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.
Milk is a mixture of fat, protein, carbohydrate and water. When raw milk sits, the fat separates from the rest of the liquid, rises to the top and forms a layer of cream. The purpose of homogenization is to keep the fat molecules dispersed throughout the milk, so separation does not occur and the milk remains uniform.