Excerpted from Pure and Modern Milk: An Environmental History Since 1900 by Kendra Smith-Howard, out now from Oxford University Press.

Do you prefer the lighter flavor of skim milk to 1 percent, 2 percent, or whole? Before World War II, skim milk—a byproduct of butter processing—was not sold in stores, but either discarded or fed to chickens, hogs, and calves as a protein-rich replacement for costlier animal feed. The development of skim milk as an attractive product for sale only came about because dairy producers, emboldened by their success selling milk to Uncle Sam during World War II, seized on postwar marketing opportunities to sell what once had been hog slop to housewives and families.

In the 1920s, the streams in rural areas with cheese factories and creameries festered with rotting dairy discharge. Excess buttermilk spilled from creamery spouts onto adjacent fields and directly into waterways. Ditches brimmed with whey, attracting flies and creating a stinky mess. Many creameries had no access to a city sewer. In 1930, only 17 percent of waste released by Wisconsin milk plants was treated, leaving 42,513 pounds of untreated whey, skim milk, and other byproducts to be drained into the state’s waterways.

The release of skim milk and whey into streams was not a new practice, but changes in the rural economy of the 1920s and 1930s—the rise of automobile tourism and the farm depression—changed the way that industrialists understood and reacted to milk plant pollution. The increase in rural tourism during the interwar period made it more difficult for dairy plants to hide the deleterious effects of dairy manufacturing from the public eye. As the number of Americans with automobiles increased and rural roads improved, motorists sought to take invigorating outdoor adventures in the countryside. When tourists who ventured to the countryside to be energized by fresh breezes found themselves breathing the foul vapors of “dairy air” instead, disappointment ensued. One manager explained in the late 1920s:

The automobile has brought the cheese factory odor and conditions to the attention of countless thousands of people, and no amount of argument or favorable advertising is going to convince the casual traveler that our cheese is of a high quality, when the attending conditions are such as to produce a feeling bordering on nausea.

The dairy industry’s long-standing use of natural scenes and ideals to promote the healthfulness of the product made the degraded landscape surrounding cheese and butter plants especially problematic. Dairy waste undermined the pastoral iconography used to sell dairy foods. The residue also appeared particularly wasteful during a time of economic strife. Ditches brimming with whey and buttermilk signaled the ecologic ills and economic imbalances threatening national prosperity.

Tourism, combined with a new attention to wasted resources in the farm depression, recast a persistent problem with dairy waste as one of national economic significance and by the interwar years, dairy plants began to consider whether they might be able to market dairy residues nationally or even internationally. Managers of cheese plants and butter factories sought to make the substances left over from the cheese- and butter-making processes into consumer goods or industrial materials.

In the 1920s and 1930s, manufacturers first used organic chemistry to remake excess whey, skim milk, and buttermilk into commercial industrial products, hoping to increase returns to farm patrons and eliminate the sanitary and legal problems such wastes generated. The transformation of agricultural byproducts into industrial resources came to be known as chemurgy, a word coined by an organic chemist with Dow Chemical Co. By 1935, 300 scientists, industrialists, and members of farm organizations gathered to form the Farm Chemurgic Council, a group devoted to studying and establishing new nonfood uses for farm crops and markets for agricultural byproducts. Unlike nutritionists, who lauded the balance of proteins, carbohydrates, and fats in milk, chemurgists sought to break down milk into its component parts and find uses for each of them. Chemurgy became a way to reconcile the growing tension between modernity and agrarian tradition.

In the dairy industry, chemurgists were especially interested in the protein of milk, called casein. Extracted from skim milk, the byproduct of butter processing, casein could be integrated into a wide variety of household goods. As early as 1900, Henry E. Alvord reported that casein could substitute for eggs, replace glue in paper sizing, and be hardened into plastic to make buttons, combs, or electrical insulators. During World War I, casein coated airplane wings.

The most dramatic transformation of casein was its reincarnation as an object of high fashion. In 1936, Italian chemists developed a process to make a wool-like fabric from casein, and soon an Italian textile firm began to produce thousands of pounds a day of the product, called “lanital.” By 1938, two USDA Bureau of Dairy Industry scientists applied for patents for its manufacture in the United States. The National Dairy Products Corp. began commercial production of casein wool bearing the trade name “aralac” in 1941. Casein fibers soon replaced fur in hats and were used to upholster car seats. In 1939 and 1941, casein fabrics were at the center of fashion reviews in New York City.

“Milk wool” symbolized chemurgists’ modernist vision. In casein fibers, scientific innovation seemed to trump natural limits; bizarrely, cows could produce wool! Food consumers might have been ambivalent about artificial flavors in their foods, but in fashion, artifice signified novelty and originality. Synthetic fibers could achieve drape and colors that natural cloth and dyes could not.

Big ideas about casein spun by the press, however, loomed larger than actual production of casein fibers. Manufacturers never devoted more than 4 or 5 percent of casein to fabric production. By 1948, even National Dairy Products Corp., the first company to make casein wool in the United States, discontinued making casein fiber.

While the vision of agrarian modernity promoted by chemists and industrialists for casein was never fully actualized, World War II successfully reincarnated skim milk from a locally utilized byproduct into one with national and international appeal. Skim milk entered World War II before American soldiers did. The U.S. secretary of agriculture asked for expanded production from dairy farmers in July 1941, months before the attack on Pearl Harbor. Dairy products quickly became essential to the lend-lease and war relief programs. Whereas evaporated milk was the preferred relief food during and after World War I, dried milk powder’s transportability and long shelf life gave it the favored spot in the lend-lease formulary. By 1941, the federal government asked for 200 million pounds of dry skim milk powder for America’s allies. Dried skim milk manufacturers struggled to keep pace with the unprecedented demand.

But as the war drew to a close, marketing, not production concerns, loomed large in the minds of dried milk companies. Manufacturers recognized that producing dried nonfat milk at record levels would not ensure the industry’s success. Without a market for the product, dairy farmers would once again be faced with the surpluses that had plagued them in the 1930s. Thus, milk companies adopted a new strategy to promote skim milk in the postwar era: selling it on its own merits.

As prices for whole milk increased in the late 1940s, milk dealers in the fluid milk market, as well as dried milk dealers, turned to skim milk as a promising product in its own right. Though many consumers were skeptical about the value of skim milk, dairy companies enticed them with promises that drinking skim milk would help them lose weight. Milk dealers secured the backing of physicians. As had been the case for certified and pasteurized milk in the Progressive Era, the recommendations of physicians gave skim milk newfound legitimacy. Although physicians had long suggested nonfat milk to patients who had difficulty digesting fats or were elderly, weight-conscious consumers became the largest sector of the skim milk market in the 1950s. Emphasizing skim milk’s role in promoting slenderness transformed skim milk’s reputation as a low-cost relief food to one that high-income dieters would embrace.

The desire to diet drew largely from consequences of postwar abundance. Equipped with deep-freezers and family-size refrigerators, Americans had access to a greater quantity of food at home than ever before. As Americans settled into the sedentary culture of drive-ins and television entertainment, many gained weight. Skim milk held a prominent place on the dieters’ menu. Gayelord Hauser’s Live Younger, Live Longer, which topped the best-sellers’ list in 1951, called skim milk a “wonder food.” Diets printed in women’s magazines and even in Hoard’s Dairyman recommended the food. A Sealtest skim milk offer gave consumers the chance to get a new bathroom scale with a proof-of-purchase tag. As concerns about cholesterol and heart disease intensified, the idea of skim milk as a healthy food would only become more entrenched.

One of the most striking features of dairy companies’ push on skim milk as a diet food was the feminization of the product. In the past, milk marketers had entreated women as mothers to buy milk to ensure children’s health and safety, but dealers seeking skim milk buyers courted women as an adult market. Skim milk advertisements emphasized not innocence and wholesomeness, but svelte sexuality. A 1950 advertisement printed in Milk Dealer, for instance, encouraged milk salesmen to pursue the low-fat milk market by picturing a bikini-clad beauty and the words “She Can Be Yours.” Advertisements for Sealtest skim milk suggested that by sipping skim milk, an overweight woman could lose pounds, regain a youthful complexion, and win back her man. Selling skim milk as a potion of slenderness, and thereby romance, certainly gave it more prestige than it carried as a mere byproduct of butter-making or as hog slop.

Excerpted from Pure and Modern Milk: An Environmental History Since 1900 by Kendra Smith-Howard, out now from Oxford University Press.