Why does Swiss cheese have holes?

The holes in Swiss cheese are gas bubbles — carbon dioxide, blown from the inside. When cheesemakers build a wheel of Emmental, they add propionibacteria — the gas-making bacteria — along with the usual starter cultures. The starter turns milk sugar into lactic acid; the gas-makers then eat that acid and exhale carbon dioxide. The wheel's rind is sealed and the paste is elastic, so the gas cannot escape. It collects into bubbles, and over weeks of warm storage those bubbles inflate into the round openings cheesemakers call eyes.

The gas works

The process runs on a schedule. After pressing and brining, wheels move into a warm room held around 20–24 °C for roughly six to eight weeks. That warmth switches the gas-makers on. They convert the lactic acid in the curd into propionic acid, acetic acid, and carbon dioxide. The acids are why Swiss-style cheese tastes sweet and nutty; the carbon dioxide is why it looks the way it does. Cool the wheel back down and the fermentation slows, freezing the eyes at their final size.

The speck that starts a bubble

Gas alone is not enough — a bubble needs a starting point. For about a century nobody could say what that starting point was. In 2015, researchers at Agroscope, the Swiss agricultural research institute, closed the case: microscopic specks of hay dust that fall into the milk during milking. Cheese made from ultra-filtered, speck-free milk produced gas but no eyes — what the trade calls a blind cheese. Add a trace of powdered hay back into the same milk and the eyes returned, in numbers the researchers could dial up or down with the dose. It also explained a real industry puzzle: as open milk buckets gave way to sealed, filtered milking lines, Swiss cheese holes had been quietly shrinking and disappearing. Cleaner milk, fewer specks, fewer eyes.

Reading the specs

So the hole is a record of the build. It tells you the right bacteria were present, the warm room did its job, the rind held pressure, and at least a little of the old farmyard made it into the vat. No mice, no mites, no drilling — just fermentation, trapped exhaust, and a speck of hay at the center of every eye.