Patrick Oconnell
The holes in Swiss cheese, also known as eyes, are caused by carbon dioxide gas bubbles that form during the cheese-making process. The gas is produced by various bacteria, including Propionibacterium freudenreichii subsp. shermanii, which breaks down lactic acid. These bacteria are naturally present in raw milk when cows are milked, and they can also be added by cheesemakers. The carbon dioxide accumulates at weak points in the curd, creating the eyes. The size of the eyes can vary, and they are typically round. In recent years, the eyes in Swiss cheese have gotten smaller due to improved hygiene and processing methods, reducing the number of dust and hay particles in the milk that contribute to eye formation.
| Characteristics | Values |
|---|---|
| What are the holes in Swiss cheese called | Eyes |
| Why are they called eyes | Because they are round holes |
| Why are they formed | Due to the breakdown of lactic acid by Propionibacterium |
| Due to the formation of gas caused by microbial metabolism | |
| Due to the presence of tiny bits of hay in the milk | |
| Size | Can vary from the size of a dime to the size of a quarter |
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What You'll Learn
The holes are called 'eyes' by cheesemakers
The holes in Swiss cheese are called "eyes" by cheesemakers. The eyes are formed by bubbles of carbon dioxide gas during the cheesemaking process. The gas is produced by various species of bacteria in the cheese, specifically a special bacterial culture called Propionibacteria, or Props, that gets added to the cheese.
Cheesemakers ensure that there is enough pliability in the cheese to allow gas bubbles to form nice round eyes. If the cheese body is too hard and brittle, the pressure from the gas will cause splits and cracks to form. The eyes develop at nucleation sites in the cheese, which are weak points in the curd where the carbon dioxide accumulates and forms bubbles.
The Propionibacteria bacteria are naturally found in hay, grasses, and soil, and can make their way into raw milk when cows are milked. However, the pasteurization process kills these bacteria, so if pasteurized milk is used as the base for Swiss cheese, cheesemakers add some back in so that eyes will form.
In recent years, the holes in Swiss cheese have gotten smaller due to increasing standards of hygiene, which have reduced the number of dust particles in the milk around which gas bubbles form. Additionally, the move from old-fashioned barns and buckets to more modern processing centers has resulted in fewer and smaller hay particulates in the milk, contributing to the reduction in eye size.
Cheesemakers have addressed this issue by adding hay flower powder (Flores graminis) to the milk during production, allowing for eyes of the typical number and size to form. This addition was approved by the Swiss Federal Administrative Court in 2025.
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They are caused by bacteria
The holes in Swiss cheese, commonly known as "eyes", are indeed caused by bacteria. Cheesemakers add a bacterium called Propionibacterium freudenreichii (subspecies shermanii) to the milk during production. This bacterium is a special bacterial culture called Propionibacteria, or Props for short, and it plays a crucial role in eye formation.
Propionibacteria break down lactic acid, which is derived from the starter culture. Specifically, the starter culture metabolizes lactose (milk sugar) into lactic acid. Propionibacteria then metabolize this lactic acid into carbon dioxide (CO2), acetic acid, and propionic acid. It is the CO2 that accumulates at weak points in the curd, forming the bubbles that become the eyes of Swiss cheese.
The size of the eyes is influenced by the amount of carbon dioxide produced. In Gouda cheese, for example, cheesemakers add Lactococcus and Leuconostoc strains of bacteria, which produce a smaller amount of CO2, resulting in smaller eyes. The process is similar but the bacteria are different, and thus the outcome varies.
The pasteurization process kills bacteria in milk, including Props. Therefore, when pasteurized milk is used as the base for Swiss cheese, cheesemakers add Propionibacteria back in to ensure eye formation. The blocks or wheels of cheese are then placed in warm rooms, aiding the bacteria in producing carbon dioxide and maintaining the desired consistency.
The presence of bacteria and the subsequent release of carbon dioxide were first linked to Swiss cheese eyes in 1912 by William Mansfield Clark, a Department of Agriculture chemist. Clark's theory was widely accepted until 2015 when the Agroscope Institute for Food Sciences attributed the eyes to the presence of hay particles in the milk. However, the more recent understanding is that both bacteria and particulate matter contribute to eye formation. The reduction in eye size over the years is likely due to improved hygiene and cleaner processing centers, resulting in fewer dust particles in the milk.
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The bacteria breaks down lactic acid
The holes in Swiss cheese, known as "eyes", are caused by bacteria breaking down lactic acid. This process releases carbon dioxide gas, which forms bubbles and creates the eyes. Cheesemakers ensure the cheese body is flexible enough during aging for gas bubbles to form nice round eyes. If the cheese body is too hard, the pressure from the gas will cause cracks instead.
The lactic acid is derived from starter cultures, which metabolize lactose (milk sugar) into lactic acid. Specifically, the bacteria responsible for the breakdown of lactic acid in Swiss cheese is a propionic acid bacterium called Propionibacterium freudenreichii subsp. shermanii. This bacterium metabolizes lactic acid into carbon dioxide (CO2), acetic acid, and propionic acid. The CO2 accumulates at weak points in the curd, forming bubbles that become the eyes.
The size of the eyes can vary, and historically, they were larger due to the presence of more and bigger hay particulates in the milk. In recent years, the holes in Swiss cheese have gotten smaller due to improved hygiene and processing, reducing the number of dust particles in the milk around which gas bubbles form. To counteract this and maintain the desired size of eyes, the Swiss Federal Administrative Court approved the addition of hay flower powder to the milk during cheesemaking in 2025.
It is important to note that not all cheeses with eyes are Swiss cheese, and other varieties like Gouda have smaller eyes formed by a different process. In Gouda, cheesemakers add Lactococcus and Leuconostoc strains of bacteria to the milk, which convert citric acid into carbon dioxide and diacetyl. The amount of carbon dioxide produced is less than in Swiss cheese, resulting in smaller eyes.
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The bacteria is added to the cheese
The holes in Swiss cheese, known as "eyes", are caused by bacteria that are added to the cheese during production. Cheesemakers add a bacterium called Propionibacterium freudenreichii (subspecies shermanii) to the milk during the production of Swiss cheese. This bacterium breaks down lactic acid, which is present in the cheese as a starter culture, into carbon dioxide (CO2), acetic acid, and propionic acid. The CO2 accumulates at weak points in the curd, forming bubbles that eventually become the eyes of the cheese.
The size of the eyes depends on the amount of carbon dioxide produced and the flexibility of the cheese body. If the cheese body is too hard and brittle, the pressure from the gas will cause cracks instead of eyes. To ensure that eyes form properly, cheesemakers must ensure that the cheese body is flexible enough to allow gas bubbles to form nice round eyes.
The pasteurization process kills bacteria in milk, including Propionibacteria, so if pasteurized milk is used as the base for Swiss cheese, cheesemakers add some of the bacteria back in so that eyes will form. After the Swiss cheese is made and brined, the blocks or wheels are placed in warm rooms, which helps the bacteria produce carbon dioxide and maintain the right consistency.
The bacteria added to the cheese also contribute to the overall flavor of Swiss cheese. The acetic acid and propionic acid produced by the bacteria give Swiss cheese its characteristic nutty flavor. In Dutch-type cheeses, such as Gouda, the eyes are smaller because cheesemakers add different bacteria, Lactococcus and Leuconostoc strains, which produce less carbon dioxide. The diacetyl produced by these bacteria also contributes to the signature flavor of Gouda.
The size of the eyes in Swiss cheese has decreased over time due to improvements in hygiene and processing, which have reduced the number of dust and hay particles in the milk. These particles can cause weaknesses in the curd structure, allowing gas to form and creating larger eyes. To counteract this, Swiss cheesemakers have started adding hay flower powder to the milk during production to promote the formation of eyes.
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The size of the eyes has reduced over time
The size of the eyes in Swiss cheese has indeed reduced over time. This phenomenon is attributed to advancements in hygiene and changes in cheese production methods.
Historically, Swiss cheese was often crafted in barns using open buckets, inadvertently allowing hay particles and dust to contaminate the milk. These tiny bits of hay created structural weaknesses in the curd, providing nucleation sites for gas to accumulate and form eyes. John Jaeggi, a researcher at the University of Wisconsin's Center for Dairy Research, likened this process to the formation of raindrops around dust particles.
However, in recent times, processing centers have become cleaner, reducing the presence of hay particulates and dust in the milk. This improvement in hygiene standards has resulted in a decrease in the number and size of eyes in Swiss cheese. The cheese variety known as ""blind Swiss"" lacks eyes altogether, although it retains the distinctive Swiss cheese flavor.
The reduction in eye size can also be attributed to variations in bacterial cultures and production techniques. For instance, Gouda, a Dutch-type cheese, typically exhibits smaller eyes compared to Swiss cheese. This difference is due to the use of Lactococcus and Leuconostoc bacteria in Gouda production, resulting in lower carbon dioxide production and, consequently, smaller eyes.
Additionally, the pasteurization process, which kills bacteria in milk, including the eye-forming Propionibacteria, has influenced eye formation. To counteract this, cheesemakers add Propionibacteria back into the pasteurized milk to ensure the development of eyes. The delicate balance of bacterial cultures and production techniques plays a crucial role in shaping the size and number of eyes in Swiss cheese.
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Frequently asked questions
Cheesemakers refer to the holes in Swiss cheese as "eyes" because they are round and specific to Swiss cheese.
The eyes in Swiss cheese are caused by bacteria in the cheese that produce carbon dioxide gas. Specifically, the bacteria are Propionibacterium freudenreichii subsp. shermanii, which are added to the milk during production.
The size of the eyes in Swiss cheese can vary from the size of a dime to the size of a quarter. In recent years, the eyes have gotten smaller due to improved hygiene and processing methods.
Yes, other cheeses that have eyes include Gouda, Havarti, Tilsit, and Esrom. The eyes in these cheeses are typically smaller than those in Swiss cheese and are caused by different types of bacteria.
