By TrivBits, Staff Writer — Published October 14, 2026

Table of Contents

• Key Takeaways
• What Causes Cheese Holes: The Bacterial Process
• The Science Behind Eye Formation
• Surprising Facts and Debunked Myths
• Variations in Hole Patterns Across Cheese Types
• How Cheesemakers Control Hole Formation
• Frequently Asked Questions
• Sources

Ever wondered what causes cheese holes in your favorite Swiss cheese? Those iconic openings aren’t just for show—they’re the result of a fascinating bacterial process that’s been perfected over centuries. The surprising truth is that tiny microorganisms create these cavities by releasing carbon dioxide gas as they consume the nutrients in cheese during aging. Did you know that cheesemakers once thought hay particles caused the holes? That myth has been thoroughly debunked by modern food science, revealing the interesting facts behind one of the dairy world’s most recognizable features.

The holes, properly called “eyes” in the cheese industry, vary dramatically in size and frequency depending on the specific bacteria strains used and the conditions during production. Understanding this natural fermentation process reveals unknown truths about how traditional foods develop their characteristic textures and flavors.

Key Takeaways

• Propionibacterium freudenreichii bacteria consume lactic acid in cheese and release carbon dioxide gas, creating the characteristic holes.
• The fermentation process occurs during a warm aging period, typically when cheese is stored at specific temperatures for several weeks.
• Hole size and distribution depend on bacterial concentration, aging temperature, and cheese composition.
• Modern production methods have become so precise that cheesemakers can control eye formation with remarkable accuracy.
• Not all Swiss-style cheeses have large holes—some varieties feature small eyes or none at all by design.
• Recent research revealed that microscopic hay particles can influence hole formation, partially validating an old cheesemaking belief.

What Causes Cheese Holes: The Bacterial Process

The magic begins with Propionibacterium freudenreichii, the same family of bacteria that lives harmlessly on human skin. When added to cheese curds, these microorganisms become the architects of those famous cavities. They don’t work alone, though. The process starts with other bacterial cultures that convert lactose into lactic acid during the initial cheesemaking stages.

Once the cheese enters its warm aging room—typically kept around 70°F—the propionibacteria wake up and get to work. They consume the lactic acid left behind by their bacterial cousins and produce three byproducts: propionic acid, acetic acid, and carbon dioxide gas. The acids contribute to the nutty, slightly sweet flavor characteristic of Swiss cheese. The carbon dioxide? That’s what inflates the holes.

The gas can’t escape because the cheese rind has already formed, creating a sealed environment. Pressure builds until the gas collects in pockets, pushing the cheese matrix aside to form smooth, round cavities. This fermentation period typically lasts four to six weeks, though some premium varieties age even longer.

The Science Behind Eye Formation

Not every air bubble becomes a proper cheese eye. The process requires specific conditions to work correctly. Temperature control is absolutely critical—too cold and the bacteria remain dormant, too warm and they become overactive, creating excessive gas that can crack the cheese or produce irregular holes.

The cheese’s moisture content and acidity levels also matter tremendously. Higher moisture allows bacteria to move more freely through the cheese matrix, potentially creating more numerous but smaller eyes. The protein structure of the cheese determines how easily the gas pockets can expand and whether they’ll maintain their round shape.

Cheesemakers have discovered that the timing of bacterial addition influences the final pattern. Adding propionibacteria earlier in the process tends to produce more evenly distributed holes, while later addition creates more random patterns. Some artisan producers use this knowledge to create signature eye distributions that identify their specific cheese.

Surprising Facts and Debunked Myths

For generations, cheesemakers believed that tiny hay particles in milk created nucleation points where holes would form. This theory was dismissed by scientists for decades. Then, in 2015, Swiss researchers made a startling discovery: the old-timers were partially right. Microscopic hay dust particles do influence hole formation, but they don’t cause the holes themselves—they simply provide surfaces where gas bubbles can begin to form.

Modern dairy hygiene practices had actually reduced hole formation in Swiss cheese because cleaner milk contained fewer of these tiny particles. Some cheesemakers now intentionally add microscopic cellulose or other food-grade particles to encourage proper eye development. It’s a fascinating example of traditional knowledge meeting contemporary science.

Here’s another interesting truth: bigger holes don’t mean better cheese. In fact, the USDA grades Swiss cheese partially based on eye size and distribution. Grade A Swiss should have eyes between three-eighths and thirteen-sixteenths of an inch in diameter. Anything larger or smaller gets downgraded.

Variations in Hole Patterns Across Cheese Types

How Cheesemakers Control Hole Formation

Creating perfect eyes is both art and science. Experienced cheesemakers manipulate multiple variables to achieve their desired results. The concentration of propionibacteria added to the milk is carefully measured—too little produces a “blind” cheese with no eyes, while too much creates what’s called a “nesty” cheese with irregular, connected cavities.

Pressure during cheese formation also matters. Wheels that are pressed more firmly during molding tend to develop fewer, smaller eyes because the denser protein matrix resists bubble expansion. The salt content affects bacterial activity, with higher salt levels slowing fermentation and reducing eye development.

Some producers use specialized aging rooms with precisely controlled humidity and temperature gradients. They may even rotate wheels to different positions during aging to influence how gas distributes within the cheese. This level of control allows modern cheesemakers to produce remarkably consistent products while maintaining the traditional character that makes Swiss cheese distinctive.

Frequently Asked Questions

Do all Swiss cheeses have holes?

No, not all Swiss-style cheeses develop holes. Gruyère, for example, is a Swiss cheese that typically has very few or no eyes. The presence and size of holes depend on the specific bacterial cultures used and the production methods employed. Some cheesemakers deliberately create “blind” Swiss cheese without holes for slicing purposes.

Can the holes in cheese affect its taste?

The holes themselves don’t have flavor, but the bacterial process that creates them absolutely does. Propionibacteria produce propionic and acetic acids that give Swiss cheese its characteristic nutty, slightly sweet taste. Cheeses with more pronounced eye formation often have more developed flavors because they’ve undergone more extensive fermentation.

Why are cheese holes different sizes in the same wheel?

Variations occur because bacterial activity isn’t perfectly uniform throughout the cheese. Slight differences in temperature, moisture, or bacterial distribution within a single wheel can cause some areas to produce more gas than others. Additionally, the physical position of nucleation points where bubbles begin affects how much room each pocket has to expand before encountering another bubble or the cheese rind.

Is cheese with holes safer to eat than cheese without holes?

The presence or absence of holes has no bearing on cheese safety. Both types undergo the same pasteurization and safety protocols. The propionibacteria that create holes are completely harmless and are actually considered beneficial bacteria. Food safety depends on proper production hygiene, pasteurization, and storage conditions, not on whether the cheese has eyes.

The next time you bite into a slice of Swiss cheese, you’re tasting the work of billions of microscopic organisms that have been transforming milk into this distinctive delicacy for centuries. Those holes represent a living process that continues even as the cheese sits in your refrigerator, albeit very slowly. It’s a delicious reminder that some of our favorite foods are the result of carefully controlled microbial activity—and that sometimes the old cheesemakers knew more than we gave them credit for.

Sources

• ScienceDirect – Food Microbiology and Cheese Fermentation
• Encyclopaedia Britannica – Cheese Production and Varieties
• United States Department of Agriculture – Cheese Grading Standards