PRESERVATION OF MILK AND ITS TREATMENT 83 



It is obvious that the sooner the individual particles of the milk 

 can be heated, the shorter will be the time occupied by the process 

 of heating, and by spraying the milk into the pasteurising vessel, 

 as is done in Oscar Lobeck's Biorisator, an effect is attained by 

 momentary heating to 75 C , which is as efficient in killing the 

 bacteria and as lenient towards the milk itself as that attained by 

 heating to 63 C for half an hour. Biorisation may thus be regarded 

 as a process intermediate in character between high and low 

 temperature pasteurisation. Unfortunately the atomiser becomes 

 choked so easily that the apparatus cannot be recommended in its 

 present form. On investigating the Biorisation process 1 ) the 

 author obtained the following remarkable results : Not only the 

 milk treated at 70 C., but also that treated at 80, 85 and 90 C., 

 kept worse than that treated at 75 C. Milk treated at 70 C. 

 rapidly became sour, while that treated at 80 C. became putrid, 

 mainly owing to the action of hay and potato bacilli. Tholstrup 

 Pedersen 2 repeated these tests, heating the milk as quickly as 

 possible in a boiling water bath. As this method of heating is 

 less rapid than biorisation, the best results were obtained at a 

 somewhat lower temperature, i.e., 70 C. The fact that hay bacilli 

 and other heat-resisting organisms grow quicker in milk heated to 

 higher temperatures can only be explained by the supposition that 

 the bactericidal constituents of the milk are killed at temperatures 

 above 70 C. ; Tholstrup Pedersen has adduced other evidence in 

 favour of this supposition. From this it follows that the bacterial 

 count of milk immediately after pasteurisation is not a perfectly 

 .reliable gauge of its keeping power, as the keeping power depends 

 just as much on the bactericidal properties of the milk as on the 

 count (and of course also as on the nature of the organisms 

 present). If the milk is kept at 10 C. or lower temperatures, 

 these properties are more noticeable in lightly pasteurised milk 

 in which the surviving organisms have become weakened by 

 heating than in raw milk, an appreciable diminution in the 

 number of organisms taking place during the first twenty-four 

 hours. 



If milk is pasteurised at a high temperature (85 to 95 C.) or 

 boiled (as in Soxhlet's apparatus for pasteurising nursery milk), 

 only the spores survive, and milk or milk foods (e.g., chocolate) 

 treated in this manner will therefore not become sour on standing, 

 but will develop putrefactive or butyric acid fermentations. In 

 most cases Bacillus mycoides and other aerobic sporing bacteria 

 will be found. The milk soon acquires an unpleasant, sickly 

 taste, a particularly dangerous feature being that poisonous sub- 



1 " Maelkeritidende," 1915, p. 483. 



2 "Maelkeritidende," 1916, p. 231. 



2 



