The holes in cheese are created by bacteria that produce carbon dioxide gas as they ferment. The gas gets trapped inside the cheese body during aging, forming bubbles that leave behind the round openings you see when the wheel is cut. Only certain types of cheese go through this specific fermentation process, which is why most cheeses are hole-free while Swiss-type varieties are full of them.
The Bacteria Behind the Bubbles
The key player is a bacterium called Propionibacterium freudenreichii, often shortened to “Props” in the dairy industry. Cheesemakers deliberately add this culture during production of Swiss-type cheeses like Emmentaler and Jarlsberg. As the cheese ripens, these bacteria feed on lactic acid that was produced earlier by other starter cultures. The byproducts of that feeding are carbon dioxide gas and compounds that give Swiss cheese its distinctive nutty, sweet flavor.
The process happens in stages. During the initial phase of cheesemaking, starter bacteria consume lactose and other milk sugars, dropping the pH of the curd from about 6.3 to 5.4. That acidic, low-nutrient environment is actually harsh for the Propionibacteria. They have to survive those early conditions before they can get to work during ripening, converting the leftover lactic acid into CO2 and flavor compounds. It’s a delayed payoff: the bacteria responsible for the holes aren’t active at the start.
Why Holes Form in Specific Spots
For decades, scientists understood that CO2 created the holes but couldn’t fully explain why bubbles formed where they did. In 2015, researchers at Agroscope, Switzerland’s federal agricultural research center, solved the puzzle. Microscopic particles of hay dust, introduced into milk during traditional open-bucket milking, act as nucleation points for gas bubbles. These tiny hay fragments contain minuscule trapped air pockets. When the bacteria release CO2, the gas migrates to those existing air pockets and enlarges them into visible holes, or “eyes.”
The amounts involved are remarkably small. As little as 5 to 10 milligrams of hay particles per 1,000 kilograms of milk is enough to produce typical eye formation. The hay particles don’t affect the number of gas molecules the bacteria produce. They simply determine where the holes appear and how many form. Without them, CO2 largely escapes from the cheese body entirely, and few or no holes develop. This discovery also explained a trend that had puzzled Swiss cheesemakers for years: as dairies modernized and switched to enclosed, sterile milking systems, their cheese started developing fewer holes. Less hay dust in the milk meant fewer nucleation points.
The Warm Room Stage
Holes don’t appear during the early days of cheesemaking. After production and brining, Swiss cheese wheels are moved into a warm aging room kept at roughly 22 to 23°C (about 72°F), significantly warmer than a typical cheese cave. They stay there for four to eight weeks. This warmth is what activates the Propionibacteria, allowing them to ramp up CO2 production. The gas slowly inflates pockets inside the cheese, and because the rind is sealed, it can’t escape. Over weeks, those pockets grow into the round eyes visible in the finished product.
After the warm room phase, the wheels move to a cooler environment where bacterial activity slows and the cheese continues to develop flavor without forming new holes. The timing of this transfer gives cheesemakers some control over the final eye size and distribution.
Why the Cheese Doesn’t Crack
Not every cheese could handle gas bubbles forming inside it. For eyes to develop properly, the cheese body needs to be flexible and elastic enough to stretch around the expanding gas, forming smooth, round cavities. Cheesemakers producing eyed varieties carefully manage moisture content and acidity to ensure enough pliability during aging. If the body is too hard or brittle, the pressure from the gas creates irregular splits and cracks instead of clean, round holes. Those cracks are considered defects, and they give the cheese a different, less desirable texture.
This is one reason cheddar, Parmesan, and other firm, dense cheeses don’t develop eyes even if small amounts of CO2 are produced during their aging. Their structure is too rigid to stretch into bubbles. The gas either escapes through tiny gaps or, in some cases, causes unwanted fissures that cheesemakers work to prevent.
What Counts as a Proper Eye
In the United States, Swiss cheese is graded by the USDA, and hole size is part of the evaluation. For top-grade Swiss, the majority of eyes need to fall between 3/8 and 13/16 of an inch in diameter, roughly the size of a cherry to a nickel. Eyes that are too large, too small, or unevenly distributed can lower the grade.
Swiss cheese that develops with no holes at all is called “blind.” It tastes the same, since the flavor compounds are produced alongside the CO2, not by the holes themselves. But consumers associate holes with quality in Swiss cheese, so blind cheese is typically downgraded commercially. Some modern producers intentionally make blind Swiss for sliced deli cheese, where holes can be inconvenient for sandwich-making.
Which Cheeses Have Holes and Which Don’t
Only cheeses made with Propionibacteria (or similar gas-producing cultures) and aged through a warm fermentation phase develop true eyes. The classic examples are Swiss-type cheeses: Emmentaler, Gruyère, Appenzeller, Jarlsberg, and Maasdam. Each varies in eye size depending on the specific bacterial strains used, aging temperature, duration, and curd texture.
- Emmentaler: The original Swiss cheese, known for the largest eyes, sometimes exceeding an inch in diameter.
- Jarlsberg: A Norwegian variety with medium, evenly distributed eyes and a milder flavor.
- Gruyère: Typically has smaller, fewer eyes or sometimes none, since it’s aged differently and at lower temperatures.
- Maasdam: A Dutch cheese developed as a faster-ripening alternative to Emmentaler, with large, smooth holes.
Some other cheeses have small, irregular openings that look like holes but form through a completely different mechanism. Havarti and Tilsit, for instance, can have small “mechanical” openings created by air trapped during the curd-washing and molding process, not by bacterial gas production. These are technically called “mechanical openings” rather than eyes.