PRESERVATION OF MILK AND ITS TREATMENT 85 



as the resulting product cannot always replace unsoured milk, 

 there is no need to go to this trouble unless facilities are lacking 

 for cooling the milk to under 14 C. l From the results of these 

 investigations it may also be understood why milk or cream 

 which has been homogenised at 60 to 70 C., and which commonly 

 has to stand for some time before it can be sterilised, may acquire 

 a bad taste. The homogenisation should rather be accomplished 

 at 80 to 90 C. , after which the milk should be sterilised at once. 

 While high temperature pasteurisation alters the relative pro- 

 portions between the numbers of the good and the harmful 

 bacteria to the advantage of the latter group, the opposite effect 

 may be produced by low temperature pasteurisation. Ayers and 

 Johnson 2 have shown that the lactic acid bacteria have, as a 

 group, greater heat-resisting powers than was formerly supposed, 

 and that a larger relative proportion of these organisms is to be 

 found in milk which has been warmed to 63 C. for half an hour 

 than in the raw milk. This would appear to be an additional 

 recommendation for low temperature pasteurisation. On the other 

 hand, it must be mentioned that the lactic acid bacteria which 

 survive this process sour the milk slowly at ordinary temperatures, 

 and that there is yet a possibility that the milk treated in this way 

 may suffer undesirable changes before it becomes sour. If the 

 raw milk is particularly good, and therefore poor in true lactic 

 acid bacteria, there is a possibility that it may eventually become 

 just as harmful after low temperature pasteurisation as after 

 heating to higher temperatures. In ordinary milk the true lactic 

 acid bacteria are only killed off entirely by heating to 77 to 82 C. 

 for half an hour. In agreement with this observation, Tholstrup 

 Pedersen 3 found that milk which had been heated to 80 C. still 

 contained many lactic acid bacteria, but that milk heated to 

 85 C. contained none. The author has found that the lactic acid 

 bacteria growing at high temperatures, i.e., the thermobacteria, 

 and in addition the microbacteria, Streptococcus thermophilus, 

 Streptococcus fcecium, Streptococcus glycerinaceus, and a few micro- 

 cocci are the lactic acid bacteria which best survive heating. 

 The heat-resisting powers of the thermobacteria have been taken 



1 The foregoing remarks, as well as those on the same subject on p. 70, 

 are of particular interest in connection with the Danish co-operative dairy 

 industry, in which separated milk is obtained as a by-product from butter 

 making and sent to the farms, where it is extensively used for feeding pigs. 

 The matter is, however, of general interest as an example of the great 

 importance of cooling after pasteurisation. In the large dairies of the 

 United Kingdom the separated milk is usually treated as recommended 

 above before sending to biscuit and other factories. Translator. 



2 U.S. Dept. of Agric. Bureau of Anim. Industry, Bull. 161, 1913 ; 

 " Journ. of Agric. Kesearch," 1914, vol. 2, No. 4. 



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