Pasteurization of Milk. 195 



a short time at a relatively low temperature, which when properly 

 applied will appropriately destroy the bacteria. 



The vegetative bacteria may be destroyed by subjecting them 

 to the influence of heat at 60-70 deg. C., for one-half hour, or to a 

 temperature of 85 deg. C. for a half minute; on the other hand 

 it is known that spores of bacteria not infrequently resist a tem- 

 perature of 100 deg. C. and over. 



In practice it is advisable to abstain from the sterilization of 

 milk with high degrees of temperature, and to apply pasteurization, 

 since, through the usual method of sterilization the destruction of 

 all germs is not attained and the disadvantages are too apparent. 



The wholesale depots may be provided with outfits for bottle 

 pasteurization and milk heating, in which flowing milk, through 

 the influence of steam on heating surfaces may be brought to a 

 temperature of 85 deg. C. In heating bottled milk it is essential 

 to observe that the milk should become uniformly heated through- 

 out ; this is attained by shaking the milk during its pasteurization. 

 Following this, rapid cooling should be undertaken, which is best 

 accomplished by atomizing pipes which cause water to fall upon 

 the hot bottles in the form of a spray. 



The apparatuses in which the milk flows over heated surfaces 

 should be so constructed that all parts of the milk will come in 

 contact with the heated surface, making the heating of the milk 

 uniform in all parts. The utilization of the heat in some of the 

 appliances is regulated in such a way that the cold inflowing milk 

 is warmed by the outflowing pasteurized milk, the latter, however, 

 being cooled subsequently. Through the exchange of heat from 

 the outgoing stream of milk about one-half of the required heat 

 may be saved. The efficiency of some of these apparatuses is 

 enormous, since they are able to treat from 5000 to 8000 liters per 

 hour. 



From a sanitary standpoint it is apparent that such appara- 

 tuses must be so constructed that they may be readily cleaned 

 mechanically, since improperly cleaned places conduct the heat 

 poorly, and may give rise to contamination of the milk with putre- 

 factive bacteria. 



In pasteurization, the same as in milk production, the greatest 

 stress should be laid on immediate and thorough cooling, and on 

 keeping the milk continuously cool until its consumption, since 

 otherwise the pasteurized milk will become spoiled, and will un- 

 dergo a form of decomposition which is very undesirable (espe- 

 cially in bottle pasteurization). Pasteurized milk decomposes 

 through multiplication of protein splitting, peptonizing bacteria 

 whose spores may have withstood the heating. The vegetative 

 bacteria, among these the lactic acid producers, are mostly de- 

 stroyed, and except for a few resistant forms of spore bearers only 

 heat-resisting organisms will remain viable, but these forms of 

 bacteria are usually harmless (Rullmann, Gerber and Wieske, 



