The Hole Story: What Actually Makes the Iconic Holes in Swiss Cheese?

From cartoon clichés to deli counter staples, Swiss cheese is instantly recognizable by one defining feature: its holes. For decades, we’ve seen animated mice dive through them and have casually referred to them as a hallmark of cheeses like Emmental and Jarlsberg. But have you ever stopped to wonder what truly creates these iconic voids, often called "eyes" by cheesemakers? The answer is a fascinating story of microbiology, tradition, and a surprisingly modern scientific discovery that solved a long-standing mystery.

This post will delve into the science behind Swiss cheese, uncovering the primary culprit responsible for the gas, and revealing the surprising accomplice that determines where the holes form, settling the debate once and for all.

The Classic Culprit: A Bacterium at Work

For over a century, the primary force behind the holes in Swiss cheese has been well understood. It’s not trapped air from stirring or tiny cheese-loving creatures, but a specific type of harmless bacteria.

The star of the show is Propionibacterium shermanii (often shortened to P. shermanii). Here’s how this microbe works its magic in the cheesemaking process:

1. Initial Fermentation: Like most cheeses, the process begins with starter cultures of bacteria that consume lactose (milk sugar) and produce lactic acid. This is what begins to acidify and solidify the milk into curds.
2. The Second Act: After the cheese is pressed into large wheels and brined, P. shermanii is added to the mix. The wheels are then moved to a relatively warm curing room, often kept around 21°C (70°F).
3. Gas Production: In this warm environment, P. shermanii gets to work. It consumes the lactic acid produced by the other bacteria and, as a byproduct of its metabolism, releases carbon dioxide (CO₂) gas.
4. Bubble Formation: The cheese's texture at this stage is a dense, pliable solid. The CO₂ gas can’t escape, so it forms small bubbles within the cheese. As the bacteria continue to produce gas, these bubbles slowly expand, creating the smooth, round holes or "eyes" that Swiss cheese is famous for. The nutty, sweet flavor of Swiss cheese is also a result of this unique fermentation process.

For decades, this was considered the complete answer. But then, cheesemakers noticed something strange.

A Modern Mystery: The Case of the Disappearing Holes

Over the last few decades, cheesemakers in Switzerland and beyond began observing that their cheeses were developing fewer holes, and sometimes none at all. The bacteria were the same, and the recipes hadn't changed, so what was going on? The cheese was still delicious, but it was losing its iconic look. This puzzle prompted Swiss researchers to investigate, leading to a groundbreaking discovery that added a crucial new chapter to the story.

The Surprising Accomplice: Microscopic Hay Particles

In 2015, a study by Agroscope, a Swiss agricultural research institute, finally solved the mystery. They discovered that the P. shermanii bacteria needed a starting point—a microscopic nucleus—to form a bubble. Without this imperfection in the cheese paste, the gas would simply dissipate or form a few tiny, unnoticeable cracks.

The source of these crucial nuclei? Hay dust.

Traditionally, cows were milked by hand in barns, often into open buckets. This method allowed microscopic particles of hay to find their way into the milk. These tiny, solid specks created the perfect anchor points for the CO₂ gas to collect and form a bubble.

The mystery of the disappearing holes was a direct result of modern, sanitized milking practices. Automated, closed-system milking machines and ultra-filtered milk meant that the milk used for cheesemaking was now exceptionally clean and free of these "impurities." With no hay particles to act as seeds for the bubbles, the famous eyes failed to develop.

Thanks to this discovery, cheesemakers can now control the number and size of the eyes in their cheese with precision by adding a minuscule, controlled amount of hay dust during the production process.

Conclusion

The iconic holes in Swiss cheese are the result of a perfect partnership. While the bacterium Propionibacterium shermanii is the engine that produces the carbon dioxide gas, it's the ghost of a traditional farm—the microscopic hay particle—that provides the crucial starting point for each hole to form. It’s a beautiful example of how science and agricultural history are intertwined in the foods we eat. So, the next time you enjoy a slice of Emmental, you can appreciate not only the complex flavor but also the fascinating story of microbiology and tradition behind every single hole.