Why does Swiss cheese have holes? Bacteria make the gas; tiny particles start the bubbles

The 30-second answer

The useful answer has four parts:

1. Bacteria produce carbon dioxide.
2. Microscopic air-filled structures start the bubbles.
3. Warm, elastic cheese allows those bubbles to grow.
4. Cooling slows the process once the eyes reach the intended size.

Bacteria make the gas — and part of the flavour

The microbial work happens in stages. Lactic-acid bacteria first acidify the milk and create the curd. During ripening, dairy propionibacteria use lactate and other available compounds and produce carbon dioxide along with propionate and acetate.

That is why the hole and the nutty flavour are related without being the same thing. The microbial fermentation contributes both the openings and flavour compounds; the empty space itself has no taste. Counting holes is therefore not a reliable shortcut to deciding which slice will taste stronger. Review of dairy propionibacteria

For Emmentaler AOP, the producer organisation describes a warm fermentation cellar of about 19–24°C. The warmth encourages propionic fermentation, while the cheese body must remain elastic enough to expand around the accumulating gas. A firm rind helps retain gas inside the wheel. Emmentaler AOP production

Hay does not replace the bacteria explanation

The modern hay finding is often reported as though scientists abandoned the carbon-dioxide explanation. They did not. The research changed the answer to a different question: not “where does the gas come from?” but “where does a visible hole begin?”

In a 2015 controlled study, researchers added between 0.0625 and 4 milligrams of powdered hay to 90 litres of microfiltered milk. The number and total volume of eyes changed with the dose. Microscopic plant particles were acting as eye nuclei. 2015 eye-formation experiment

The proposed mechanism was physical. Capillaries in fragments of leaf or stem can retain tiny pockets of air. Carbon dioxide dissolved in the cheese then diffuses towards those pockets, allowing an eye to begin at a lower gas pressure than it would need in an uninterrupted cheese matrix.

Hay is therefore not feeding a special hole-making colony or punching a cavity through the curd. Its useful feature is a microscopic structure that already contains air.

The 2026 experiment isolated the physical step

The 2015 result still left a fair question: was something chemically special about hay doing the work? A 2026 Agroscope study tested the structure more directly.

Researchers made experimental Emmental-style cheeses from microfiltered milk and introduced standardised hollow fibres or small PEEK tube pieces. These inert structures modelled the air-holding capillaries in plant matter. X-ray images showed eyes beginning exactly at the added structures. Cross-sections later found the structures at the eye sites. 2026 Agroscope eye-nuclei experiment

Controls without suitable nuclei produced very few eyes. Some of the gas instead contributed to cracks near the surface. That distinction matters: an eye is not simply any gas damage inside cheese. A suitable nucleus plus an elastic matrix helps produce a controlled, rounded opening; poorly managed gas can create splits and other defects.

The synthetic fibres and tubes were laboratory tools, not proposed food ingredients. The experimental cheeses were marked as unfit for consumption. What the experiment supports is the capillary-and-trapped-air mechanism, not the industrial addition of plastic to cheese.

Why cleaner milk made the holes shrink

Traditional open milking could let minute plant particles enter the milk naturally. Modern closed milking systems, filtration and high hygiene standards are better at removing them. That is good food handling, but it also made reliable eye formation harder.

The change became commercially important because the size, number and distribution of eyes are part of Emmentaler’s expected appearance. In April 2025, the Swiss Federal Administrative Court upheld an appeal allowing natural hay-flower powder to be included in the Emmentaler AOP product specification. The point was to restore controlled nuclei that older production supplied less predictably. Swiss Federal Administrative Court decision

That decision applies to the Swiss AOP specification. It does not mean every cheese sold as “Swiss” around the world contains added hay-flower powder or follows the same rules.

Cheesemakers control more than one variable

Adding nuclei is only one control. Eye formation also depends on how much carbon dioxide the bacterial culture produces, how readily gas can move through the cheese, salt, acidity, moisture, fat, texture, temperature and time. Too little gas or too few nuclei can leave a nearly “blind” cheese; excessive gas or a weak, brittle body can produce cracks rather than attractive eyes.

Temperature gives cheesemakers a practical brake. Emmentaler AOP matures for at least 120 days. Its eyes typically reach the intended size after about six to eight weeks in the warmer phase. The wheels then move to storage at about 12°C, which slows propionic fermentation and further eye growth while flavour development continues. Emmentaler AOP maturation process

This is why “more time means bigger holes” is incomplete. Time matters only alongside temperature, microbial activity, gas loss, available nuclei and the changing structure of the cheese.

What the holes can tell you — and what they cannot

A regular pattern of rounded eyes is an expected quality characteristic in Emmentaler. It shows that gas production, nuclei and cheese texture worked together well enough to create the intended structure.

It does not prove that one cheese is safer, older or more flavourful than another. Nor is a hole-free cheese automatically defective: many Swiss cheeses are made without Emmentaler-style eyes, and “Swiss cheese” in North American shops often names a family of Emmental-inspired cheeses rather than every cheese produced in Switzerland.

If you are choosing between two packages, use the named variety, age and producer description to predict flavour. Treat the holes as part of that cheese’s expected structure — not as a universal quality score.

The shortest correct answer is that bacteria supply the gas and tiny air pockets organise it. The holes appear only when fermentation, physical nuclei, temperature and texture cooperate.