Swiss cheese has holes because special bacteria release gas inside the cheese, and that gas collects around tiny particles in the curd and forms round “eyes” as the cheese ripens.

Quick Scoop

The simple science answer

  • Cheesemakers add specific bacteria (often Propionibacteria) to the milk when making Swiss-style cheeses like Emmental.
  • As the cheese ages in warm rooms, these bacteria “eat” acids in the cheese and release carbon dioxide gas.
  • The gas cannot easily escape, so it gathers in pockets and pushes the cheese apart, creating the familiar holes (called “eyes”).

So what’s with the hay and “dirty buckets”?

  • For about a century, people thought it was only the bacteria and their carbon dioxide that made the holes.
  • Around 2015, Swiss researchers showed that tiny specks of hay or barn dust in the milk act as “seed points” where these gas bubbles can start and grow.
  • Modern milking systems are much cleaner, so fewer hay particles get into the milk, which is why many modern Swiss cheeses have smaller or fewer holes than older, traditional ones.

In other words, the classic cartoon holes are a mix of busy bacteria and old‑school, slightly “dirty” barn milk.

What are the holes actually called?

  • Cheesemakers call the holes “eyes” rather than holes.
  • Swiss-style cheeses with no holes are known as “blind” cheeses.
  • In the U.S., even the size and number of these eyes in Swiss cheese are regulated as a quality feature.

Mini timeline of the explanation

  1. Early 1900s: Scientists link the holes mainly to carbon dioxide from bacteria in the cheese.
  1. 20th century: That explanation becomes the standard story for why Swiss cheese has holes.
  1. 2015: Swiss institute Agroscope shows hay dust and barn particles in the milk are key to starting those holes, helping explain why the holes have been shrinking in recent decades.

Fun “today” angle

  • Online food and forum discussions now often mention both causes: bacteria gas + tiny particles like hay dust , rather than just “bacteria bubbles.”
  • People also notice that some packaged “Swiss” slices barely have holes at all, which fits with cleaner, industrial milk collection and different processing choices.

Bottom note: Information gathered from public forums or data available on the internet and portrayed here.