As infant mortality rose in cities, standards and practices for making milk safer, like pasteurisation, began more important. Around the same time, there was a growing interest in treating people through food, giving the sick only the purest, simplest things to eat. It was an idea espoused by John Harvey Kellogg, the inventor of the corn flake and head doctor of a famed sanitarium in Michigan, and many others like him.

“You might say it was an age of health foods and health movements,” Valenze says. Modern life and food were complicated and corrupt, but milk, consumed by everyone early in life, was simple, earthy, and natural, in the logic of the time. (The fact that it was white – the colour of purity – didn't hurt either.) To boot, milk had fat, carbohydrates, and protein, all three of the components needed by the human body.

Temperance effect

Different countries have their own interesting variations on this tale: around the turn of the 20th Century in Germany, writes University of Basel historian Barbara Orland, the advent of grown men drinking milk was closely intertwined with temperance movements, as well as with the search for simple, healthy foods. In an attempt to shift the culture of drinking beer and spirits, especially in factory workers, temperance groups opposed to drinking alcohol pressed for the serving of milk in factories and even set up milk booths in towns, with some success.

As researchers deepened their knowledge of nutrition in the early decades of the 20th Century, the idea that milk had fat, carbohydrates, and protein waned as a justification for milk's perfection. But its role as a source of newly discovered vitamins and the idea that it was somehow able to correct the deficiencies in any diet more than took up the slack. Elmer McCollum, a biochemist and dietary scientist, wrote an influential book in 1918 called The Newer Knowledge of Nutrition, in which he called it “without a doubt our most important foodstuff”.