266 ANNUAL. REPORT OF THE Off. Doc. 



PASTEURIZATION OF CITY MILK. 



Pasteurization lua^- be defined as a process of applying heat to 

 milk for the purpose of destroying the vegetable or growing forms 

 of bacteria. It is really partial sterilization and may be accom- 

 plished in two ways: 



1. By heating the milk to a comparatively high temperature for 

 a few seconds only. 



2. By heating the milk to a lower temperature for some minutes. 



The first is known as the continuous, the second as the intermittent 

 process. The time required will bear some relation to the degree of 

 temperature used. 



It is not advisable to use the first metliod for treating market milk. 

 The normal milk fermentation may be conveniently divided into three 

 classes; namely: 



1. The lactic acid. 



2. The butyric acid. 



3. The peptogenic or putrefactive. 



The peptogenic and butyric organisms are spore-bearing, that is, 

 when introduced into an unfavorable environment, as may result 

 from the application of high or very low temperature, they are capa- 

 ble of producing very resistant, hard, glistening bodies which de- 

 velop into bacteria when again introduced into a favorable environ- 

 ment. Because of these spores, these two classes of bacteria are 

 very hard to destroy. A temperature of 185 degrees for a few 

 moments, or 155 degrees F. for 15 or 20 minutes will destroy most 

 of the vegetative forms, but will not destroy the spores. These 

 spores will become bacteria if the temperature of the media in which 

 they are found is reduced to 70 to 90 degrees F. They correspond 

 somewhat to the seeds of the higher plants in their action. The 

 latic acid bacteria are destroyed very easily — a temperature of 140 

 degrees F. applied for a few minutes accomplishing this result. In 

 addition to these organisms producing normal 'sno^i3:^n8rajc>j there 

 are sometimes found in milk what are called "pathogenic," or dis- 

 ease-producing organisms, such as the germs of typhoid fever, tuber- 

 culosis, diphtheria, etc. These pathogenic bacteria are rendered 

 non-injurious by continuous pasteurization at 185 degrees F., or by 

 an intermittent exposure for 15 to 20 minutes at 155 degrees F. 



From the above facts concerning bacteria and their relation to 

 temperature, it is apparent that an application of heat to the ex- 

 tent of 185 degrees F. for a few minutes, will destroy or render non- 

 injurious most of the bacteria found in milk; but on the other hand 

 such a high temperature will give the milk a decided cooked taste^ 

 which renders it unpalatable to most consumers. To avoid this 

 cooked flavor, milk should not be heated above 158 degrees F. This 



