Holes in cheese have a habit of making people suspicious. They look deliberate. They look engineered. They look like something went wrong and everyone collectively agreed not to talk about it.

And yet, when you slice into a good Havarti and see those small, irregular openings scattered through the paste, you’re not looking at a flaw. You’re looking at a record of microbial activity. A frozen moment of fermentation, captured mid-conversation between bacteria, milk, salt, and time.

Havarti doesn’t have holes because someone poked them in. It has holes because the cheese was alive while it was being made.

Let’s unpack what’s actually going on inside Havarti. And why its holes are smaller, softer, and very different from the famous eyes of Swiss-style cheeses.

First, what kind of cheese is Havarti?

Havarti is a washed-curd, semi-soft cheese, traditionally made from cow’s milk. It originated in Denmark in the 19th century and was designed to be supple, sliceable, and gently aromatic rather than firm or crumbly.

From a structural point of view, Havarti sits in an interesting middle ground.

• It’s not elastic like Mozzarella.
• It’s not dense like Cheddar.
• And it’s not engineered for big, dramatic eyes like Emmental.

That middle position matters. Because holes only appear when a cheese’s internal structure is soft enough to stretch, but firm enough to trap gas.

Havarti is exactly that.

Holes are made by gas, not by air

This is the single most important thing to understand.

Cheese holes are not pockets of trapped air. They are bubbles of gas created inside the cheese after the curd has formed.

That gas comes from bacteria.

During fermentation, certain bacteria metabolise compounds in the cheese and release carbon dioxide as a by-product. If that gas can’t escape, it accumulates. Slowly. Quietly. Pushing against the surrounding protein network.

Eventually, a void forms. That void is a hole.

In Havarti, the process is subtle. The bacteria involved are not aggressive gas producers. The curd is not designed to stretch dramatically. The result is small, irregular openings rather than big round eyes.

The washed-curd step sets the stage

To understand why Havarti gets holes at all, we need to look at how it’s made.

Havarti is a washed-curd cheese. After the curd is cut, some of the whey is drained and replaced with warm water. This step does three important things:

1. It removes lactose from the curd
2. It raises the moisture content
3. It softens the final texture

Less lactose means less fuel for acid-producing bacteria. That’s why Havarti is mild rather than tangy. But the increased moisture also creates a looser protein matrix.

That looser structure matters later.

When gas begins to form during ageing, the paste can deform slightly instead of cracking. It stretches just enough to form small cavities.

If Havarti were drier, the gas would escape or create splits. If it were wetter, the bubbles would collapse.

Holes require balance.

Which bacteria are responsible?

Havarti doesn’t rely on the classic “eye-forming” bacteria used in Swiss-type cheeses. Those cheeses use Propionibacteria, which produce large amounts of carbon dioxide and create big, round eyes.

Havarti uses a more modest microbial cast.

The primary cultures are lactic acid bacteria, which convert lactose into lactic acid early in the make. But during ageing, secondary fermentation can occur. This involves bacteria that metabolise residual compounds such as lactate and citrate.

Some of those metabolic pathways release small amounts of carbon dioxide.

Not enough for dramatic holes. Just enough for gentle openings.

This is why Havarti holes tend to be:

• Small
• Irregular
• Unevenly distributed

They’re not symmetrical. They’re not planned. They’re opportunistic.

Temperature matters more than you think

Holes don’t form instantly. They develop during ageing, and temperature plays a critical role.

If Havarti is aged too cold, bacterial activity slows. Gas production drops. The paste sets before holes can form.

If it’s aged too warm, gas production increases too quickly. The cheese can swell, crack, or develop mechanical openings instead of clean holes.

Traditional Havarti ageing temperatures allow slow fermentation. That gives gas time to accumulate gradually. The curd relaxes. The protein network stretches. Small cavities remain intact.

This slow pace is why Havarti’s holes feel integrated into the cheese, not punched through it.

Why Havarti holes are irregular

Compare Havarti to Emmental and the difference is obvious.

Swiss-style cheeses have:

• Uniform eye size
• Rounded, glossy holes
• Predictable distribution

Havarti does not.

That’s because Havarti’s gas production is inconsistent by design. The bacteria responsible are not specialised gas formers. They produce carbon dioxide as a side effect, not a primary goal.

Gas accumulates wherever the protein network is weakest. That might be near a curd junction. Or around a slightly wetter pocket. Or next to a microfracture from pressing.

Each hole tells a slightly different story.

Pressing plays a quiet role

Havarti is lightly pressed. Enough to knit the curd together, but not enough to expel all internal spaces.

That gentle pressing leaves behind micro-channels and weak points in the structure. These act as starting points for gas accumulation later.

Heavy pressing would close those spaces. No pressing would leave the cheese too fragile.

Again, Havarti sits in the middle.

Are holes a sign of quality?

In Havarti, small holes are normal. They’re expected. They’re part of the style.

That said, more holes is not always better.

Too many holes can indicate:

• Excess gas production
• Poor temperature control
• Imbalanced cultures

Too few holes can suggest:

• Over-pressing
• Over-acidification
• Ageing that’s too cold

Commercial Havarti often aims for a restrained, consistent hole pattern. Artisanal versions may show more variation. Both can be excellent.

The key is integration. The holes should feel like they belong there.

Do flavoured Havarti cheeses still get holes?

Yes. And sometimes more so.

When herbs, spices, or flavour inclusions are added, they disrupt the protein network. Each inclusion creates a local weakness where gas can collect.

This is why dill Havarti, caraway Havarti, or pepper Havarti often show more pronounced openings around the inclusions.

The cheese hasn’t changed its biology. Its structure has.

Why Havarti doesn’t get “eyes” like Swiss cheese

This is a common misconception.

All holes are not created equal.

Swiss-style eyes are the result of intentional propionic fermentation. The cheese is designed to trap large volumes of carbon dioxide. The curd is elastic. The ageing environment is warm. Everything points toward big holes.

Havarti is not built for that.

• Its bacteria produce less gas.
• Its curd is softer, not elastic.
• Its ageing temperatures are lower.

So the gas that does form has nowhere dramatic to go.

It settles. Gently.

Holes and flavour are connected

Those small holes aren’t just visual. They affect flavour.

Holes increase internal surface area. That allows oxygen to interact with the paste in tiny amounts. It also changes how volatile aroma compounds move through the cheese.

This contributes to Havarti’s mild, buttery aroma and soft dairy notes. The cheese breathes, just a little.

It’s subtle. But it matters.

What happens if Havarti has no holes?

It can still be Havarti. Especially in industrial production, where consistency is prized.

But a completely hole-free Havarti often feels denser. Less expressive. Slightly flatter in flavour.

The presence of small openings suggests that fermentation ran its course naturally. That the cheese had time to settle into itself.

Holes aren’t mandatory. But they’re telling.

A quiet record of fermentation

Havarti’s holes aren’t there to impress you.

They’re not a spectacle. They’re not a party trick. They’re a record.

Each one marks a place where bacteria exhaled. Where gas pushed gently against protein. Where the cheese yielded without breaking.

That’s what good cheesemaking looks like. Not control, but guidance.

If you enjoyed this deep dive into the quiet mechanics of cheese, you’ll love my email list. I share new articles, experiments, and behind-the-scenes cheese science straight to your inbox. No spam. Just more reasons to look twice at what’s on your plate.

Jonah Kincaid

Cheese lover. Scientist. Created a website and a Youtube channel about cheese science because he could not find answers to his questions online. 

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The post Why Havarti Has Holes (And Why They’re Not An Accident) appeared first on Cheese Scientist.

If you’ve ever cut into a wedge of blue cheese and noticed the tiny tunnels, cracks, or pinprick holes running through it, you’ve already met one of the most important features in blue cheesemaking.

Those holes are not mistakes. They are not “bad ageing”. And they’re definitely not there by accident.

In fact, without them, most blue cheeses simply wouldn’t be blue at all.

This post unpacks why blue cheeses have holes, how they form, what they do for flavour and texture, and why cheesemakers work surprisingly hard to control something that looks so chaotic.

Blue cheese is an oxygen problem (and a solution)

At its core, blue cheese is an exercise in oxygen management.

The mould that gives blue cheese its colour, aroma, and bite is Penicillium roqueforti. This mould is aerobic. That means it needs oxygen to grow.

Milk, curds, and pressed cheese are not exactly oxygen-rich environments. Once curds are formed and drained, they quickly become dense and low-oxygen. That’s great for many cheeses. It’s terrible for blue mould.

So cheesemakers had to solve a problem:

How do you get oxygen deep inside a cheese without breaking it apart?

The answer is holes.

The holes are air highways for mould

Those small openings inside blue cheese act as oxygen channels.

They allow air to move from the outside of the cheese into the interior. Along those air paths, Penicillium roqueforti wakes up, grows, and produces the familiar blue-green veins.

Where there is oxygen, mould grows.
Where there isn’t, it doesn’t.

That’s why blue cheese doesn’t turn uniformly blue. Instead, it forms veins, streaks, and pockets that follow cracks and air spaces. The mould is literally tracing the cheese’s internal airflow.

Holes come before veins

A common assumption is that blue mould somehow creates the holes.

It doesn’t.

The holes come first. The mould follows.

During early cheesemaking, blue cheeses are handled much more gently than pressed cheeses like Cheddar. Curds are often loosely packed into moulds rather than pressed hard together.

This leaves behind:

• Small gaps between curds
• Irregular cracks
• Micro-pockets of trapped air

These spaces later become the scaffolding for blue mould growth.

If the curds were pressed tightly and fully knit together, oxygen would be excluded. The mould would suffocate. You’d end up with a dense white cheese with no blue character.

Piercing: the moment the holes really matter

Most blue cheeses are pierced during ageing.

This is when long stainless-steel needles are pushed through the wheel or cylinder of cheese. Dozens of holes are made in a deliberate pattern.

This piercing step serves two purposes:

1. It introduces fresh oxygen into the interior
2. It connects existing air pockets into continuous channels

Think of it like ventilation.

Once pierced, air can move freely through the cheese. Dormant mould spores inside the paste suddenly have access to oxygen. Growth accelerates. Veins expand outward from the pierced holes.

Without piercing, blue development would be weak, patchy, or confined to the surface.

Not all holes look the same

Blue cheese holes aren’t uniform, and that’s intentional.

Different styles aim for different internal structures.

Some blues have:

• Fine, hairline cracks
• Small pinholes
• Delicate marbling

Others have:

• Large cavities
• Chunky blue pockets
• Dramatic internal landscapes

These differences come down to curd size, moisture, handling, and how aggressively the cheese is pierced.

A more open structure allows faster mould growth and bolder flavour. A tighter structure slows things down and keeps the blue more restrained.

Holes shape flavour, not just appearance

Blue cheese flavour isn’t only about mould being present. It’s about what the mould does once it has oxygen.

As Penicillium roqueforti grows, it produces enzymes that break down:

• Milk fats (lipolysis)
• Milk proteins (proteolysis)

These reactions generate many of the compounds we associate with blue cheese:

• Peppery notes
• Savoury depth
• Mushroomy aromas
• That unmistakable piquant, tingling finish on the palate

The more oxygen the mould gets, the more active these reactions become.

That means holes don’t just enable blue veins. They actively control flavour intensity.

Fewer holes. Milder blue. More airflow. Bigger personality.

Texture depends on those air pockets too

Blue cheese texture is closely tied to its internal openness.

The breakdown of fats and proteins near air channels softens the paste. That’s why blue cheeses often feel:

• Creamy near veins
• Crumbly yet yielding
• Softening from the inside out

If oxygen were evenly distributed (which it never is), the cheese would mature uniformly. Instead, you get contrast. Firmer areas sit next to buttery, breakdown-rich pockets.

Those textural shifts are part of the appeal. Each bite changes depending on where it lands relative to a vein or cavity.

Why blue cheese holes aren’t “eyes”

It’s worth clearing up a common misconception.

The holes in blue cheese are not the same as the eyes in Alpine-style cheeses.

Eyes in cheeses like Emmental are formed by carbon dioxide produced by bacteria during fermentation. Gas builds up, stretches the paste, and creates round, glossy holes.

Blue cheese holes are different:

• They’re irregular, not spherical
• They’re formed mechanically and structurally
• They’re designed for airflow, not gas expansion

If blue cheese relied on gas production to create holes, the structure would be unpredictable and often destructive. Instead, cheesemakers build openness into the curd from the start.

Too many holes can be a problem

More holes are not always better.

If a blue cheese is too open, several things can go wrong:

• Excessive moisture loss
• Overly aggressive mould growth
• Bitter or metallic flavours
• Structural weakness

Cheesemakers walk a fine line. They want enough airflow for healthy blue development, but not so much that the cheese collapses under its own enzymatic enthusiasm.

This is why blue cheesemaking is as much about restraint as it is about encouraging mould.

Some blue cheeses hide their holes better

Not all blue cheeses advertise their internal architecture.

Some styles have:

• Tighter pastes
• Smaller, more evenly distributed air channels
• Subtle veining

Others are proudly chaotic inside.

The difference often comes down to milk type, moisture, and ageing conditions rather than mould strain alone.

A denser blue still needs oxygen. It just gets it through finer cracks rather than dramatic cavities.

What happens if you remove oxygen entirely?

If you vacuum-seal a young blue cheese before mould has fully developed, the result is telling.

Blue growth stalls. Veins stop expanding. Flavour development slows dramatically.

The cheese doesn’t spoil. It just pauses.

That’s because the mould can’t breathe.

Those holes and channels aren’t optional extras. They’re the difference between a living, evolving cheese and a frozen snapshot of one moment in time.

Blue cheese is engineered chaos

From the outside, blue cheese looks rustic and unruly. Inside, it’s even more so.

But the chaos is carefully engineered.

• Curd size.
• Packing style.
• Piercing patterns.
• Ageing humidity.
• Oxygen availability.

All of these variables determine where holes form and how the mould uses them.

What looks accidental is actually the result of hundreds of tiny decisions made by the cheesemaker.

So why do most blue cheeses have holes?

Because without them:

• The mould couldn’t grow
• The veins wouldn’t form
• The flavour wouldn’t develop
• The texture wouldn’t soften correctly

The holes are not flaws. They’re infrastructure. They are the breathing system of blue cheese.

And every vein you see is simply mould following the path of air, doing exactly what it has evolved to do.

The takeaway

Blue cheese holes aren’t there to look pretty. They aren’t signs of poor craftsmanship. They’re deliberate, functional, and essential.

They let oxygen in. They guide mould growth. They shape flavour and texture. Remove the holes, and you remove the blue.

If you enjoyed this kind of deep-dive into how cheese really works, you’ll probably like what I send by email. I share new posts, weird cheese science, and the occasional rabbit hole worth falling into.

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Jonah Kincaid

Cheese lover. Scientist. Created a website and a Youtube channel about cheese science because he could not find answers to his questions online. 

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The post Why Blue Cheese Has Holes (It’s Not What You Think) appeared first on Cheese Scientist.

If you’ve ever cut into a wheel of Jarlsberg and felt quietly pleased by those neat, cartoon-perfect holes, you’re not alone. Cheese with holes just looks clever. It suggests intention. Precision. Confidence. Nobody accidentally makes a cheese that looks like Swiss engineering.

But those holes are not decoration. They’re not punched out. And they’re definitely not there “to make the cheese lighter” (a persistent myth that refuses to die).

They’re the visible fingerprint of microbiology doing exactly what it’s meant to do.

So let’s talk about why Jarlsberg has holes. And why those holes tell a much bigger story about bacteria, fermentation, and one very Norwegian solution to a Swiss problem.

Jarlsberg is a Swiss-style cheese (with a Norwegian twist)

First, a quick bit of context.

Jarlsberg is what cheesemakers call a Swiss-type or Alpine-style cheese. It sits in the same broad family as Emmental and Gruyère. Firm. Elastic. Mildly sweet. Designed to melt beautifully and slice cleanly.

But Jarlsberg isn’t Swiss. It was developed in Norway in the 1950s by dairy scientist Ole Martin Ystgaard at the Agricultural University of Norway. The goal wasn’t romance or tradition. It was functionality.

Norway wanted a cheese that:

• Used local milk
• Stored well
• Appealed to international markets
• Had the nutty sweetness people loved in Swiss cheese

And yes, it needed holes. Because at the time, cheese without holes didn’t scream “premium” in export markets.

Those holes weren’t an aesthetic afterthought. They were a biochemical requirement.

Holes are gas bubbles, not empty space

Let’s get the most important point out of the way early:

Cheese holes are pockets of gas.

Specifically, carbon dioxide.

During fermentation, certain bacteria break down lactic acid and release CO₂ as a byproduct. That gas has nowhere to go. The cheese is semi-solid. Elastic. Trapped.

So the gas collects into bubbles.

As the cheese ages, those bubbles grow, merge, and stabilise. When the wheel is finally cut open, the gas escapes, leaving behind smooth, round holes.

No drilling. No moulds. No Swiss elves with tiny spoons.

Just bacteria breathing out.

The star of the show: Propionic acid bacteria

Jarlsberg owes its holes to a group of microbes called Propionibacterium freudenreichii.

These bacteria kick in after the main lactic acid fermentation has already done its job. At this stage, the cheese is pressed, salted, and moved into warm ripening rooms.

Here’s what Propionibacteria do:

• They consume lactic acid
• They produce propionic acid, acetic acid, and carbon dioxide

That propionic acid is what gives Swiss-style cheeses their sweet, nutty aroma. The CO₂ is what inflates the cheese from the inside.

No Propionibacteria? No gas. And no holes.

This is why holey cheeses are not an accident. They are designed ecosystems.

Why the holes are round (and not weirdly shaped)

Gas in cheese behaves like gas in dough or balloons. It follows the path of least resistance.

Jarlsberg has a supple, elastic paste. As CO₂ accumulates, pressure builds evenly in all directions. That creates spherical holes, because spheres are the most stable shape under pressure.

If the cheese paste is too brittle, the gas escapes through cracks. If it’s too soft, the bubbles collapse.

Jarlsberg sits in that Goldilocks zone where the curd structure stretches without tearing.

That’s why the holes look so polite.

Milk composition matters more than you think

Here’s where things get quietly nerdy.

Jarlsberg’s milk is partially skimmed. That lower fat content creates a firmer protein network. Casein proteins link up more tightly, forming a strong but flexible matrix.

This matters because:

• Strong protein networks trap gas better
• Even curd structure promotes evenly sized holes
• Excess fat would weaken the walls of the bubbles

In other words, milk chemistry determines hole architecture.

Same bacteria. Same process. Different milk. Different hole pattern.

This is why not every Swiss-style cheese looks the same inside.

Temperature controls hole size

If you want to control holes, you control heat.

Jarlsberg wheels are typically aged in warm rooms, often around 20–24°C, during the eye-forming stage. That warmth wakes up Propionibacteria and speeds up gas production.

Warmer aging:

• Faster fermentation
• Larger holes

Cooler aging:

• Slower gas release
• Smaller or fewer holes

Too warm, and the cheese can split or form “blind eyes” where gas escapes unpredictably. Too cold, and you get a disappointingly flat interior.

Jarlsberg producers are extremely precise here. Holes sell consistency. Consistency sells trust.

Timing is everything

Holes don’t form immediately.

First, starter cultures convert lactose into lactic acid. This lowers the pH and sets the cheese structure. Only after this stage do Propionibacteria get involved.

If they start too early:

• The curd is too soft
• Gas escapes
• Holes fail to form

If they start too late:

• The curd is too rigid
• Gas builds pressure
• Cracks form instead of eyes

Jarlsberg’s production schedule is calibrated to give Propionibacteria the perfect window to do their thing.

This is microbial choreography.

Why Jarlsberg holes are smaller than Emmental’s

If you’ve ever compared Jarlsberg to Emmental, you’ll notice something subtle.

Jarlsberg holes tend to be:

• More numerous
• Slightly smaller
• More evenly distributed

That’s deliberate.

Jarlsberg uses:

• A slightly different bacterial balance
• Tighter curd structure
• Shorter or more controlled warm aging

The result is a cheese that looks Swiss but behaves more predictably on the slicer.

Which brings us to a very modern reason holes matter.

Holes are a slicing problem

In industrial cheesemaking, holes are not universally loved.

Large holes cause:

• Uneven slices
• Weak points
• Crumbling

Jarlsberg was engineered to strike a compromise. Enough holes to signal “Alpine-style cheese,” but not so many that the cheese falls apart or frustrates consumers.

This is why Jarlsberg performs beautifully in sandwiches and toasties. The holes don’t dominate the structure. They cooperate.

Holes affect flavour, not just appearance

Those holes aren’t inert. They change how flavour develops.

More internal surface area means:

• Increased aroma release
• Faster perception of sweetness
• More oxygen exposure during cutting

That slightly sweet, nutty aroma you smell the moment you slice Jarlsberg?
That’s volatile compounds escaping from those gas pockets.

No holes would mean a flatter sensory experience.

This is also why pre-sliced Jarlsberg can smell stronger than a solid wedge. Every cut releases trapped flavour.

Why some Jarlsberg has fewer holes

If you’ve ever noticed variation from wheel to wheel, you’re not imagining it.

Hole formation can be influenced by:

• Seasonal milk changes
• Cow diet
• Minor temperature fluctuations
• Small shifts in bacterial activity

Modern producers aim for consistency, but cheese is still a biological product. Perfection is managed, not guaranteed.

Interestingly, many cheesemakers would prefer slightly fewer holes. Consumers, however, expect them. When Jarlsberg looks “too smooth,” people assume something went wrong.

The holes have become part of the brand promise.

The myth of “holes for air” or “holes for weight”

Two myths worth retiring permanently.

Myth one: Holes let the cheese breathe.
Cheese does not need internal ventilation. The rind handles gas exchange.

Myth two: Holes reduce weight or cost.
Gas pockets don’t meaningfully change yield. In fact, failed eye formation often leads to rejected wheels.

Holes are not a shortcut. They’re a risk.

What happens when holes go wrong

When eye formation fails, cheesemakers see issues like:

• Blind cheese (no holes)
• Slits and cracks
• Irregular, misshapen eyes

These cheeses are often downgraded or diverted into processing. You don’t see them neatly wrapped at the supermarket.

Jarlsberg’s reputation depends on getting this right, every time.

Why not all cheeses want holes

It’s worth saying this plainly.

Most cheeses do not want gas.

Cheddar, Brie, Blue, Feta, Mozzarella. All of these aim to suppress gas production. Holes in those cheeses signal contamination or process failure.

Jarlsberg is the exception because its flavour and identity depend on controlled gas.

In cheesemaking, holes are either a feature or a flaw. There is no neutral.

The quiet brilliance of Jarlsberg

Jarlsberg looks simple. Friendly. Mild. Inoffensive.

But inside that tidy yellow wheel is one of the most carefully managed microbial systems in modern cheesemaking. Those holes are evidence of:

• Bacterial precision
• Milk chemistry control
• Temperature mastery
• Decades of refinement

They are not rustic accidents. They are industrial elegance.

So the next time you see those perfect little eyes staring back at you, remember:
That cheese isn’t empty. It’s alive with intent.

And frankly, that’s a pretty good reason to eat it.

And if you enjoy digging into the science behind everyday cheeses like this, make sure you subscribe to the Cheese Scientist email list. It’s where I share deeper dives, fresh research, and the stories hiding inside your fridge—straight to your inbox.

Jonah Kincaid

Cheese lover. Scientist. Created a website and a Youtube channel about cheese science because he could not find answers to his questions online. 

cheesescientist.com

The post Why Does Jarlsberg Cheese Have Holes? The Science Behind Those Perfect Eyes appeared first on Cheese Scientist.

Let’s talk holes.

Big ones. Small ones. Neat little rounds. Occasional chaotic craters that look like something went very wrong in the ageing room.

Because holes in cheese aren’t decorative. They’re not air bubbles. And they’re definitely not there “to make the cheese lighter”.

They’re biology under pressure.

Cheeses like Swiss-style cheeses, Emmentaler, and some styles of Gouda don’t just have holes. They create them, slowly and deliberately, as part of fermentation.

Once you understand why, you stop seeing holes as quirks and start seeing them as evidence.

Evidence that microbes did exactly what they were supposed to do.

First things first: those holes have a name

Cheesemakers don’t call them holes. They call them eyes.

And that’s important, because eyes aren’t random voids. They’re a specific structural feature caused by gas formation inside the cheese during ripening.

Whether a cheese has eyes, tiny pinholes, cracks, or no openings at all depends on a surprisingly precise balance of factors:

• Which bacteria are present
• How much lactic acid is available
• How elastic the curd structure is
• Temperature during ageing
• Time

Change one variable and the entire outcome shifts.

That’s why two cheeses made from the same milk, on the same day, can age very differently.

The short explanation: gas made the holes

At its simplest, eye formation comes down to one thing.

Gas.

Certain bacteria inside the cheese produce carbon dioxide as part of fermentation. That gas builds up inside the cheese paste. If the paste is flexible enough, it stretches and forms bubbles.

Those bubbles become eyes.

If the paste is too rigid, the gas escapes. Or worse, it tears the cheese apart.

So when you see beautiful, round eyes, you’re looking at a cheese where fermentation, structure, and timing lined up perfectly.

The real hero: propionic acid bacteria

Most cheeses start with lactic acid bacteria. They convert lactose into lactic acid and drop the pH. That’s step one.

Eye-forming cheeses have a second act.

Enter Propionibacterium freudenreichii.

This bacterium consumes lactic acid and converts it into:

• Propionic acid
• Acetic acid
• Carbon dioxide

The carbon dioxide forms the eyes. The propionic acid creates the nutty, sweet aroma we associate with Swiss-style cheeses.

No propionic bacteria, no eyes. It’s that simple.

This is why eye formation happens after pressing, after initial fermentation, and often weeks into ageing.

Why Emmentaler has those iconic giant holes

Emmentaler didn’t become the visual shorthand for “cheese” by accident. Its eyes are large, glossy, and evenly distributed. That’s not luck. It’s engineering.

Raw milk plays a role

Traditional Emmentaler is made from raw milk. Raw milk carries a richer microbial ecosystem, including components that support propionic fermentation later on.

Pasteurised versions can still form eyes, but they tend to be smaller and more uniform.

The curd is designed to stretch

Emmentaler curds are cooked at relatively high temperatures and stirred extensively. This expels whey and creates a smooth, elastic protein network.

Think balloon, not brittle shell.

When gas builds up, the curd stretches instead of cracking.

Warm ageing is essential

After pressing, wheels are moved into warm cellars, typically around 20–24°C.

This warmth activates propionic bacteria. Carbon dioxide production increases. Eyes slowly expand from the inside.

Too cold, and nothing happens. Too warm, and the cheese risks splitting.

Time finishes the job

Eye formation isn’t immediate. It takes weeks. Sometimes months.

Cut an Emmentaler wheel too early and the eyes will be small or incomplete. The cheese simply hasn’t finished inflating yet.

Swiss cheese isn’t a single cheese

“Swiss cheese” is a style, not a protected name.

In Europe, it covers multiple Alpine cheeses. Outside Europe, it usually refers to a mild, industrial Emmental-style product.

The science is the same. The execution differs.

Industrial Swiss-style cheeses often use:

• Highly standardised starter cultures
• Shorter ageing periods
• More controlled textures

The result is smaller, more predictable eyes and a milder flavour.

Still cheese. Just less dramatic.

A quick note on Jarlsberg and its holes

Jarlsberg is often grouped with Swiss-style cheeses, but it’s a much more deliberately engineered example of eye formation.

Developed in Norway in the mid-20th century, Jarlsberg uses propionic acid bacteria to produce carbon dioxide during ripening, just like Emmentaler. That gas creates the holes and contributes to the cheese’s mild, slightly sweet flavour.

The difference is control.

Jarlsberg is typically made from pasteurised milk with tightly managed starter cultures and ageing conditions. This leads to smaller, more uniform eyes and far fewer structural surprises.

Same biology. Less chaos.

Why Gouda sometimes has holes and sometimes doesn’t

Gouda is where things get interesting. Some Gouda wheels have small, tidy eyes. Others are completely closed. Both can be correct.

That’s because Gouda sits right on the edge of eye-forming conditions.

Washed curds change the chemistry

Gouda is made using a washed-curd process. Whey is removed and replaced with warm water. This reduces lactose levels in the curd. Less lactose means less lactic acid later on.

And less lactic acid means less fuel for propionic bacteria.

Propionic bacteria are optional

Some Gouda styles include them. Some don’t.

Even when they are present, the environment isn’t ideal for large gas production.

The result is usually:

• Small eyes
• Irregular openings
• Or no eyes at all

Young Gouda may show eyes. Aged Gouda almost never does.

Same cheese family. Very different outcomes.

Why many cheeses never form holes

If gas causes holes, why don’t all cheeses end up full of them?

Because many cheeses are built to prevent it.

Dense curd structures

Cheeses like Cheddar or Parmigiano Reggiano are pressed firmly and expel more moisture.

The structure doesn’t stretch. Gas either escapes or stays dissolved.

Different microbial pathways

If propionic bacteria aren’t present, no carbon dioxide is produced at that stage.

Plenty of cheeses rely solely on lactic fermentation.

Cold ageing environments

Low temperatures slow bacterial metabolism. Less activity. Less gas. Fewer eyes.

Eye-forming cheeses require a deliberate warm phase. Without it, nothing happens.

When eye formation goes wrong

Eyes should be round, smooth, and evenly distributed. When they’re not, it usually means something went off script.

Cracks and slits

Long, jagged openings suggest gas formed too quickly or the curd wasn’t elastic enough.

Often caused by:

• Excessive fermentation
• Poor curd knitting
• Temperature fluctuations

Late blowing defects

This is a serious fault.

Certain unwanted bacteria, like Clostridium species, can ferment residual sugars late in ageing, producing gas when the cheese can no longer stretch.

The result is splitting, off-flavours, and structural failure.

No eyes at all

Sometimes propionic bacteria simply don’t activate. This can happen if the cheese is too acidic, the cellar is too cold, or the microbial balance is off.

You still get cheese. Just not the one you planned.

The persistent myth: “holes come from trapped air”

They don’t.

Air pockets from pressing would be irregular, shallow, and inconsistent. True eyes are smooth, spherical, and distributed throughout the paste.

They form slowly, after pressing, from gas produced in place. Cheese doesn’t trap air. It ferments itself open.

The recent claim: is hay responsible for cheese holes?

Over the past few years, a headline has made the rounds claiming that hay particles, not bacteria, are responsible for holes in Swiss-style cheeses.

The story usually goes like this: microscopic hay dust from traditional barns introduces particles that act as nucleation points for gas bubbles, helping eyes form.

There is some truth here. And a lot of misunderstanding.

What the research actually showed

Studies from Swiss researchers found that modern, ultra-clean milking systems reduced the number of natural particles in milk.

Fewer particles meant fewer places for gas bubbles to start forming.

When tiny particles from hay dust were reintroduced, eye formation became more predictable.

So yes, particles can influence where eyes start.

What hay does not do

Hay does not produce gas. It does not replace bacteria. It does not create eyes on its own.

Without propionic bacteria producing carbon dioxide, nothing happens.

Hay particles are scaffolding, not engines.

Why the claim got exaggerated

“Holes caused by bacteria” isn’t a catchy headline.

“Holes caused by hay” is.

But stripping the context makes it sound like centuries of cheese science were wrong, which simply isn’t true.

Eyes still come from fermentation. Hay just helps guide bubble formation in traditional systems.

It’s a nuance, not a revolution.

Eyes influence flavour, not just appearance

Those holes aren’t neutral.

Gas formation affects moisture distribution, texture, and aroma development.

Eye-forming cheeses tend to have:

• Sweeter notes
• Nutty aromas
• Softer textures around the eyes

That’s propionic acid doing its work.

Remove the eyes and you remove part of the cheese’s identity.

Why holes became iconic

In Alpine cheesemaking, well-formed eyes were proof of skill. They showed the cheesemaker understood milk, microbes, and time.

Even today, eye quality is used in grading traditional Swiss-style cheeses.

They’re not decoration.

They’re evidence.

So why do some cheeses have holes?

Because:

• The right bacteria were present
• The curd could stretch
• Temperature allowed gas production
• Time did the rest

Swiss-style cheeses, Emmentaler, and some Gouda are built to make space for fermentation.

Others are built to resist it.

Neither approach is better. They’re just telling different microbial stories.

Final thought

The next time you see holes in cheese, don’t think “Swiss”.

Think controlled fermentation. Think bacteria inflating protein networks from the inside out. And think centuries of cheesemakers learning how to let gas exist without letting chaos take over.

Cheese doesn’t get holes by accident.

It earns them.

Want more cheese science like this?

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Jonah Kincaid

Cheese lover. Scientist. Created a website and a Youtube channel about cheese science because he could not find answers to his questions online. 

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