Why Does Swiss Cheese Have Holes The Science Explained

Swiss cheese, with its iconic round openings scattered throughout its pale yellow body, is one of the most recognizable cheeses in the world. These holes, technically known as \"eyes,\" are more than just a visual curiosity—they’re the result of a precise biological and chemical process that occurs during cheesemaking. For decades, scientists debated what exactly caused these formations. While folklore once blamed mice or poor straining, modern research has revealed a far more fascinating explanation rooted in microbiology and physics.

Understanding why Swiss cheese has holes isn’t just a matter of culinary trivia—it sheds light on the delicate balance required in artisanal food production and how even minor changes in environment or technique can alter the final product. From the alpine dairies of Switzerland to industrial cheese factories, the formation of eyes remains a hallmark of quality Emmental and other Swiss-style cheeses.

The Role of Bacteria in Eye Formation

The primary agent behind the holes in Swiss cheese is a specific bacterium: Propionibacterium freudenreichii. This microorganism is added to milk during the early stages of cheesemaking and plays a crucial role in flavor development and gas production. Unlike many bacteria that spoil food, this strain is carefully cultivated for its beneficial effects.

During the ripening (or aging) phase, which lasts several weeks to months, P. freudenreichii consumes lactic acid produced earlier by other starter bacteria like Lactobacillus. As it metabolizes the acid, it releases carbon dioxide gas as a byproduct. Since the curd structure is firm but still slightly elastic, the gas cannot escape easily and instead forms small bubbles within the cheese matrix. Over time, these bubbles expand and coalesce into the characteristic eyes seen in Swiss cheese.

“Without Propionibacterium freudenreichii, there would be no gas, no pressure buildup, and ultimately no eyes. It’s a perfect example of controlled microbial activity shaping food texture.” — Dr. Hans Meier, Dairy Microbiologist, ETH Zurich

Environmental Factors That Influence Hole Size and Number

While bacteria initiate eye formation, environmental conditions during cheesemaking determine their size, distribution, and quantity. Temperature, humidity, acidity, and even the physical handling of the curds all play pivotal roles.

• Temperature: The cheese must be warmed to around 22–24°C (72–75°F) during the \"cooking\" phase to activate P. freudenreichii. Too cold, and the bacteria remain dormant; too hot, and they may die off prematurely.
• Aging Duration: Eyes develop slowly over weeks. Cheeses aged for shorter periods tend to have fewer or smaller eyes.
• Milk Quality: Raw milk traditionally used in Swiss Emmental contains microscopic hay particles that act as nucleation sites—tiny starting points where gas bubbles can form. This insight overturned earlier assumptions that only bacterial activity was responsible.
• Humidity Control: High humidity during aging prevents the rind from forming too quickly, allowing internal gases to migrate and form uniform eyes rather than bursting through weak spots.

Tip: If you're tasting Swiss cheese at home, let it sit at room temperature for 20–30 minutes before serving. This enhances both aroma and texture, revealing subtle nutty notes developed during fermentation.

Historical Misconceptions and Scientific Breakthroughs

For much of the 20th century, scientists believed that tiny fragments of debris—such as straw or dust—were responsible for initiating bubble formation. However, attempts to replicate hole formation by introducing foreign particles yielded inconsistent results. It wasn’t until a landmark study conducted by Agroscope, a Swiss agricultural research institute, in 2015 that the full picture emerged.

Using high-resolution X-ray tomography, researchers observed that microscopic hay particles present in raw milk served as critical nuclei for gas accumulation. When modern filtration methods removed these particles completely, the resulting cheese had significantly fewer or no eyes—even when the same bacterial cultures were used. This explained why some industrial producers using ultra-filtered or pasteurized milk struggled to produce authentic-looking Swiss cheese.

This discovery bridged a long-standing gap between traditional craftsmanship and scientific understanding. It confirmed that while bacteria generate the gas, natural impurities in raw milk provide the structural foundation for eyes to form.

Step-by-Step: How Swiss Cheese Is Made (and Holes Form)

1. Milking and Milk Preparation: Fresh cow’s milk is collected, often without ultra-filtration to preserve micro-particles from the barn environment.
2. Adding Starter Cultures: Lactobacillus bacteria are introduced to convert lactose into lactic acid.
3. Curdling: Rennet is added to coagulate the milk into solid curds and liquid whey.
4. Cutting and Cooking: Curds are cut into small pieces and gently heated to release more whey and prepare the texture.
5. Pressing: The curds are pressed into large wheels, typically 80–100 kg each, forming a dense but porous structure.
6. Salt Bath: Wheels are soaked in brine to enhance flavor and form a protective rind.
7. Initial Aging (Cool Room): Stored at ~15°C for about two weeks; minimal gas production.
8. Warm Room Fermentation: Moved to a warmer room (~24°C) for 4–6 weeks. Propionibacterium becomes active, producing CO₂.
9. Eye Development: Gas accumulates around hay particles, forming spherical cavities. Pressure builds gradually.
10. Final Aging: Cheese matures for several months, developing complex flavors and stabilizing eye structure.

Types of Swiss Cheese and Their Eye Characteristics

Cheese Type	Typical Eye Size	Eye Density	Bacterial Strain Used
Emmental (Switzerland)	2–4 cm	Moderate (10–20 per wheel)	Propionibacterium freudenreichii
Gruyère	Pinhead to pea-sized	Low (often nearly smooth)	Naturally occurring strains
Appenzeller	Small, irregular	Moderate	Propionic bacteria + surface cultures
Industrial Swiss-style	1–2 cm	High and uniform	Lab-selected P. freudenreichii

Note that not all Swiss cheeses have large holes. Gruyère, also from Switzerland, typically has very small or no visible eyes due to differences in culture use and pressing techniques. The term \"Swiss cheese\" in North America often refers specifically to Emmental-style cheese with prominent eyes.

Frequently Asked Questions

Why don't all Swiss-style cheeses have holes?

Modern processing techniques, especially ultra-filtration and the use of pasteurized milk, remove the microscopic hay particles needed for gas bubbles to form. Without these nucleation sites, even with active bacteria, eyes fail to develop properly.

Are the holes a sign of freshness or spoilage?

No. The holes are formed during controlled aging and are a sign of proper fermentation, not spoilage. Spoiled cheese typically shows mold, sliminess, or off-odors unrelated to eye presence.

Can I make hole-free Swiss cheese at home?

Yes. By using highly filtered or pasteurized milk and adjusting fermentation temperatures, home cheesemakers can produce a denser version. Alternatively, skipping the propionic bacteria culture will prevent gas formation entirely.

Conclusion: Appreciating the Science Behind Tradition

The holes in Swiss cheese are a beautiful intersection of nature, tradition, and science. What once seemed like a simple quirk of food design turns out to be the result of centuries of empirical knowledge refined by modern microbiology. Each eye tells a story—of mountain pastures, raw milk, and bacterial teamwork working in harmony under precise conditions.

Next time you slice into a wheel of Swiss cheese, take a moment to appreciate the invisible forces at work. The holes aren’t flaws—they’re proof of a living, breathing food system shaped by both human hands and microbial life.

🚀 Want to explore more food science mysteries? Share this article with fellow curious minds and spark a conversation about the hidden biology in your kitchen!