320 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



the keeping quality of the milk, the method used is that known as the 

 continuous 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. With all these factors to be 

 considered it is not surprising that the germ content in the pasteurized 

 milk varies widely. In the instantaneous method of pasteurization, the 

 lactic organisms are killed first, while any spores which are present 

 are allowed to pass through the machine and remain in the milk. These 

 spores may result in the rapid development of certain putrefactive types- 

 of organisms in the pasteurized milk, until its bacterial condition at the 

 end of a few hours may be worse than before pasteurization. If, however, 

 the milk was of reasonably good quality, the process carefully done and 

 the milk properly cooled, the bacterial quality of the milk may be materi- 

 ally improved as a result of the pasteurization. This method of pasteur- 

 ization cannot be depended upon to kill all of the disease organisms which 

 may be in the 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, all of the organisms 

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

 the pasteurizing process contain but few organisms compared with its 

 original germ content. 



It is believed by many that heating milk to a high temperature hastens 

 the growth of any organisms which may remain in it or afterward gain 

 entrance. Others maintain that this is not true. The weight of evidence, 

 however, seems to indicate that it is a fact. This being the case it is 

 evident that extreme care should be taken with pasteurized milk to pre- 

 vent subsequent contamination and also the development of any organ- 

 isms which may not have been killed by the pasteurizing process. 



THE USE or CHEMICALS. The addition of certain chemicals to milk 

 will retard the growth of bacteria. The chemicals most commonly used 

 for this purpose are borax and formalin. While the keeping quality 

 of milk may be materially increased by the use of such chemicals, their 



