Why Does Swiss Cheese Have Holes? A Wisconsin Cheesemaker Explains the Eyes

Swiss cheese has holes. Everyone knows this. It is one of the first cheese-related facts every American learns, right after “cheese comes from cows.”

What most people don't know is that the actual scientific answer changed in 2015, & most of what you were taught about it in school is now wrong. Here is the real story, from cheesemakers who make holey cheese for a living, plus everything you need to know about why the holes are there, how they form, & why they've gotten smaller over the last hundred years.

Key Takeaways

• The holes in swiss cheese are called “eyes,” & the cheese without them is called “blind.”

• They form during aging when specific bacteria release carbon dioxide gas.

• A 2015 discovery showed tiny hay particles in the milk are what trigger where the holes actually form.

• Modern milking cleanliness is why swiss cheese holes have gotten smaller over the last century.

• Our Holey-Smoked Swiss & Cheddar carries the classic hole pattern with a Wisconsin twist.

Why Does Swiss Cheese Have Holes?

Short answer: bacteria release carbon dioxide gas during aging, & that gas gets trapped in pockets inside the cheese as it firms up. Those trapped gas pockets are the holes.

Long answer: it is more interesting than that. Three specific bacterial cultures work together in the making of Swiss-style cheese. Two of them (Lactobacillus & Streptococcus) do the initial acidification & flavor development. The third one, Propionibacterium freudenreichii (usually just called “propi” by cheesemakers), does the hole-making. As the cheese ages in a warm cellar, the propi bacteria eat lactic acid & release two byproducts: propionic acid (which creates that classic nutty-sweet Swiss flavor) & carbon dioxide gas (which forms the holes).

The cheese wheel is warm enough that the gas expands & pushes outward, forming those characteristic bubbles. The wheel is firm enough on the outside that the gas can't escape. So the bubbles stay put. The cheese ages, cools, & sets. Holes forever.

What Are Swiss Cheese Holes Called?

In cheesemaker terms, the holes are called “eyes.” Swiss cheese without any holes is called “blind.” A wheel with lots of small, even eyes is called “eyed properly.” A wheel with a few enormous eyes is a mistake, & is usually sold cheaper.

There is even a name for the ideal eye distribution: “regular & pea-sized.” That is what a great emmentaler wheel should look like when sliced open.

The 2015 Discovery That Changed Everything

Here is where it gets interesting. For nearly a hundred years, the standard textbook explanation of Swiss cheese holes was “carbon dioxide from bacteria.” That explanation is technically correct but incomplete. It never explained why the holes formed in specific spots, or why they had been getting smaller over the decades.

In 2015, researchers at Agroscope (Switzerland's federal agricultural research institute) figured it out. The gas needs a nucleation site to form a bubble. Something for the CO2 to gather around. For over a century, that nucleation site was microscopic particles of hay dust that fell into the milk during milking. As modern dairies got cleaner (closed milking systems, sanitized equipment, no more hay drifting into buckets), the hay-dust particles disappeared. Fewer particles meant fewer nucleation sites. Fewer sites meant fewer, smaller holes. (Related quirk of aged cheese: the crunchy white specks you sometimes find inside are cheese crystals, not mold, & they are one of the great signs of a well-aged wheel.)

Modern Swiss cheesemakers now sometimes deliberately add tiny amounts of hay dust to milk to restore the traditional hole pattern. Not because the hay does anything to the flavor, but because customers expect holes, & the cleaner milking systems had accidentally engineered them out.

Real answer, from real research, that most food-fact articles still get wrong.

Which Swiss Cheeses Have Holes?

Cheeses With Holes

• Emmentaler (the original Swiss with the biggest holes)

• Jarlsberg (Norwegian Swiss-style; medium holes)

• Baby Swiss (smaller wheels, smaller holes)

• American “Swiss” (mild, small holes)

• Our Holey-Smoked Swiss & Cheddar (Swiss-cheddar blend with the classic hole pattern & wood-smoked flavor)

Cheeses That Look Like They Should, But Don't

• Gruyère (looks like Swiss cousin, but no holes)

• Comté (French cousin of gruyère, no holes)

• Appenzeller (Swiss, but no holes)

The distinction is that Swiss-style cheeses use the propi bacteria specifically to create holes; gruyère & comté don't add that culture, so no gas & no holes.

Why Are Swiss Cheese Holes Getting Smaller?

Two reasons:

First, the hay-dust discovery above. Cleaner modern milking means fewer nucleation sites.

Second, market preference. Some modern eaters find enormous holes hard to slice, hard to store, & hard to build a sandwich around. Cheesemakers responded by dialing back the propi cultures & selecting for smaller, more regular holes. The classic “cartoon” swiss cheese with huge round holes is now less common than it was in 1950.

The Complete Swiss Cheese Holes Cheat Sheet

How to Actually Use Swiss Cheese

Now that you know why the holes are there, here is what to do with the cheese:

On Sandwiches

Swiss is the classic partner for ham, turkey & roast beef. The nutty flavor cuts through cured meats without overpowering them. Our Holey-Smoked Swiss & Cheddar adds a wood-smoked depth that makes a plain ham sandwich into something people remember.

Melted

Swiss melts beautifully at moderate heat. Classic uses: French onion soup, fondue, croque monsieur, patty melts. The nutty flavor amplifies when heated.

On a Cheese Board

Cube it or slice it thin. Pair with cured meats, apples, whole-grain crackers, & a bit of Hot Honey Drizzle for the sweet-nutty combination that ruins other cheese boards.

The Wisconsin Swiss Cheese Story

Wisconsin has been making Swiss-style cheese since the 1840s, when Swiss immigrants brought the tradition to New Glarus & the surrounding towns. Green County, Wisconsin still calls itself “America's Swiss Cheese Capital.” Modern Wisconsin cheesemakers have taken the traditional Swiss holes & experimented with them. Our own Holey-Smoked Swiss & Cheddar is one example: it blends the classic Swiss hole pattern with sharper cheddar flavor & real wood smoke. Not traditional, but authentically Wisconsin. For more on the aged-cheese magic that happens over years of patient cellar time, our aged gouda deep dive is a good companion read.

FAQ

What causes holes in swiss cheese?

Bacteria called Propionibacterium freudenreichii release carbon dioxide gas during aging. That gas gets trapped inside the cheese as it firms up, forming the characteristic holes (or “eyes”). The 2015 research showed hay-dust particles in the milk are what determine where the holes actually form.

What are the holes in swiss cheese called?

They are called “eyes.” Cheesemakers describe a good wheel as “eyed properly.” A wheel with no holes is called “blind.” The ideal size is “regular & pea-sized.”

How does swiss cheese get its holes?

During aging, propionic acid bacteria eat lactic acid & release carbon dioxide gas. The gas expands in the warm cellar & gets trapped by the firm outer rind. Those trapped gas pockets become the holes. Tiny hay-dust particles in the milk serve as nucleation sites where the bubbles start forming.

Why are swiss cheese holes getting smaller?

Modern milking equipment is much cleaner than it used to be, which means fewer hay-dust particles fall into the milk. Fewer particles means fewer places for gas bubbles to form. Some cheesemakers now deliberately add small amounts of hay dust to restore the traditional hole pattern.

Does all swiss cheese have holes?

No. Cheeses that use the propi bacterial culture (emmentaler, jarlsberg, American swiss) have holes. Related cheeses that don't use propi (gruyère, comté, appenzeller) look similar but have no holes.

Is swiss cheese safe to eat?

Absolutely. The holes are just gas bubbles, & the bacteria that make them are food-safe cultures used for centuries. Swiss cheese is also naturally low in lactose because of the long aging time.

What is the difference between swiss cheese & emmentaler?

Emmentaler is the original Swiss cheese. “Swiss cheese” in American grocery stores is usually a milder, mass-produced version of emmentaler with smaller holes & shorter aging. True imported emmentaler has bigger holes, more complex nutty flavor, & takes longer to age.

Swiss cheese is the rare food where the fun fact everyone knows turns out to have a way better story behind it. If you want to taste a Wisconsin twist on the classic, our Holey-Smoked Swiss & Cheddar adds wood-smoked flavor to the traditional hole pattern. Browse the full cheese block collection to see what else deserves a spot on your board.