Swiss cheese is one of the most popular and beloved types of cheese in the world. Even those who don’t know much about cheese can typically spot this iconic type thanks to the many holes that it has going through it.

This hole-filled cheese has been made since at least the 1200s, but it likely goes back even further than that. Ever since it was first produced, people have undoubtedly been wondering what causes the holes to form.

For a long time, it was believed that the holes formed due to a bacterium in the cheese that produced gases. Those gases would prevent the formation of cheese in specific areas, resulting in the holes forming as the cheese hardened.

In a 1917 paper, USDA researcher William Mansfield Clark wrote:

“The quality of a prime Emmental cheese is determined not alone by its sweet, nutty flavor and pliant texture, but also by the character of its holes. Various attempts have been made to distinguish the fermentations responsible for various types of holes. The biological origin of normal eyes is still in doubt.”

This theory made a lot of sense and did give an explanation of the holes that had stood for decades. Unfortunately, scientists over the years have tried to pinpoint what type of bacteria caused the gases, or even what the gases were, without success.

As modern techniques were used to make cheese, the number and size of the holes were shrinking. Cheesemakers even noticed that the composition of the holes would change depending on whether it was made in the summer months or the winter.

This was confusing to many because modern practices allowed much more precision to be used throughout the process, which should have led to more standardized results.

Many connoisseurs were worried that Swiss cheese would eventually lose the holes entirely, which would mean that it no longer stood out from the crowd of other cheeses.

Fortunately, in 2015, the mystery was finally solved, but the answer was not at all what people expected.

Researchers found that the reason the holes were becoming smaller and less common was that the process of making the cheese was becoming much cleaner. In the past, the milk for the cheese was put directly into buckets before being brought in and undergoing the rest of the steps needed.

During the milking or transportation of the milk, microscopic specs of hay and grass would land in the milk. While larger pieces would be filtered out, those tiny ones remained. It was these tiny particles that caused the holes to form, or at least that was the theory.

To prove the hypothesis, the team harvested pristine milk and sprinkled ground-up hay powder into it before putting it through the 130-day cheese-making process. The end results were just what they expected. Delicious Swiss cheese with plenty of holes.

Now, cheese makers are able to more precisely control how many holes and how large they will be by adding hay particles to the milk before making the cheese.

Thankfully, this will secure the popularity of Swiss cheese for generations to come.

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