392 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



tion' of milk are to increase its keeping quality and to destroy any 

 pathogenic organisms which the milk may contain. The purpose 

 for which the pasteurization is done will determine 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 instantaneous 

 method, where the milk is subjected to a high temperature for a few 

 seconds only and then cooled. In this method of pasteurization 

 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. 



Where the chief purpose of pasteurization is to render the milk free 

 from disease-producing organisms, the so-called "holding" 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 

 flora of milk, pasteurized by the "holding" method, have shown that 

 some strains of the lactic acid bacteria can survive the relatively lower 

 temperatures used in this method, and that the later development of 

 the different groups of bacteria is similar to that in raw milk of equal 

 bacterial grade. 



Pasteurization at the temperatures used in the holding process does 

 not seem to cause any injuj-ious chemical changes in the milk constitu- 

 ents, or affect its digestibility. 



Proper pasteurization gives a valuable means of rendering the milk 



