THE RELATION OF MICROORGANISMS TO MILK 459 



PASTEURIZATION. The term pasteurization is used to designate 

 the process of heating milk to a temperature sufficient to destroy 

 a portion of the bacteria, including the pathogens, and then cooling it 

 to a temperature which will prevent the rapid development of the 

 organisms that are left. The temperatures commonly used for this 

 purpose vary from 60 to 85. The length of time the milk is exposed 

 to the high temperature may also vary from a few seconds to thirty 

 minutes, depending upon the method employed. The two chief 

 purposes for the pasteurization of milk are to destroy any pathogenic 

 organisms which the milk may contain and to increase its keeping 

 quality. The purpose for which the pasteurization is done will deter- 

 mine the method used. In commercial pasteurization, where the chief 

 purpose is to destroy the lactic organisms and thus improve the keeping 

 quality of the milk, the method used is that known as the "flash" or in- 

 stantaneous method, where the milk is subjected to a high temperature 

 for a few seconds only and then cooled. In this method of pasteuriza- 

 tion varying degrees of efficiency are obtained, depending upon a 

 number of factors, chiefly the bacterial condition of the milk to be 

 pasteurized, the degree of heat and the length of the exposure and the 

 temperature to which the milk is cooled. By this method, it is possible 

 to destroy a large percentage of the organisms in the raw milk, and 

 materially increase its keeping quality, but the temperature and time to 

 which any particle of milk is exposed cannot be accurately controlled, 

 and this method cannot be depended upon to kill all of the disease-pro- 

 ducing organisms which may be in the milk. This method has been 

 largely abandoned for the pasteurization of market milk. 



Under present conditions of the market milk business where the 

 chief purpose of pasteurization is to render the milk free from disease- 

 producing organisms, the so-called "holding' 1 method is employed. 

 This consists in raising the temperature of the milk to about 60 

 to 63 and holding it at this temperature for a period of twenty to 

 thirty minutes. If this method is properly done, most of the organisms 

 except certain spore forms should be killed and the milk at the end of 

 the pasteurizing process contain only a small percentage of its original 

 germ content. 



Formerly it was believed that heating milk to a high temperature 

 killed all the lactic acid organisms, and favored the subsequent growth of 

 other more undesirable species, but more recent studies on the bacterial 



