THE CELLULAR CONTENT OF MILK 



141 



When milk is heated for a second time no appreciable further 

 increase in the cell content was found, viz : 



He suggests that this fact might be utilised in determining 

 whether milk had been previously heated or not. 



The increased leucocyte count obtained by Russell and Hoff- 

 mann was believed by them to be due to alterations in the fat 

 globule clusters. They state that milk when heated for ten minutes 

 to 60 C. exhibits a great reduction in the creaming-power, 

 which reduction appears to be due to the breakdown of the fat 

 clusters. In rising to the surface these fat globules ordinarily 

 entangle with them many leucocytes. When the fat-globule clusters 

 are broken down by heat they are no longer capable of enmeshing 

 the leucocytes as they rise in the milk on centrifuging. Campbell 

 agrees with this explanation of the results obtained by heating milk, 

 and believes also that the leucocytes settle more freely with the 

 sediment as a result of becoming disintegrated from the fat globules 

 due to the effect of heat. In unheated milk he believed that only 

 a small proportion of the leucocytes settle with the sediment. 



Prescott and Breed (1910) investigated further methods for 

 the estimation of the cell content of milk, and believed that accurate 

 results can only be obtained when the milk is used without centri- 

 fuging. They compared the methods in which the sediment is 

 used and the results obtained when milk is used without previous 

 manipulation. 



The method employed by them was as follows : A small quantity 

 of milk (o-oi c.c.), which has been carefully mixed but not centri- 

 fugalised, is spread on an area of definitely known extent (i sq. cm.). 

 The film is allowed to dry and the fat removed by washing with 

 alcohol. The film is then stained with methylene blue and the cells 

 counted. It is desirable to count 100 fields at random. Each cell 

 found will represent 5000 cells per c.c. in the milk. These authors 

 show that any method which involves the use of the centrifuge 

 is very inaccurate, since a great number of the cells go up in the 

 cream and many also lie in the fluid just below the cream. Only 

 from 2-5-50 per cent, of the cells are found in the sediment as 

 a result of centrifuging, and the variations obtained are too great 



