Pasteurization has been around since Louis Pasteur discovered it in the min-1800s. The treatment also destroys most of the microorganisms that cause spoilage and so prolongs the storage time of food. Packaged in sterile, hermetically sealed containers, UHT milk may be stored without refrigeration for months. Prior to pasteurization, many diseases were transmitted through raw milk to children and adults alike. That problem has been completely eliminated.
Homogenization is more recent.
If you take a gallon of fresh milk straight from a cow and allow it to sit in the refrigerator, all of the cream will completely separate, leaving you with skim milk and a layer of cream. Homogenization is the process of breaking up the fat globules in cream to such a small size that they remain suspended evenly in the milk rather than separating out and floating to the surface. This page describes the actual process used in great detail. Inc. All rights reserved.