Homogenization disrupted the chemical structure of the milkfat so drastically as to release a torrent of enzymes that promptly turned raw milk rancid. Even when dairy chemists learned to sidestep rancidity by combining the steps of pasteurizing (which inactivated the enzymes) and homogenizing, there remained the age-old consumer habit of judging milk by its richness—i.e., the thickness of the cream layer on top. When packaging in glass bottles came in toward the start of the twentieth century, one of its advantages from a buyer’s point of view was the plainly visible ‘creamline.’ The fact that homogenized milk in glass tended to acquire an unpleasant oxidized flavor on exposure to light more rapidly than creamline milk was another strike against it.

As a result, until shortly after World War II few people saw any reason to want homogenized milk. Milk for drinking was almost without exception available in only two degrees of richness: with or without all the original fat. Skim milk, or what was left when the cream was separated for other purposes, was the ugly sister. Health experts warned mothers that it was paltry stuff, deficient in crucial nutrients. (Most states required that it be fortified with vitamin A to replace the fat-soluble beta-carotene that disappeared along with the cream; this step is still mandatory for fat-free and most reduced-fat milk.) At the nation’s creameries skim milk was an unvalued by-product, often dumped for lack of any profitable use.

As early as the late 1930s a few dairy processors had been trying to win people over to homogenized milk. The turning point came with a postwar shift to opaque or cardboard containers in place of returnable milk bottles. This in turn accompanied another shift away from home delivery and toward supermarket purchases of milk. Consumers and supermarket managers adored the convenience of throwaway packaging, Milk processors and distributors loved the fact that cardboard couldn’t be seen through, which incidentally solved the oxidation problem. It was the perfect moment for abolishing creamline milk and substituting a product whose appearance had previously weighed against it.