Homogenized milk is milk that has been processed in milk plants to break up fat globules into smaller fat particles that will not separate or rise to the milk surface to form a cream layer. Milk is pumped through a restricted orifice or valve under high pressure, which causes the fat globules (3-4 microns) to fragment into small fat particles (< 1 micron). Homogenization occurs just prior to the pasteurization of milk. If homogenized milk is not heated to the pasteurization temperature, an enzyme in milk will cause the milk fat to break apart to create a bitter, soapy taste. Homogenization increases the number of fat particles and causes them to remain in a homogeneous dispersion throughout the milk container.

The chemical composition of milk is not changed since nothing has been added or removed during the homogenization process. Homogenized milk, as sold with various levels of fat, is a safe and healthy food for human diets, including those for pregnant women. (See pp. 190-191 in Patton reference below.)

Mike Hutjens, Extension Dairy Specialist, University of Illinois - Illini DairyNET

Additional reading:
Stuart Patton. Milk. Transaction Publs., New Brunswick, NJ. 2005.