Swiss cheese is arguably one of the most iconic dairy products known worldwide, not just for its creamy flavor but also for the large, round holes that make it unique. If you’ve ever wondered, “Why does Swiss cheese have big holes?”—you’re not alone. This curious feature has fascinated both cheese lovers and scientists alike for over a century. The answer lies in a fascinating combination of microbiology, chemistry, and cheesemaking tradition. In this article, we’ll explore the science behind the formation of these holes, the crucial microbe involved, and how this process contributes to the distinct qualities of Swiss cheese.
What Causes the Holes in Swiss Cheese?
The holes in Swiss cheese—also called “eyes”—are a natural result of the fermentation process that takes place during the cheese’s ripening. These eyes form as gases are released within the cheese mass. Specifically, carbon dioxide (CO₂) is produced inside the solidifying curd, and because it cannot escape easily, it accumulates and creates bubbles. These bubbles become the holes we see in the final product.
But why does this happen only in Swiss cheese? And what exactly causes the release of carbon dioxide in this particular way?
Carbon Dioxide Release During Cheese Ripening
Cheese is made by coagulating milk, separating the solids (curds) from the liquid (whey), and then allowing it to rest and ferment. The formation of gas bubbles begins during a phase known as the “ripening” or “aging” phase. During this stage, specific microbes present in the cheese begin to consume and break down lactic acid, releasing carbon dioxide in the process. The structure of the cheese at this point is still firm enough to hold these gas bubbles, rather than allowing them to escape.
This process occurs in certain types of Swiss cheese including Emmental and Gruyère, both of which are known for their pronounced eye formation.
Optimal Temperature and Timing
The formation of holes in Swiss cheese depends critically on the internal temperature and the duration of the ripening period. Typically, Swiss cheeses are aged in temperature-controlled environments ranging from 20 to 24°C (68–75°F). This warm environment is ideal for microbial activity and gas production.
Furthermore, the cheese must age long enough to allow for sufficient lactic acid metabolism and gas formation while maintaining a structure that does not collapse or allow the gas to escape too quickly.
The Microbe Behind Swiss Cheese Holes
Every Swiss cheese owes its signature holes to a specific bacteria known for gas production: Propionibacterium freudenreichii. This bacterium, particularly the subspecies Propionibacterium freudenreichii subsp. shermanii, plays a crucial role in cheese development.
Role of Propionibacterium freudenreichii
This specific bacteria is responsible for producing several important reactions inside the cheese:
- Metabolism of lactic acid, which is present from earlier stages of cheesemaking
- Production of carbon dioxide, resulting in the formation of bubbles or holes
- Contribution to the nutty aroma and flavor associated with Swiss cheeses
The process through which the bacteria convert lactic acid into acetic acid, propionic acid, and carbon dioxide is known as homofermentative propionic acid fermentation.
Biology and Habitat of Propionibacterium freudenreichii
Propionibacterium freudenreichii is a Gram-positive, rod-shaped, anaerobic bacterium. It naturally occurs in raw milk and dairy fermentations and is intentionally added during the cheesemaking process to aid in flavor and structural development.
The bacteria are capable of surviving in the low-oxygen environment of a cheese wheel and gradually begin their fermentation process after most immediate acid-producing microbes (such as lactococci) have completed their role.
Propionate Fermentation Process Equation
The chemical process that Propionibacterium freudenreichii undergoes can be summarized by the following equation:
C₃H₆O₃ (lactic acid) → C₂H₄O₂ (acetic acid) + C₃H₆O₂ (propionic acid) + CO₂ (carbon dioxide)
This reaction results in not only gas formation (causing eyes), but also imparts distinct flavor compounds that make Swiss cheese delicious.
Historical Discovery of the Holes in Swiss Cheese
The scientific explanation for why Swiss cheese develops holes became clearer in the early 20th century. However, the phenomenon was something cheesemakers had observed for centuries. It wasn’t until advances in microbiology in the 1910s that researchers were able to identify the specific bacterium responsible for causing eye formation.
The Scientific Breakthrough
In 1915, Swiss scientist Walter Moeller discovered that specific bacteria played a role in gas formation in Emmental cheese. His work led to the isolation and identification of Propionibacterium freudenreichii.
Moeller and others found that if the microbe was absent or inactive, no gas bubbles formed in Swiss cheese, meaning no “eyes” appeared. From this, the scientific community understood that cheese development is not just dependent on the physical properties of milk and curd, but also heavily influenced by the microbial ecosystem in which it ages.
Early Cheesemaking and Observation
Even before microbiological theories explained this phenomenon, Swiss cheesemakers in the canton of Emmental had already noted the effect of cheesemaking temperature and fermentation conditions on eye formation. These producers often observed that certain wheels developed large, well-developed eyes, while others were more dense and compact. Though they may not have known the exact reason at the time, their meticulous control of conditions helped sustain and optimize this visual and sensory feature of the cheese.
Factors That Influence Eye Formation in Swiss Cheese
Although the presence of Propionibacterium freudenreichii is central to eye formation, several external factors can influence whether the holes actually develop in a visually appealing manner.
Milk Quality and Composition
The base milk used to make Swiss cheese must have the correct levels of fat, protein, and lactic acid content. Milk from different sources, animals, or seasons can impact the cheese’s composition and thus affect how gas is trapped and distributed.
Temperature and Aging Time
Maintaining the right temperature during aging is essential. Cool temperatures can stall bacterial growth, limiting gas production. Hot temperatures can cause the cheese structure to break down too early, allowing gas to escape unevenly or entirely. For ideal hole formation, aging time must also be optimized, typically lasting several months.
Ripening Starter Cultures
In modern cheesemaking, specific starter cultures are added to the milk in controlled quantities before the cheese sets. Not all cheeses use all the same starter strains—but for Swiss cheese, a combination of lactic acid bacteria and Propionibacterium freudenreichii is typically employed to ensure flavor development, texture, and eye formation proceed as desired.
How are Starters Added?
Starter cultures may be introduced in raw milk or milk that has been pasteurized, often in combination with rennet. Proper mixing and distribution ensure a uniform microbial presence throughout the cheese matrix, helping ensure a consistent pattern of holes and flavor development.
Types of Swiss Cheese Known for Their Holes
While “Swiss cheese” is often used colloquially to refer to any cheese with holes, not all cheeses labeled as “Swiss-style” actually come from Switzerland—or contain holes! Let’s explore some key regional cheeses known for their distinctive eyes.
Emmental (Emmentaler)
This is the prototypical Swiss cheese known for its large holes, sometimes referred to as giant Swiss eyes. Emmental cheese originates from the Emmental region of Switzerland. Traditionally aged for up to 14 months, it has a mild, nutty flavor with a texture that balances elasticity and firmness.
Gruyère
While Gruyère does contain eyes, they tend to be smaller and less numerous than those found in Emmental cheese. Gruyère is valued more for its complex flavor and granular texture. It originates from the canton of Fribourg and has Protected Designation of Origin (PDO) status, meaning only cheese made in specific regions can be labeled as true Gruyère.
Sbrinz
Sbrinz is a hard, aged Swiss cheese with a flavor that can be described as similar to Parmesan. It also contains small to medium eyes when produced using traditional methods. Sbrinz is less known internationally than Emmental or Gruyère, but remains a favorite in Swiss households for grating and cooking.
The Myth of the Hay Mite Theory
Before scientists isolated Propionibacterium freudenreichii, there was a popular theory in the 1990s and earlier that tiny hay mites might be responsible for introducing the bacteria into the milk, causing the fermentation process to begin. This theory suggested that raw milk from Alpine cows included trace elements of hay, including microscopic particles, that might have seeded the microbial ecosystem early on.
While compelling, this theory was eventually dispelled as it was shown that the main actors were already present in the starter cultures deliberately added during cheesemaking—and not necessarily introduced by environmental mites.
However, it underscored an important point: the fermentation process in cheese is deeply tied to the surrounding microbiome and traditional farming practices.
Practical Applications and Industrial Considerations
Cheesemakers today utilize controlled microbiology to ensure consistent eye formation. Whether producing cheese in artisanal dairies or large-scale industrial setups, understanding the function of Propionibacterium freudenreichii allows cheesemakers to standardize processes for optimal flavor and texture.
Use in Industrial Cheese Production
In industrial settings, the use of freeze-dried or liquid cultures ensures uniformity in bacterial contribution. Propionibacterium freudenreichii may not always grow well in environments that don’t mimic the traditional alpine environment, so cheesemakers carefully monitor fermentation conditions.
Precision in Eye Distribution and Size
Modern cheesemaking allows for adjustments in:
- Cheese moisture content
- Temperature modulation in aging chambers
- Length of secondary fermentation
These parameters can be optimized to ensure the development of uniformly distributed eyes of desired sizes—making each wheel visually appealing and consistent in taste and texture.
Why Eyes Matter: Not Just Visual Appeal
While the holes are visually striking and often what draws first-time tasters to Swiss cheese, they are not just cosmetic. The presence of eyes influences how the cheese ages, breathes, and delivers flavor.
Impact on Cheese Texture
The holes contribute to the cheese’s overall structure. A cheese with well-developed eyes is generally more aerated and less dense, making it easier to chew and more enjoyable for consumers.
Influence on Flavor Development
As previously mentioned, Propionibacterium freudenreichii produces flavor compounds such as propionic and acetic acid. These add to the distinctive nutty and slightly sweet profile that Swiss cheeses are known for. Without microbial intervention, the cheese would be more sour and less complex in flavor.
Historical Identity and Market Appeal
The iconic holes in Swiss cheese have become a symbol of authenticity and artisanal heritage. The eyes are not only unique to certain cheeses made in Switzerland but are a quality consumers now expect when purchasing Swiss varieties from around the world.
Conclusion: The Science and Art Behind Swiss Cheese
Swiss cheese, with its distinctive holes, brings together centuries of tradition and modern science. The formation of these “eyes’ is not accidental—it results from the careful orchestration of fermentation conditions using a key bacterium, Propionibacterium freudenreichii. It is this microbe that ensures Swiss cheese’s characteristic visual signature, flavor complexity, and texture.
Understanding the relationship between microbial activity, cheese composition, and environmental factors helps both cheesemakers and lovers appreciate Swiss cheese in a new light. So, the next time you see a slice of Swiss cheese with holes, take a moment to appreciate the invisible work of these microbes, turning simple milk into a global culinary treasure.
What causes the holes in Swiss cheese?
The distinctive holes in Swiss cheese, known scientifically as “eyes,” are formed during the cheese-making process by specific bacteria. The primary microbe responsible is Propionibacterium freudenreichii, which consumes lactic acid in the cheese and produces carbon dioxide gas as a byproduct. This gas becomes trapped within the cheese curds and gradually forms bubbles, which eventually become the round openings characteristic of Swiss cheese.
The size and number of holes depend on several factors, including the temperature, acidity, and moisture content of the cheese during its ripening phase. In controlled conditions, cheesemakers can influence how many eyes develop and how large they become. Cheeses made during different seasons or under varying environmental conditions may exhibit different eye patterns, which is why the appearance of Swiss cheese can vary from wheel to wheel.
How does *Propionibacterium freudenreichii* end up in Swiss cheese?
Propionibacterium freudenreichii occurs naturally in raw milk, especially milk from mountainous regions where traditional Swiss cheese like Emmental is produced. During the cheese-making process, this microbe is introduced along with other starter cultures that help ferment the milk. Once in the cheese, it remains dormant until the later stages of ripening, when it becomes active in the cheese’s warm curing environment.
Modern cheesemaking often involves pasteurized milk, which may require the deliberate addition of this bacterium to ensure consistent eye formation. Cheesemakers have learned to control and optimize the amount of Propionibacterium freudenreichii in the cheese, allowing them to reproduce the signature holes reliably regardless of the milk’s origin or season.
Why is eye formation important to Swiss cheese quality?
Eye formation is not only an aesthetic feature but also contributes to the texture and flavor of Swiss cheese. The carbon dioxide gas that creates the holes plays a role in the overall ripening process, influencing how the cheese matures. The presence of holes indicates that the cheese has undergone proper fermentation and development by the right microbial activity.
In the cheese industry, certain size, quantity, and distribution of eyes are considered markers of high quality, especially for traditional varieties such as Emmental. These characteristics are often associated with the artisanal skill of the cheesemaker and the authenticity of the cheese. Without the proper microbial development, Swiss cheese would lack its unique appearance and some of the nuanced flavors associated with its texture.
Can other cheeses develop similar holes?
While Swiss cheese is the most famous for its holes, other cheeses can develop similar, though typically smaller or less pronounced, eyes under specific conditions. The formation of gas bubbles in cheese during fermentation can occur in various types of cheese, especially those made with similar microbial cultures. For example, cheeses like Gouda or even some semi-hard cheeses have been known to develop small eyes when ripened under similar warm and slow fermentation conditions.
However, other factors such as the cheese’s moisture levels, salt content, and pressing process can prevent eye formation. Swiss cheese like Emmental has a relatively low salt and high moisture content, which allows Propionibacterium freudenreichii to thrive and produce gas freely. Most cheeses are either too dense or too acidic to allow for the same distinctive eye development seen in classic Swiss varieties.
How do environmental factors affect the size and number of holes in Swiss cheese?
Environmental conditions such as temperature, humidity, and the duration of ripening play a significant role in determining the number and size of holes in Swiss cheese. Higher temperatures during the ripening phase can stimulate microbial activity, increasing gas production, and allowing the holes to form more readily. The time the cheese spends in controlled environments also influences eye development, with longer ripening periods usually resulting in more pronounced holes.
The consistency of the cheese curd and the rate at which it solidifies during pressing also affect how gas escapes or accumulates. If the curd is too firm, bubbles might not form properly, leading to a cheese with few or no eyes. Conversely, if the curd is too soft or unstable, the bubbles may coalesce into larger voids or even collapse, creating uneven or undesirable eye structures.
Are the holes in Swiss cheese considered defects or desired features?
In the case of traditional Swiss cheeses like Emmental, the presence of holes is a desired feature and is considered an indicator of proper fermentation and texture development. Swiss cheesemakers value the appearance of eyes because they are linked to the cheese’s characteristic flavor profile and its artisanal heritage.
However, in modern mass production, there are differing opinions. Some commercial cheesemakers intentionally reduce the number of holes to accommodate slicing machines or consumer preferences for a more uniform product. In such cases, efforts are made to limit microbial growth that creates the eyes. Still, among connoisseurs and traditional consumers, the presence of well-formed, evenly distributed holes remains a hallmark of quality Swiss cheese.
What might happen if no holes form in Swiss cheese?
If no holes form in Swiss cheese, it could indicate a disruption in the fermentation process, particularly the activity of Propionibacterium freudenreichii. This could result from overly acidic or salty conditions, pasteurization that eliminated beneficial microbes, or an unsuitable ripening temperature. When the desired gas production doesn’t occur, the cheese will lack the signature visual appeal of traditional Swiss cheese.
From a flavor and texture perspective, the absence of holes might suggest that the cheese underwent a faster, less complex ripening process. This could affect the development of its nutty, slightly sweet flavor profile and creamy texture. While the cheese may still be edible and safe, its identity as Swiss cheese—particularly of the Emmental type—would be diminished without the formation of eyes.