Preservation of Milk. 117 



of creaming property, and especially in cream of a marked 

 decrease in thickness. Both of these conditions seriously 

 interfere with the general extension of pasteurization, be- 

 cause the consumer does not like a milk on which the 

 cream does not rise thoroughly. The reasons which under- 

 lie this physical change have already been noted (p. 109), 

 and it should be further observed that if milk is not 

 pasteurized at a temperature exceeding 140 F., this 

 change in condition does not obtain. Milk heated to this 

 temperature satisfactorily fills the biological requirement, in 

 that the vegetating forms of the acid-producing as well as 

 the disease bacteria are destroyed. The consequence is 

 that the keeping quality of such milk is practically as good 

 as if it was heated to a temperature of 155 F. The appli- 

 cation of this temperature results in the preparation of a 

 milk or cream that more closely approximates the condi- 

 tion of the normal product, while at the same time such 

 milk possesses practically all of the advantages that are 

 found in that heated to a higher temperature. 



Bacteriological studies. The following bacteriological 

 studies as to the effect which a variation in temperature 

 exerts on bacterial life in milk are of importance as indi- 

 cating the proper temperature limits to be selected. In 

 the following table the exposures were made for a uniform 

 period (20 minutes): 



The bacterial content of milk heated at different temperatures. 



Number of bacteria per cc. in milk. 



45C. 50C. 55C. 60C. 65C. 70C. 



Unheated 113F. 122F. 131F. 140F. 149F. 158F. 



Series I. 2,895,000 1,260,000 798,000 82,000 5,770 3,900 



Series II. 750,000 665,000 262,400 201,000 950 700 705 



Series III. 1,350,000 1,100,000 260.000 215,000 575 610 650 



Series IV. 1,750,000 87,3ftO 4,000 3,500 3,600 



