4o8 THE BACTERIAL ASPECTS OF PASTEURIZATION 



man. The temperature is a function of the time; i.e., for every degree above 140° F. 

 the time may be reduced, so that at 145° F. all the foregoing pathogenic micro-organ- 

 isms are killed in six minutes; at 155° C. within thirty seconds. The exposure under 

 commercial conditions should always be in excess of the minimum requirements so 

 as to provide a factor of safety, for it is of signal health moment in practical pasteuri- 

 zation to be certain that all the milk is uniformly heated to the temperature required. 

 By conmion consent, therefore, holding methods requiring thirty minutes have uni- 

 formly been adopted. Automatic records of the temperature and time should be kept 

 of each run. 



A factor of safety is necessary in commercial practice, and it has been found by 

 long experience in great cities handling enormous quantities of milk that i42°-i45° F. 

 for thirty minutes is safe and satisfactory, using equipment free from serious engineer- 

 ing defects. Milk thus pasteurized protects the public health and satisfies our knowl- 

 edge of the thermal death-points of the particular viruses as disclosed by bacteri- 

 ologists. Most cities and many states have adopted this definition of pasteurization, 

 but some still insist upon a minimum of 145° F. for thirty minutes. The dairy in- 

 dustry objects to heating milk above 145° F. because the cream line is affected, but 

 there is no sanitary harm from these temperatures. This, therefore, is clearly a com- 

 mercial and economic question rather than a health problem. 



No pasteurizing device has been made that will automatically heat all the milk 

 passing through under practical conditions to a given temperature and hold it pre- 

 cisely on the mark. There is a plus or minus deviation in the best designed heating 

 apparatus, often of 1.5° F. To pasteurize milk between 142° and 145° F., it is neces- 

 sary to set the thermoregulator at 143.5° F. When the sanitary code requires a 

 minimum of 145° F., the apparatus must be set to at least 146.5° F., and the milk 

 will then range between 145° and 148° F., thereby affecting the creaming ability of 

 the milk. 



As the result of years of extensive experience with commercial pasteurization 

 and of recent researches, it has been demonstrated that a temperature of i42°-i45° F., 

 with a holding period of 30 minutes, serves the purpose of protecting the public 

 health and preserving the integrity of the milk. This provides a factor of safety of 

 6° F. or twenty minutes' time at 142° F.; in fact. New York and other large cities and 

 several states have for some years used this standard for pasteurization with reason- 

 ably good results. Massachusetts requires a minimum of only 140° F. for thirty min- 

 utes; other states and cities permit or countenance the newer time and temperature 

 recommended; Pennsylvania insists upon a temperature of 145° F. for thirty minutes. 



THE EFFECTS OF HEAT UPON MILK 



The changes produced in milk by heat depend upon the degree of heat and the 

 length of time of exposure. Milk heated to 145° F. for thirty minutes does not undergo 

 any appreciable physical or chemical change. Higher temperatures, however, pro- 

 duce pronounced changes. In the main these consist of a partial decomposition of 

 the proteins and other complex nitrogenous derivatives; diminution of the organic 

 phosphorus and an increase of inorganic phosphorus; precipitation of the calcium and 

 magnesium salts and the greater part of the phosphates; expulsion of the greater 

 part of the carbon dioxide; caramelization or burning of a certain portion of the milk 



