Kamil Perry
Swiss cheese, also known as Emmental, is famous for its holes, which are called eyes in the cheese industry. The eyes are caused by carbon dioxide gas produced by bacteria in the cheese. However, the presence of bacteria is not the whole story. Swiss researchers at Agroscope have recently discovered that the size of the holes is determined by the amount of hay particles present in the milk.
| Characteristics | Values |
|---|---|
| Gases | Carbon dioxide |
| Bacteria | Propionibacteria, or Props |
| Other causes | Tiny bits of hay present in the milk |
| Size of holes | From the size of a dime to the size of a quarter |
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What You'll Learn
Propionibacteria bacteria added during cheesemaking
The holes in Swiss cheese, or "eyes" as they are known in the cheese industry, are caused by a special bacterial culture called Propionibacteria, or Props for short. This bacterium is commonly found in milk and dairy products, as well as in hay, grasses, and soil, where it can find its way into raw milk naturally during the milking process.
During cheesemaking, Propionibacteria bacteria are added to Swiss cheese to give it its distinctive characteristics, especially its holes and flavour. The pasteurization process kills the bacteria in milk, including Propionibacteria, so if pasteurized milk is used as the base for Swiss cheese, cheesemakers add some Propionibacteria 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 Propionibacteria produce carbon dioxide and also keeps the cheese at the right consistency. The carbon dioxide produced by the Propionibacteria forms bubbles in the cheese, and when the cheese cools, the bubbles leave behind holes. The Propionibacteria also contribute to the characteristic nutty and sweet flavour of Swiss cheese through the production of acetate, propionic acid, and acetic acid.
Cheesemakers can control the size of the holes in Swiss cheese by changing the acidity, temperature, and curing time of the mixture. The concentration of Propionibacteria in Swiss cheese is higher than in any other cheese, with an estimated one billion living cells of Propionibacteria present in one gram of Emmental cheese.
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Bits of hay in milk cause weakness in curd structure
The holes in Swiss cheese, also known as "eyes", have long been attributed to carbon dioxide released by bacteria. However, it has been discovered that the actual cause of these eyes is the presence of tiny bits of hay in the milk.
When cheese is made in barns using open buckets, hay particles can easily find their way into the collected milk. These bits of hay then cause weaknesses in the structure of the curd, allowing gas to form and create the eyes characteristic of Swiss cheese.
The traditional method of milking, which involved using buckets, allowed more and larger hay particulates to enter the milk, resulting in bigger eyes. As the dairy industry transitioned from using buckets to more modern equipment, the size of the eyes in Swiss cheese decreased due to reduced hay particle contamination.
The role of hay in the formation of eyes was confirmed by Swiss researchers at the Agroscope Institute for Food Sciences. They attributed the decreasing size and occasional absence of eyes in Swiss cheese to the disappearance of the traditional bucket used during milking, which previously allowed hay particles to fall into the milk.
The presence of hay particles in the milk creates weaknesses in the curd structure, providing spaces for gas to form and expand, ultimately resulting in the characteristic eyes of Swiss cheese. This phenomenon is similar to the formation of raindrops around dust particles, as explained by John Jaeggi, a researcher at the University of Wisconsin's Center for Dairy Research.
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Carbon dioxide gas produced by propionic acid bacteria
The holes in Swiss cheese, also known as "eyes", are caused by a specific bacteria called Propionibacterium freudenrichii subspecies shermanii, or Propionibacteria for short. This bacteria is found in hay, grasses, and soil, and can make its way into raw milk naturally when cows are milked. The pasteurization process kills the bacteria in milk, including Propionibacteria, so if pasteurized milk is used as the base for Swiss cheese, cheesemakers add some Propionibacteria back in so that the eyes will form. After the Swiss cheese is made and brined, the blocks or wheels are placed in warm rooms, which helps the Propionibacteria produce carbon dioxide and also keeps the cheese at the right consistency.
Propionibacteria produce carbon dioxide through the process of glycerol fermentation. During this process, propionibacteria convert lactic acid and carbohydrates from biomass into propionic acid using Zn as a reducing agent and Co as a catalyst. Propionibacteria can also produce propionic acid through bacterial fermentation, where they produce propionic acid, carbon dioxide, and hydrogen from lactate and pyruvate.
In the context of Swiss cheese, the carbon dioxide produced by Propionibacteria forms bubbles in the cheese when it is kept in a warm room. When the cheese cools, the bubbles leave behind air pockets, forming the characteristic eyes of Swiss cheese.
The production of carbon dioxide by Propionibacteria is also important in other industrial processes. For example, the heterotrophic CO2 fixation of Propionibacterium acidipropionici has great potential in the industry for propionic acid production, as CO2 is one of the byproducts during fermentation. Additionally, CO2 supplementation has been shown to increase the productivity of propionic acid in glycerol fermentation.
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Pasteurization kills bacteria, so must be re-added
The holes in Swiss cheese, also known as "eyes", are caused by a specific bacterial culture called Propionibacteria, or Props, that gets added to the cheese. Props are found in hay, grasses, and soil, and they can make their way into raw milk naturally when cows are milked. The pasteurization process, however, kills the bacteria in milk, including Props. Therefore, if pasteurized milk is used as the base for Swiss cheese, cheesemakers will add some Props back in so that eyes will form.
After the Swiss cheese is made and brined, the blocks or wheels are placed in warm rooms. This helps the Props produce carbon dioxide and also keeps the cheese at the right consistency. The production of carbon dioxide by the bacteria is what leads to the formation of the holes in Swiss cheese.
The traditional process of making Swiss cheese involved the use of open buckets in barns, which allowed hay particles to mix with the milk. These hay particles caused weaknesses in the structure of the curd, allowing gas to form and create the holes. As the traditional milk bucket method has been replaced by cleaner processing centers, the holes in Swiss cheese have become smaller or may not form at all.
While the presence of hay particles is a significant factor in the formation of holes, the bacterial culture is still essential. The pasteurization process kills the bacteria, so it must be re-added to the cheese to ensure the production of carbon dioxide and the formation of the characteristic eyes in Swiss cheese.
In summary, the combination of the bacterial culture and the presence of hay particles in the milk contributes to the unique eyes in Swiss cheese. The pasteurization process kills the bacteria, so it is necessary to re-introduce it to achieve the desired consistency and hole formation in the final product.
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Warm rooms help bacteria produce carbon dioxide
The holes in Swiss cheese, also known as "eyes", are caused by a specific bacterial culture called Propionibacteria, or Props, that gets added to the cheese. These bacteria are commonly found in hay, grasses, and soil, and they can make their way into raw milk naturally when cows are being milked. The pasteurization process kills these bacteria, so if pasteurized milk is used as the base for Swiss cheese, cheesemakers add some Props 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 Props produce carbon dioxide and also keeps the cheese at the right consistency. The carbon dioxide produced by the bacteria forms bubbles in the cheese, and when the cheese cools, the bubbles leave air pockets behind, creating the eyes characteristic of Swiss cheese.
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 because processing centers have become cleaner, resulting in fewer hay particulates in the cheese. According to Agroscope researchers, the eyes are caused by tiny bits of hay present in the milk, which create weaknesses in the structure of the curd, allowing gas to form and create the eyes. As the traditional milk bucket method disappears, Swiss cheese varieties are slowly losing their eyes.
While the presence of carbon dioxide and its role in forming the eyes are well-established, the source of the carbon dioxide is still a subject of some debate. The traditional understanding is that the carbon dioxide is produced by the Propionibacteria bacteria. However, some recent studies have challenged this theory, suggesting that the carbon dioxide may come from other sources, such as the conversion of propionic acid by bacteria.
Warm rooms are essential in the Swiss cheese-making process because they provide the optimal environment for the Props to produce carbon dioxide and maintain the desired cheese consistency. By controlling the temperature and humidity in these rooms, cheesemakers can ensure that the bacteria remain active and continue to generate carbon dioxide, contributing to the formation of the eyes.
The warm environment also influences the overall flavour and texture of Swiss cheese. The Props bacteria not only contribute to the formation of eyes but also play a crucial role in developing the characteristic nutty flavour of Swiss cheese. Additionally, the warm rooms help maintain the right moisture level in the cheese, impacting its final texture and mouthfeel.
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Frequently asked questions
The holes in Swiss cheese, also known as "eyes", are caused by a special bacterial culture called Propionibacteria, or Props, that gets added to the cheese. Bits of hay present in the milk can also cause holes by creating weaknesses in the structure of the curd, allowing gas to form and create the holes.
Cheesemakers have identified three types of holes in Swiss cheese: Nissler holes (the smallest), “eyes” (the medium-sized and most abundant holes), and “large blow holes” (which can make blocks of cheese undesirable because they make slicing difficult).
Propionibacteria produce carbon dioxide at warm temperatures of around 70°F. When the cheese cools, the carbon dioxide gets trapped, leaving air bubbles behind and creating holes in the cheese.
