Centuries after the first wheel of Emmentaler rolled into the Alps, scientists are still learning about how Swiss cheese is made. PHOTOGRAPH BY BERLINER ILLUSTRATIONS GESELLSCHAFT / ULLSTEIN BILD VIA GETTY

“On earth I was a manufacturer of Imported Holes for American Swiss Cheese,” the braided man tells Dorothy, in her fourth adventure in Oz. “I will acknowledge that I supplied a superior article.” Ultimately, though, the man explains, he fell into one of his own holes and got stranded down near the center of the planet. Which leaves the big question unanswered: Where, in fact, do the holes in Swiss cheese come from?

The long version of the story, according to the cheese historian Paul Kindstedt, is that they come from the Alps. Although it looks picturesque on the packet, the mountainous landscape of Switzerland actually made medieval cheese-making quite a chore. Salt had to be hauled a long way uphill from a faraway coast, and the Swiss authorities, for reasons best known to themselves, levied taxes according to the number of cheeses sold rather than their net weight, which incentivized the production of oversized wheels (the typical Emmentaler is three feet in diameter, weighs two hundred and twenty pounds, and uses one and a half tons of milk). In response to these challenges, fromagers developed work-arounds—new curd-cutting tools, higher cooking temperatures, specialized presses—to make sure that the cheese came out dry enough. The result was a product with just the right elasticity and low enough acid and salt levels to favor the growth of the carbon-dioxide-burping microbe Propionibacterium freudenreichii, whose metabolic by-products give Swiss cheese both its nutty, sweet flavor and its holes.

Eventually, this accident of culture turned into a desirable quality; a cheese’s negative space acquired value. “The holes became something that people expected,” Mike Tunick, a research chemist for the U.S. Department of Agriculture, and the author of “The Science of Cheese,” told me. Imagine the horror, then, when the holes began to disappear—or, to put it scientifically, given that the holes are properly known as eyes, when the cheese began to go blind. “It was during the last ten or fifteen years,” Walter Bisig, a scientist at Agroscope, the Swiss government’s food-research center, said. “The farmers told us, and we saw it in the cheese-association grading reports: there were just fewer eyes.”

Naturally, Bisig and his colleagues in the Food Processing Technology Group took an interest. The main problem was how many potential problems there were. “It’s the hardest cheese to make well,” Stephanie Clark, a dairy specialist at Iowa State University, told me. “There are so many steps in the process, and the list of things that can go wrong is almost endless.” In addition to blindness, Swiss cheese can suffer from streuble (an overabundance of small eyes just under the rind); cabbaging (overcrowded, irregular eyes); frogmouth (spindle-shaped eyes); or any one of about a dozen other defects, including dull or dead eyes, in which the eye walls lack what Clark described as the “preferred shiny lustre.” Each of these flaws has a range of possible sources, from a curd that is too acidic or too firm to fluctuating storage temperatures to bacterial underperformance. Isolating a single cause is an exercise in frustration, Clark said, because it’s all but impossible to peer into a cheese without ruining it. Scientists have sometimes resorted to MRI machines or CT scanners. “It’s not like cheddar,” she said. “Once you cut into your Swiss, you’re done. You don’t have any pressure left to hold the carbon dioxide in.”

Another issue, besides the fickleness of the cheese itself, was that no one knew how the eyes decided where to show up. One theory, to which Mike Tunick subscribed, held that the carbon dioxide simply accumulated at “weaker spots in the cheese matrix.” Other experts felt sure that the eyes formed around some kind of nuclei, in much the same way that raindrops condense onto specks of dust. The clue, finally, came from the way that eyes wax and wane throughout the year. More blind cheeses are born in summer, when cows are out grazing on grass, than in winter, when they are in the barn feeding on hay. “That is how we came up with the hay hypothesis,” Bisig said—the idea that microscopic hay particles in the milk were serving as nuclei.

Bisig and his colleagues tested this hypothesis by sprinkling varying amounts of the particles, each one smaller than a tenth of a millimetre, into their test cheeses, which were made from ultra-pure, microfiltered milk. The results were clear and reproducible: the more hay they added, the more eyes formed. The Swiss scientists speculate that hay’s particular potency comes from its structure—its tiny capillaries provide the perfect shelter for bubbles of CO 2 . And, according to Dominic Guggisberg, a colleague of Bisig’s at Agroscope, hay dust gives fromagers control over not only the presence or absence of eyes but also their characteristics. Less hay makes for large holes, which Italian consumers apparently prefer, whereas more hay makes for many small holes, which cheese-slicing machines prefer.

Switzerland’s cheese-blindness epidemic seems to have been caused by excessively clean milk. In the nineteen-nineties, when the problem first became widespread, many dairies had recently modernized to meet European Union standards, replacing the traditional hand-and-bucket setup with state-of-the-art automatic-milking parlors, which admit less hay dust. But this points to yet another mystery. In the United States, where cows are commonly fed a mixture of corn, alfalfa, and spent distillers’ grains, and milked using a closed system of tubes, valves, and tanks, blindness isn’t a significant problem. “We see blind cheeses in competition, sure,” Clark said, but not enough to be concerned about. (The real problem for U.S. Swiss, she said, is “late-blowing” cheeses, in which “a big huge gush of gas” let off by spores of Clostridium tyrobutyricum creates blowholes, cracks, splits, and other disastrous structural flaws.) But if bits of hay aren’t serving as nuclei, what are? “I don’t know where the microparticles come from in your country,” Bisig said. Perhaps the braided man has a successor.