of treatment has a less destruc- 

 tive effect on the proteins 3 , and is therefore to be recommended 

 for the treatment of milk for infants or for cheese making, where 

 it is of importance to preserve the natural properties of the milk. 

 The following table shows that on heating to over 70 C., albumin 

 coagulation occurs fairly rapidly, while the milk loses its power of 

 coagulation with rennet 4 . 



" Forsogslaboratoriets 22 Beretning," 1891. 



2 .Harding and Eoqers, New York Experimental Station, Bulletin 172, 

 1899. 



3 " Landwirtschaftliches Jahrbuch der Schweiz," 1905. 



4 It must be added that the extent of the change suffered varies greatly 

 with different samples of milk. In the experiments recorded in the table 

 the same samples .were used throughout for the same times of heating, but 

 different samples for different times of heating ; a maximum effect was 

 soon reached, which was first exceeded when the milk was heated to a 

 temperature sufficiently high to turn it brown. 



