84 DAIRY BACTERIOLOGY 



stances may be formed before any apparent change has set in. 

 Immediate cooling to under 14 C., or better still to under 10 C., 

 delays these changes considerably. As many of the sporing 

 bacilli can grow and even thrive well at high temperatures, 

 the slow cooling of milk pasteurised by the methods under 

 consideration may have disastrous results ; Tholstrup Pedersen 

 has shown l that milk treated thus is affected most rapidly at 

 70 to 60 C. At these temperatures non -motile, aerobic rods 

 will appear after only three hours' standing ; after four hours 

 the milk is so changed that it will no longer stand boiling, and 

 after six to eight hours it coagulates owing to the action of 

 fermentation acid and bacterial rennet. At 60 to 50 C., the 

 bacteria in question do not grow so rapidly, but in their place 

 anaerobic plectridium forms develop freely. It is only under 50 C. 

 that the common hay and potato bacilli appear, while at 40 to 

 30 C. the true butyric acid bacteria appear as welL The above 

 outline gives an idea of the possible bacterial developments in 

 milk pasteurised at high temperatures and allowed to cool spon- 

 taneously. It has an important bearing on the treatment of skim 

 milk in the Danish co-operative dairies ; as was mentioned above, 

 the skim milk is sent back warm from the pasteuriser to the farms ; 

 it is contained in cans holding up to 50 litres, and after three to 

 four hours its temperature may be above 50 C., and in summer 

 sometimes above 60 C. 2 Formerly it was thought that the holding 

 of milk at high temperatures over long periods enhanced the 

 effect of pasteurisation, but now it is known that while this 

 treatment certainly tends to destroy the normal flora of the milk, 

 especially the lactic acid bacteria, it also encourages the growth 

 of another and far more dangerous group of organisms. For this 

 reason skim milk should be thoroughly cooled in the dairy ; there 

 is no reason for heating it to temperatures above 80 to 85 C., the 

 limit set by the Danish pasteurisation law. If this procedure is 

 adopted, the cans must be cleaned before receiving the cooled 

 milk ; the extra trouble thus involved is amply paid for by the 

 saving in coal which may be effected by the use of the regenerative 

 system of cooling ; moreover, the quality of the separated milk 

 will be improved both as regards taste and keeping powers, while 

 the tinning on the cans will last longer. It should be pointed out 

 that the cleaning of the cans by the dairy by no means relieves 

 the consumers from the obligations in this connection. The 

 harmful bacteria may also be suppressed by souring the milk, but 



1 " Maelkeritidende," 1915, p. 817, and 1916, p. 35. 



2 These investigations also point to the necessity for caution in hay-box 

 cookery, the food being kept for many hours at temperatures favourable 

 to the development of thermophilic bacteria. 



