374 



PROTOPLASM 



change the sign of the charge of the protein ions, provided that 

 the point of change (specific pH value) is passed. 



Fat globules in milk are coated with a stabilization membrane 

 which is probably protein (caseinogen) . If this is true, then 

 acid milk should be positively charged, and basic milk negatively 

 charged. The fat droplets of pure milk are usually negative. 

 The pH value of fresh milk is about 6.6. If acid is added until 

 the pH value falls below 4.5, the globules become positively 

 charged and migrate to the cathode. The direction of migration 

 reverses at a definite pH value (4.55) and the rate of migration 

 becomes progressively slower as this value is approached. In 

 the following table is recorded the change in rate of travel of the 

 fat (protein coated) particles in milk with change in pH. 



Cataphoresis of Milk 



The foregoing table may be expressed as a curve (Fig. 162). 



The point at which particles in colloidal suspension reverse 

 the sign of their charge is a very characteristic one and is specific 

 for many substances. It is known as the isoelectric point. Pro- 

 teins can sometimes be identified by it. It is usually expressed 

 in terms of acidity (pH). The isoelectric point of milk is 

 pH 4.55. This may be taken as convincing evidence that the 



