Why Does Swiss Cheese Have Holes?
The answer involves bacteria, carbon dioxide, and hay particles. Mice had nothing to do with it.
Swiss cheese holes look like they were designed by a whimsical architect, but the truth is way more specific than “a gas got trapped.” It starts the moment the milk hits the vat, when the right mix of bacteria kicks off a chemistry chain that turns liquid into curds, then into a wheel that can breathe.
Here’s where it gets complicated: Swiss cheese, properly called Emmental, uses an extra bacteria, P. shermanii, which loves the lactic acid left behind by starter cultures. That swap in the microbial lineup changes everything, because P. shermanii gobbles up the acid and releases carbon dioxide, which forms pockets once the rind shows up.
And then, just when everyone thought the hole story was locked in, a 2015 discovery tied the shrinking “eyes” to something less obvious than temperature and time.
What Causes Holes in Swiss Cheese?
Making cheese starts with adding bacteria to milk. The bacteria trigger chemical reactions that separate the milk into solid curds and liquid whey. The curds get pressed, salted, and aged into cheese. Different bacteria produce different flavors and textures, which is why there are thousands of cheese varieties worldwide.
Swiss cheese, properly called Emmental after the Emmental region of Switzerland where it originated, uses an extra bacterial culture that most other cheeses don't: Propionibacterium freudenreichii subspecies shermanii, or P. shermanii for short. This bacterium is naturally found in hay, grasses, and soil, and it shows up in raw milk from cows that graze on pasture.
Here's the sequence. Standard starter bacteria (Lactobacillus and Streptococcus thermophilus) first convert the lactose in milk into lactic acid. Then P. shermanii takes over, consuming the lactic acid and producing three things: propionic acid (which gives Swiss cheese its nutty, slightly sweet flavor), acetic acid, and carbon dioxide gas.
The carbon dioxide doesn't escape because by this point the cheese has formed a rind. The gas collects in pockets and pushes outward, creating spherical voids. The cheese is soft enough during this warm-aging phase, around 70 degrees Fahrenheit, that it stretches rather than cracks. The result: smooth, round holes distributed throughout the wheel.
Cheesemakers control hole size by adjusting temperature, acidity, and aging time. Higher temperatures and longer aging mean more bacterial activity, more gas, and bigger holes.
magnificThat’s the classic chain, starter bacteria make lactic acid, P. shermanii eats it, and carbon dioxide starts pushing outward inside the forming rind.
Swiss Cheese Holes and the Hay Dust Discovery
In 2015, researchers at Agroscope, a government agricultural institute in Bern, published findings that upended a hundred years of conventional wisdom about what causes holes in Swiss cheese.
They had noticed that Swiss cheese holes were shrinking. Wheels of Emmental produced in modern, sealed milking facilities had noticeably fewer and smaller eyes than cheese made from milk collected in traditional open buckets. Same bacteria. Same recipes. Same aging conditions. Different holes.
The culprit turned out to be hay. When milk is collected in open buckets in a barn, microscopic particles of hay dust fall in. These particles are too small to see or taste, but they serve as nucleation sites, tiny irregularities where gas bubbles preferentially form. Without them, the carbon dioxide still gets produced, but it disperses more evenly through the cheese instead of collecting into distinct bubbles.
The researchers tested this by adding measured amounts of hay dust to milk before making cheese. More hay particles produced more holes. The correlation was direct and reproducible.
John Jaeggi, a researcher at the University of Wisconsin's Center for Dairy Research and a third-generation cheesemaker with Swiss ancestors, confirmed the finding: "The Swiss are saying that the fine dust from hay powder is forming the nuclei and curd texture. Those are the spots where the gas forms and you get the eye development."
So the answer to "why does Swiss cheese have holes" is actually two mechanisms working together. The bacteria produce the gas. The hay dust determines where the gas collects.
Cheese With Holes: Not Just Swiss
Swiss cheese is the most famous cheese with holes, but it's not the only one. Several other varieties develop eyes through similar mechanisms.
Jarlsberg, a Norwegian cheese, has medium-sized holes and a milder flavor than Emmental. It uses a proprietary bacterial culture that produces carbon dioxide under slightly different conditions. Baby Swiss, an American invention, is made with a shorter aging period that produces smaller, more numerous holes and a creamier texture.
Gouda can develop small, pea-sized holes, especially when made from raw milk. Some varieties of Gruyère show tiny irregular openings. Even certain aged cheddars occasionally develop gas pockets, though these are considered defects rather than features.
The USDA regulates the size and distribution of holes in American-made Swiss cheese. Grade A Swiss must have eyes that are evenly distributed and between 11/16 and 13/16 of an inch in diameter. Cheese without holes is called "blind" Swiss. It tastes the same but looks wrong to consumers who expect the holes.
magnificSo when the 2015 Agroscope team noticed Emmental holes shrinking in sealed milking facilities, the whole “just age it longer” explanation suddenly felt incomplete.
If you think Swiss cheese holes are tricky, these food trivia questions start easy, then catch you off guard.
The hay-and-grass angle matters too, because P. shermanii is naturally found in those environments, and changing the cows’ setup can quietly change the bacteria mix.
The "Eyes" of Swiss Cheese: What They Tell You
Cheesemakers use the term "eyes" for holes because they're indicators of what's happening inside the wheel. A wheel with large, evenly spaced eyes means the P. shermanii was active and healthy, the aging temperature was right, and the cheese matured properly.
Small or uneven eyes suggest the bacteria were stressed, the temperature fluctuated, or the milk chemistry was off. No eyes at all means the bacterial culture failed or the aging conditions didn't support gas production. In professional cheese grading, the eyes are as important as flavor.
There's a direct relationship between hole size and taste intensity. Larger eyes mean longer fermentation, which means more propionic acid production, which means a stronger, nuttier flavor. This is why American Swiss cheese, with its smaller holes, tastes milder and creamier than European Emmental with its golf-ball-sized cavities.
Some cheesemakers deliberately produce blind Swiss for slicing, since holes make the cheese harder to cut cleanly for sandwiches. The Sargento and Boar's Head brands both sell "lacey" Swiss that's thin-sliced with small holes, optimized for deli use rather than cheese boards.
Why Does Swiss Cheese Have Holes? The Myths
The mice theory is the most persistent myth about Swiss cheese holes, but it's not the only one. Here are the main misconceptions people carry around:
Mice eat through the cheese. No. Mice would eat the entire cheese, not bore neat holes through it. The myth probably comes from cartoons that show mice carrying wedges of holey cheese, which is just artists drawing what they think cheese looks like.
The holes are air pockets from the pressing process. No. Pressing squeezes air out. The holes form after pressing, during the warm-aging phase, when bacteria produce carbon dioxide weeks or months into the process.
All cheese with holes is Swiss. No. As noted above, several cheese varieties develop eyes. And not all Swiss cheese has holes. Blind Swiss exists and is commercially common.
The holes have been getting bigger. Actually the opposite. Swiss cheese holes have been getting smaller since the mid-20th century because modern dairy facilities produce cleaner milk with fewer hay particles. If anything, cheesemakers now have to work harder to get holes of the traditional size.
magnificThat’s why this isn’t just a fun cheese fact, it’s a real-world mystery about how modern milking conditions can rewrite the hole pattern.
Emmental: The Original Swiss Cheese With Holes
The word "Swiss" is a regional shorthand. In Switzerland, the cheese is called Emmental (or Emmentaler), and it carries a geographical indication similar to Champagne or Parmesan. To be labeled Emmental in Switzerland, the cheese must be made with natural ingredients, aged in cellars for at least four months, have a round shape with a natural rind, and come from the Emmental region.
The Emmental valley sits in the canton of Bern, in the heart of Switzerland. Cheesemaking there dates back to at least the 13th century, though the specific techniques for producing holed cheese evolved over time as cheesemakers learned to control bacterial cultures and aging conditions.
A full wheel of Emmental weighs about 200 pounds and takes roughly four months to age. During that time, the cheese goes through three temperature phases: a warm phase that encourages bacterial growth and gas production, a cooler phase that slows the process and lets the rind develop, and final cold storage.
The flavor develops in layers. Young Emmental is mild and slightly sweet. As it ages, the propionic acid builds, adding nutty and butterscotch notes. Very aged Emmental, over 12 months, can develop an almost crystalline texture similar to aged Parmesan, with intense umami depth.
The holes are just the most visible part of a process that's been refined over centuries. Every eye in a wheel of Emmental represents a pocket of carbon dioxide that formed around a microscopic particle of hay, pushed outward by bacteria that have been eating lactic acid in the dark for months.
That's why Swiss cheese has holes. Not mice. Not air. Bacteria and hay dust, working together in a cellar in the Emmental valley, the same way they've been working for at least 700 years.
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