116 PASTEURIZATION OF MILK. 



point, bring about profound changes. The .significance of 

 these, in connection with the digestibility of the altered con- 

 stituents, has been widely discussed. Important manifestations 

 of the effect of heat on milk concern temperatures higher than 

 those necessary for pasteurization. 



Creaming quality . Exposure to heat above certain limits so 

 alters the physical constitution of milk that the cream does not 

 rise naturally but remains more completely emulsified. The 

 public judges milk largely by the amount of cream that rises, 

 especially in the case of bottled milk where the location of the 

 "cream line" is so easily detected. The same causes that 

 operate in this regard in milk affect market cream. The 

 " body " is lessened, the cream looks of a poorer quality than 

 it really is, its whipping quality is affected, and consequently 

 its salability is impaired. These are weighty considerations 

 and the application of heat must conform to them. Exposure 

 to 160F. (71C.) for one minute, or long exposures at 140F. 

 (60C.) are both safe (2). 



Vital qualities of milk. Milk has certain vital qualities, 

 whose sensitiveness to heat deserves especial consideration with 

 respect to the use of heated milk as infant food. Among these 

 are the proteolytic, oxidizing, and fat splitting ferments. 

 Here, too, may be mentioned the alexins, according to Von 

 Behring, and the agents causing the so-called " germicidal " 

 phenomenon, probably agglutinins. An exposure to not more 

 than 140F. (60C.) for forty minutes avoids the impairment of 

 all these qualities of milk (3, 5). 



Spore- bear ing bacteria. Spores of bacteria in milk are not 

 killed by one exposure to heat unless it is above 212 C F. Any 

 of the processes of pasteurization will fail to kill bacterial 

 spores, that ordinarily constitute less than \% of the bac- 

 teria present. The hygienic significance of spore-bearing 

 bacteria in such small numbers in fresh pasteurized milk is of 

 no consequence, and the fact of their survival has been of 

 necessity ignored in formulating methods of pasteurization. 

 Their significance in old pasteurized milk has been the subject 

 of considerable discussion. The varying percentage of spores 

 in different lots of milk results in the variation of the bacteri- 



