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