Kamil Perry
Cheese is a beloved food, incorporated into many cuisines and consumed directly or used as an ingredient in cooking. It is one of the few foods that contains extraordinarily high numbers of living, metabolizing microbes. The magic of cheese happens with just four ingredients: milk, salt, rennet (or another coagulant), and microbes. The broad groups of cheese-making microbes include many varieties of bacteria, yeast, and filamentous fungi (molds). Lactic acid bacteria (LAB) are often called starter cultures as they play a crucial role in converting milk sugar (lactose) into lactic acid, lowering the cheese's pH and inhibiting spoilage. As cheese ages, Non-Starter Lactic Acid Bacteria (NSLAB) increase while starter cultures die off, and their exact role in flavour development is still being understood. The complex ecosystems created among these microbes give rise to the unique flavours, smells, and textures of different cheeses.
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What You'll Learn
The role of bacteria in cheese-making
Cheese is made with milk, salt, rennet (or some other coagulant), and microbes. The role of bacteria in cheese-making is complex and multifaceted. Bacteria are essential to the fermentation process, converting lactose to lactic acid, which acidifies the milk and inhibits the growth of undesirable microorganisms. This process, known as rapid acidification, is carried out by lactic acid bacteria (LAB), which play a crucial role in the development of flavour and texture in cheese.
There are two main families of LAB: lactococci (sphere-shaped, lactic-acid producing bacteria) and lactobacilli (rod-shaped, lactic-acid producing bacteria). Streptococci can also play an important role in initial cheese ripening and are very important in yoghurt-making. As cheese ages, the number of non-starter LAB, which are not added for the express purpose of acidifying the milk, increases while starter cultures die off. These non-starter LAB are present naturally in the milk or are picked up during the cheesemaking process.
The bacteria Propionibacterium freudenreichii ssp. shermanii is responsible for the holes in Swiss cheeses such as Emmental and Gruyere, as well as the sharp smell of these cheeses. This is due to its ability to convert lactic acid into carbon dioxide, propionic acid, and acetic acid. The carbon dioxide seeps into the cheese and creates holes.
The white mould found on the outside of soft-ripened cheeses is a subspecies of P. camembertii (also called P. candidum). These moulds produce enzymes that break down milk proteins, creating the characteristic ripened layer surrounding a firm interior. The flavour compounds produced by this process are often garlicky or earthy.
Smear bacteria, such as Brevibacter linens, are responsible for the strong smell of cheeses such as Epoisses, Münster, and Limburger. This bacterium produces a multitude of compounds that give rise to the distinctive aroma.
The microbial composition of cheese is important in the dairy industry, as it contributes to the safety, quality, and sensory properties of the cheese. The process of cheesemaking has become cleaner and more controlled over time, which has reduced the diversity of microbes in cheese. However, even in modern cheesemaking, there are still many microbes that are introduced unintentionally, through the milk or the surrounding environment, which contribute to the unique characteristics of the cheese.
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How bacteria contribute to the flavour of cheese
The flavour of cheese depends on several factors, including the type of milk used and the bacteria present during the fermentation process.
Cheese is a traditional food that is incorporated into many cuisines and is one of the few foods containing extraordinarily high numbers of living, metabolising microbes. The broad groups of cheese-making microbes include many varieties of bacteria, yeast, and filamentous fungi (moulds).
Bacteria play a crucial role in converting milk sugar (lactose) into lactic acid, lowering the pH of the cheese and inhibiting the growth of spoilage organisms. This process is known as fermentation and is essential for preserving milk and preventing it from spoiling. There are two main families of lactic acid bacteria: lactococci (sphere-shaped) and lactobacilli (rod-shaped). Additionally, streptococci play a significant role in initial cheese ripening and yoghurt-making.
During the cheese-making process, starter bacteria are added to initiate fermentation. As the cheese ripens, whole communities of microbes, including bacteria, yeast, and mould, further develop and contribute to the flavour profile. The specific types of bacteria present can influence the production of flavour compounds, resulting in distinct flavours such as fruity, nutty, buttery, or musty notes found in different cheeses.
For example, in Swiss cheeses like Emmental, the presence of Propionibacterium freudenreichii ssp. shermanii converts lactic acid into carbon dioxide, propionic acid, and acetic acid. The carbon dioxide leads to the formation of characteristic "holes", while the propionic acid contributes to the sharp, complex flavour profile of these cheeses. In contrast, Gouda cheese often exhibits smaller "eyes" due to the action of bacteria such as Leuconostoc mesenteroides and Lactococcus lactis ssp. lactis biovar. diacetylactis, which convert citric acid into carbon dioxide and diacetyl, imparting a buttery flavour.
The white moulds found on soft-ripened cheeses, such as Camembert and Brie, are subspecies of Penicillium camemberti (formerly known as P. candidum). These moulds produce enzymes that break down milk proteins, resulting in a ripened layer surrounding a firm interior. The flavour compounds produced by this enzymatic breakdown can be garlicky or earthy, while also producing ammonia as a by-product.
Additionally, smear bacteria, such as Brevibacter linens, are responsible for the strong aroma and flavour of cheeses like Epoisses, Münster, and Limburger. This bacterium produces a range of compounds that contribute to the distinctive aroma and flavour profile of these cheeses.
While modern microbiology has yet to fully elucidate the role of all microbes in cheese flavour development, it is clear that bacteria play a pivotal role in shaping the unique flavours of different cheeses.
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The different types of bacteria used in cheese-making
Cheese is made from milk, salt, rennet (or another coagulant), and microbes. The microbes used in cheese-making include bacteria, yeast, and filamentous fungi (molds). The type of microbes used depends on the type of cheese being made.
Lactic Acid Bacteria
Lactic acid bacteria (LAB) are often called "starter cultures" because they initiate the process of converting milk sugar (lactose) into lactic acid, which lowers the pH of the cheese and inhibits spoilage organisms. The two main families of lactic acid bacteria are lactococci (sphere-shaped) and lactobacilli (rod-shaped). Streptococci are also important in initial cheese ripening and yogurt-making. Examples of lactic acid bacteria include:
- Lactococcus lactis ssp. lactis biovar. diacetylactis, which is used in the production of Gouda cheese
- Lactobacillus casei ssp. casei, an example of Non-Starter Lactic Acid Bacteria (NSLAB) that are present naturally in the milk or picked up during cheesemaking
Propionibacteria
Propionibacteria are responsible for the distinctive holes and sharp flavor of Swiss cheeses such as Emmental. They digest lactic acid and convert it into carbon dioxide, propionic acid, and acetic acid. Examples include:
- Propionibacterium freudenreichii ssp. shermanii, which is also known as Propionobacter shermanii
- Propionibacterium freundenreichii
White Molds
White molds, or P. camembertii (also called P. candidum), are found on the outside of soft-ripened cheeses like Camembert and Brie. They produce enzymes that break down milk proteins, creating a ripened layer surrounding a firm interior. They also contribute to the flavor and texture of these cheeses.
Smear Bacteria
Smear bacteria, or Brevibacter linens, are responsible for the strong smell of cheeses like Epoisses, Münster, and Limburger. They produce a variety of compounds that give rise to the distinctive aroma of these cheeses.
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The impact of pasteurisation on cheese bacteria
Cheese is made from four main ingredients: milk, salt, rennet (or another coagulant), and microbes. The microbes used in the cheese-making process include bacteria, yeast, and filamentous fungi (molds). The bacteria in cheese are responsible for its flavour, texture, and smell.
Prior to the widespread adoption of pasteurisation, cheesemakers relied on natural bacteria and other microorganisms present in raw milk to initiate fermentation, or "ripening". However, with the introduction of pasteurisation, cheesemakers found it challenging to produce cheese with the same complex flavour and texture. This is because pasteurisation destroys not only harmful bacteria but also beneficial bacteria and enzymes.
There are several methods of pasteurisation, each with varying effects on the milk's bacteria. Low-temperature long-time (LTLT) pasteurisation, for example, involves heating the milk to 63°C for 30 minutes and then cooling it. This method is less damaging to the milk's bacteria and may result in better cheesemaking results. However, it requires more time and energy than high-temperature short-time (HTST) pasteurisation, where milk is heated to 72°C for 15 seconds.
Another method is thermalisation, which falls between raw milk and pasteurised milk in terms of treatment intensity. Thermalisation involves heating the milk to a lower temperature than HTST and LTLT for a more extended period, usually between 57°C and 68°C for 15 to 30 seconds. This range is sufficient to kill some harmful bacteria but not all, preserving more of the natural bacterial flora present in raw milk.
The final method is ultra-high-temperature (UHT) pasteurisation, where milk is heated to 135°C for a few seconds and then rapidly cooled. UHT pasteurisation is commonly used for milk that will be stored at room temperature, such as boxed long-life milk or canned milk.
The introduction of pasteurisation has had a significant impact on cheese production. While it has contributed to safer commercial cheesemaking, it has also made it more difficult to achieve the same complex flavours and textures as those obtained with raw milk. As a result, some cheesemakers have turned to alternative methods to introduce beneficial bacteria, such as using starter cultures.
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The effect of bacteria on the texture of cheese
Bacteria play a crucial role in shaping the texture of cheese. The type of bacteria, along with the milk type, feed given to the providing animal, manufacturing practices, ripening environment, and duration of ripening, all influence the final texture of the cheese.
Lactic acid bacteria (LAB) are the most common type of bacteria used in cheese-making and are often called "starter cultures". They play a vital role in converting milk sugar (lactose) into lactic acid, lowering the cheese's pH and inhibiting the growth of spoilage organisms. This process of fermentation preserves milk as cheese, allowing it to be consumed months or years later. Different strains of LAB, such as Lactococcus lactis and Streptococcus thermophilus, contribute to varying textures and flavours in the final product. For example, Lactococcus lactis, in combination with other bacteria, gives cheddar its firm texture, while a different combination of bacteria, including Penicillium candidum, gives brie its soft texture.
The aging process further influences the texture of cheese. As cheese ages, certain bacteria, such as Propionibacterium freudenreichii ssp. shermanii, continue to grow and ferment, producing carbon dioxide, propionic acid, and acetic acid. The carbon dioxide seeps into the cheese body, creating the characteristic "eyes" or holes found in Swiss cheeses like Emmental. The propionic acid contributes to the sharp flavour and complex bouquet of these cheeses. Additionally, as cheese ages, the number of Non-Starter Lactic Acid Bacteria (NSLAB) increases, further impacting the texture and flavour development.
In plant-based cheeses, bacterial blends are used to improve texture and flavour. Different blends of bacteria can enhance acidification, contribute to gel hardness, and develop dairy-like flavours. For example, the presence of L. plantarum and L. casei in certain blends reduces gel firmness and produces dairy-like compounds.
The environmental factors, or "terroir," can also influence the texture of cheese. The type of pasture that cows or goats graze on, as well as the local microorganisms present in the environment, can impact the final texture of the cheese.
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Frequently asked questions
Milk, salt, rennet (or some other coagulant), and microbes.
The broad groups of cheese-making microbes include many varieties of bacteria, yeast, and filamentous fungi (molds).
Lactic acid bacteria (LAB) are often called "starter cultures" as they play a main role in converting milk sugar, lactose, into lactic acid. Other examples include Propionibacterium freudenreichii ssp. shermanii, Brevibacterium linens, and Streptococcus.
Microbes contribute to the flavour, smell, texture, safety, and quality of the cheese.
Some cheeses with microbes include Cheddar, Mozzarella, Camembert, Grana Padana, Pecorino Romano, Emmental, Gruyere, Epoisses, Münster, Limburger, and Swiss cheese.
