CHAPTER IX 

 PASTEURISED OR HEATED MILK 



In addition to the usual bacteriological tests it is occa- 

 sionally advisable to examine pasteurised milk with a view to 

 determining the nature of the heat treatment to which it has 

 been subjected. Prolonged heating at temperatures exceeding 

 150° F. results in the destruction of the enzymes and the loss 

 of albumin and soluble phosphates; the fat globules may also 

 be so altered that they do not rise normally and so affect what 

 is commercially known as the " cream Hne.'^ 



The effect of time and temperature, the two factors con- 

 trolKng the general effect, have been admirably expressed by 

 Dr. North of New York, in a diagram which, with slight modi- 

 fications to bring it into harmony with the author's results, is 

 reproduced on page 187. . 



For the detection of overheated milk, several methods are 

 available: (1) determination of the cream hne, (2) enzyme 

 reactions, and (3) estimation of the albumin. 



Cream Line. Place 100 c.cms. of the sample in a cream- 

 ometer or graduated cylinder and observe the percentage of 

 cream obtained after standing for six hours at 60° F. If less 

 than 2.5 per cent of cream rises for each 1 per cent of fat con- 

 tained in the original milk, the presence of heated milk must be 

 suspected. If less than 2.5 per cent of cream is found for each 1 

 per cent of fat, the sample may either be milk pasteurised at a 

 temperature ej^ceeding 150° F., or a mixture of sterilised and 

 fresh milk. 



Bnzymes. The effect of heat on milk enzymes has been 

 studied by many workers and the more important results are 

 given in Table LXI. 



185 



