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 made blue cheese, you already know the mould is doing most of the heavy lifting. That pungent, savoury aroma. The peppery bite. The veins that look chaotic but behave with precision.

That’s Penicillium roqueforti at work.

Today, most cheesemakers buy freeze-dried cultures in neat little sachets. They’re clean, predictable, and boring in the best possible way. But for most of blue cheese history, that wasn’t how it worked at all.

Instead, cheesemakers grew their mould on… bread.

Not metaphorically. Not accidentally. Very deliberately.

And unsurprisingly, the next question is one that a lot of home cheesemakers have asked me:

Can stale bread be used to grow Penicillium roqueforti for blue cheese?

The answer is yes. Historically, that was the norm. But doing it well requires far more understanding than most modern retellings admit.

Why Penicillium roqueforti needs help in the first place

Unlike surface moulds used on Brie or Camembert, blue cheese moulds don’t just politely bloom on the outside.

Penicillium roqueforti is an internal mould.

It needs:

• Oxygen
• Moisture
• A food source
• A way to survive being mixed into curds

Milk alone doesn’t give it all of that upfront. So historically, cheesemakers cultivated the mould separately before introducing it into the cheese.

Bread turned out to be the perfect medium.

Why bread works so well as a mould substrate

Stale bread offers almost everything Penicillium roqueforti wants:

• Starch that can be broken down into simple sugars
• Low moisture, which discourages many competing bacteria
• Porous structure, allowing oxygen to penetrate
• Neutral flavour, so it doesn’t dominate the cheese

Crucially, bread doesn’t contain fats that would inhibit mould growth. It’s basically a fungal gym.

This is why bread has been used for centuries to cultivate moulds, not just for cheese but also for fermentation starters more broadly.

The historical method: how blue mould was traditionally grown

In regions like Roquefort-sur-Soulzon, cheesemakers didn’t isolate moulds under microscopes. They worked by observation, repetition, and brutal natural selection.

The traditional method looked roughly like this:

1. Bake simple bread
   No salt. No fat. No sugar. Just flour and water.
2. Dry it thoroughly
   Stale wasn’t enough. The bread needed to be hard.
3. Expose it to the environment
   Often caves already rich in Penicillium roqueforti spores.
4. Wait for blue-green mould growth
   Not white. Not black. Not fuzzy grey.
5. Dry the mouldy bread again
   This stopped unwanted microbes from taking over.
6. Powder the bread
   The mould spores were now shelf-stable.

That powder was then added to milk or curds to inoculate blue cheese.

This wasn’t folk magic. It was empirical microbiology without the lab coat.

Why Penicillium roqueforti thrives on bread but not milk alone

Milk is rich, but it’s also competitive.

Fresh milk contains:

• Lactic acid bacteria
• Enzymes
• Dissolved oxygen that disappears quickly

Penicillium roqueforti prefers an environment where it can establish itself first, without being bullied by faster-growing microbes.

Bread gives it that head start.

Once introduced into cheese curds, the mould is already robust enough to survive salting, draining, and early acidification.

That’s the key. Bread isn’t feeding the cheese. It’s training the mould.

From bread to blue cheese: how the mould enters the curd

Once the bread-grown mould is powdered, it’s typically added in one of three ways:

1. Added directly to milk

The spores disperse evenly before coagulation. This creates fine, even veining.

2. Mixed into curds

More traditional. Results in patchier, bolder veins.

3. Combined with whey or water

Creates a slurry for more controlled distribution.

In all cases, the bread itself never becomes part of the cheese. Only the spores move forward.

Why piercing matters more than the bread ever did

Growing the mould is only half the battle.

Penicillium roqueforti is aerobic. It needs oxygen. Cheese interiors don’t provide that naturally.

That’s why blue cheeses are pierced.

Those little holes aren’t decoration. They’re ventilation shafts.

Once oxygen enters the cheese, the dormant spores wake up and spread through the curd, digesting fats and proteins and releasing the compounds we associate with blue cheese flavour.

Without piercing, even the best bread-grown mould does nothing.

Does bread-grown mould change flavour?

Yes. And this is where things get genuinely interesting.

Traditional bread-grown cultures tend to be less uniform than commercial strains. That can lead to:

• Greater aromatic complexity
• More savoury, meaty notes
• Less predictable intensity
• Occasional earthy or mushroomy undertones

Some of the world’s most distinctive blue cheeses owe their character to this microbial diversity.

But unpredictability cuts both ways.

The modern safety reality

Here’s where I need to be very clear.

Growing mould on bread can be done safely, but it requires:

1. Controlled environments
2. Careful strain selection
3. Experience identifying moulds visually and aromatically

Bread will happily grow things you do not want in cheese.

• Black moulds.
• Yeasts that produce off flavours.
• Moulds that produce mycotoxins.

Historically, cheesemakers lost batches. Sometimes entire seasons. The survivors passed on knowledge. The failures rarely wrote cookbooks.

Modern starter cultures exist because they reduce risk. Not because tradition was wrong, but because consistency matters when people aren’t expecting roulette with their cheese board.

Can home cheesemakers do this today?

Technically? Yes.

Practically? Only if you know what you’re doing.

Most home experiments fail because:

• The bread isn’t dry enough
• The environment isn’t selective
• The wrong mould dominates
• The spores are introduced too late

And once unwanted moulds are present, you can’t “edit” them out later.

That’s why most modern blue cheese recipes still recommend commercial cultures — even when following traditional styles.

What bread-based mould cultivation teaches us about cheese

This isn’t just a quirky historical footnote. It reveals something fundamental about cheesemaking.

Cheese isn’t made in isolation. It’s made in dialogue with its environment.

Bread acted as a bridge between cave and cheese. A way to carry invisible life from place to place, batch to batch.

When we talk about terroir in cheese, this is part of it. Not just the milk. Not just the pasture. But the microbial memory embedded in tools, walls, and yes — stale bread.

So, can you use stale bread to make blue cheese?

If we’re being precise:

You cannot make blue cheese from bread. But you absolutely can make blue cheese with mould grown on bread.

That’s not a hack. That’s history.

Modern cheesemaking has cleaned up the process. It hasn’t erased the truth behind it.

Bread was never the cheese.
It was the mould’s classroom.

Final takeaway

Growing Penicillium roqueforti on bread is one of those practices that sounds strange until you understand the biology. Then it feels inevitable.

Bread provides structure. Mould provides flavour. Milk provides the canvas.

When those three align, you don’t get a gimmick. You get blue cheese.

And if that doesn’t make you appreciate how much invisible life shapes what we eat, nothing will.

If you enjoyed this deep dive into the strange, beautiful intersection of mould, bread, and blue cheese, I share this kind of research regularly.

Join my email list for weekly cheese science, fermentation history, and myth-busting that goes deeper than the surface rind.

Because the best cheese stories always start where the microbes live.

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 The Strange Reason Cheesemakers Once Grew Blue Mould on Bread appeared first on Cheese Scientist.