Definition of substitution. 1  1a : the act, process, or result of substituting one thing for anotherb : replacement of one mathematical entity by another of equal value. 2  2 : one that is substituted for another.
Homogenized milk is any sort of milk that has been mechanically treated to ensure that it has a smooth, even consistency. The homogenization process typically involves high temperatures, agitation, and filtration, all aimed at breaking down milk's naturally occurring fat molecules.
Homogenization today is usually a two step process. The first stage, similar to Gaulin's early device, pushes milk through small, tapered tubes or pores. As the diameter shrinks and the flow of milk remains constant, pressure builds up and fat globules break apart in the turbulence.