122 PROTOPLASM 



milk is churned, the protective membrane is broken, and the fat 

 globules coalesce, forming butter. Among the numerous con- 

 stituents of milk, which include water, butterfat, salts, sugars, 

 albumin (lactalbumin), casein (caseinogen), and phosphatides 

 (lecithin), any one of the last three could function as the stabi- 

 lizer. Lectalbumin or caseinogen has usually been regarded as 

 the emulsifying agent in milk. The fact that the fat globules 

 in milk cannot be easily stained with Sudan III, which is an oil- 

 soluble dye, suggests that the membrane is protein, but it is 

 believed by some that the membrane material in milk is chiefly 

 phosphatide, a nitrogenous fat, rather than a protein. Perhaps 

 the stabilizer is a complex of fat and protein; caseinogen is itself 

 a phosphoprotein. 



Newer technique involving cataphoresis (page 361) gives an 

 ingenious method for ascertaining, with a fair degree of 

 certainty, the nature of the covering on colloidal particles. 

 Because of the charge which they possess, colloidal particles 

 travel in an electrical field. They reverse their charge, or, 

 rather, they are at zero charge, at the so-called "isoelectric 

 point," which is often expressed in terms of the acidity (pH) of 

 the solution. This point of zero charge is very specific. A sub- 

 stance can be characterized by its isoelectric point or point of no 

 migration in an electrical field. If the isoelectric point of milk 

 globules is at a pH, or acidity, value which is specific for protein 

 and not near the isoelectric point of phosphatides or other fatlike 

 substances, then it is likely that the coating on the fat globules 

 is of protein. The pH value at which the suspended particles 

 in milk show no movement in an electrical field is 4.6, which is 

 the isoelectric point of casein. On the basis of this evidence, it 

 would seem that the coating on fat globules in milk, i.e., the 

 stabilizer, is protein rather than some other substance. 



The stabilization membrane quite completely isolates the 

 dispersed phase of an emulsion from the dispersion medium. 

 It should, therefore, be possible to emulsify two miscible sub- 

 stances, such as an oil in an oil, if the stabilizer could be got 

 around the dispersed oil before it is put into the dispersion 

 medium. This has actually been accomplished with casein for 

 the membrane. 



The distinguishing properties of colloidal systems reside pri- 

 marily in the interface. Nowhere is this more true than in 



