The Good Bacteria In Cheese

Cheese is a food produced by removing water from milk, yielding concentrated milk proteins, fats, and other nutrients and compounds conferring flavour and aroma. The four main ingredients required to make cheese are milk, salt, rennet (or another coagulant), and microbes. The microbes used in the cheese-making process are responsible for the uniqueness and character of the cheese. The most common microbes used are bacteria, yeasts, and moulds.

The most common types of bacteria used in the cheese-making process are lactic acid bacteria, which are often referred to as starter cultures. Lactic acid bacteria play a crucial role in converting the basic milk sugar, lactose, into lactic acid, which lowers the cheese's pH and makes the cheese inhospitable to spoilage organisms. There are two main families of lactic acid bacteria: lactococci (sphere-shaped, lactic-acid-producing bacteria) and lactobacilli (rod-shaped, lactic-acid-producing bacteria).

Other types of bacteria used in the cheese-making process include propionic acid bacteria, which can digest acetic acid and convert it to sharp, sweaty-smelling propionic acid and carbon dioxide. The carbon dioxide is what gives Swiss cheeses, such as Emmental, their characteristic holes.

Moulds are also commonly used in the cheese-making process. Moulds love cheese, and if left unprotected in a fridge, cheese will quickly be colonised by a fuzzy carpet of interestingly coloured spores. The most common moulds that grow on cheese are species of Penicillium. Only two species of blue mould, P. roqueforti and P. glaucum, give rise to the unique flavour and texture of hundreds of blue cheeses, such as Roquefort, Stilton, and Gorgonzola.

Lactic Acid Bacteria: Converts milk sugar to lactic acid, preventing the growth of spoilage organisms

Lactic Acid Bacteria (LAB) are often referred to as "starter cultures" as they kickstart the process of converting milk sugar (lactose) to lactic acid. This is a crucial step in cheesemaking as it lowers the pH of the cheese, making it inhospitable to spoilage organisms.

There are two main families of LAB: Lactococci (sphere-shaped) and Lactobacilli (rod-shaped). Streptococci, while not as common, can also play an important role in the initial stages of cheese ripening and are very important in yoghurt-making.

LAB are added to milk very early in the cheesemaking process to induce fermentation. This process is called acidification, where the pH of the milk is lowered through the conversion of lactose to lactic acid. This step is crucial as it prevents the growth of unwanted microorganisms and gives the cheese its distinctive sharp taste.

While many LAB die off after the initial cheesemaking steps, some survive and continue to contribute to the flavour of the cheese, especially in alpine and Italian hard cheeses.

The role of LAB in cheesemaking cannot be overstated. They are responsible for preventing spoilage and giving cheese its distinctive taste and texture. By producing lactic acid, they create an environment that inhibits the growth of unwanted bacteria, ensuring that the cheese remains safe for consumption.

Propionic Acid Bacteria: Produces carbon dioxide, creating the holes in Swiss cheese

Propionic acid bacteria (PAB) are a group of bacteria with the ability to produce propionic acid. In cheese, they are responsible for the characteristic eyes and nutty flavour of Swiss-type cheeses.

Propionic acid bacteria are added to milk as a starter culture, along with other bacteria, to initiate the fermentation process. The main reaction taking place here is the conversion of lactose to lactic acid, acidifying the milk. This process is essential to the formation of cheese, as it lowers the pH of the milk, allowing the milk proteins to bind to one another and form a gel.

Propionic acid bacteria are specifically responsible for the formation of the eyes in Swiss-type cheeses. They convert lactic acid into carbon dioxide, propionic acid, and acetic acid. The carbon dioxide collects at weak spots in the cheese, building up pressure until holes form. The propionic acid and acetic acid also contribute to the unique flavour of Swiss-type cheeses.

Moulds: Blue and white moulds are responsible for the unique flavour and texture of blue cheese

Blue and white moulds are responsible for the unique flavour and texture of blue cheese. Blue moulds, including Penicillium roqueforti and Penicillium glaucum, are added to the milk during the preliminary steps of cheese-making or to the curds before they are hooped. They are then encouraged to grow by piercing the cheese with needles, which create air channels. Blue moulds accelerate the breakdown of proteins and fats, resulting in an extra-creamy texture and a tangy, spicy, sharp and strong flavour.

White moulds, on the other hand, are found on the outside of soft-ripened cheeses and are a subspecies of Penicillium camemberti (also called Penicillium candidum). They produce enzymes that break down milk proteins, leading to a ripened layer surrounding a firm interior. The flavour compounds produced by this process are generally garlicky or earthy, but ammonia is also a by-product, so these cheeses should be allowed to breathe to allow the ammonia to dissipate.

Microbial Succession: The process by which microbes interact and succeed one another during cheese ripening

Microbial succession is a process that occurs during cheese ripening, where different microorganisms interact and replace one another. This succession of microbial populations plays a crucial role in developing the desired characteristics of the final cheese product, such as its taste, flavour, texture, and colour.

During the first few days of cheese production, the microbial population is primarily composed of lactic acid bacteria (LAB) from the starter culture. These bacteria play a crucial role in converting lactose, the basic milk sugar, into lactic acid, which lowers the cheese's pH and inhibits the growth of spoilage organisms. After this initial phase, the relative proportion of LAB decreases as other bacteria, originating from the production environment, begin to appear and dominate.

In the case of smear-ripened cheese, the production and ripening process can be divided into three stages, each with distinct microbial populations. During the first week of production, the cheese microbiota is mainly composed of LAB, specifically Lactococcus and Leuconostoc species. In the second week, the relative abundance of LAB decreases, and bacteria from the production environment, such as Psychrobacter, Pseudoalteromonas, and Vibrio, become more prominent. From the third week onwards, other adventitious bacteria, such as Malaciobacter and Psychrilyobacter, appear and contribute to the ripening process.

The formation of a rind further influences the microbial composition of the cheese. The cheese core, with its reduced oxygen level and lower pH, favours the growth of LAB, while the rind, exposed to the external environment, is colonised by aerobic and halophilic bacteria.

The microbial interactions and successions during cheese ripening are complex and involve a variety of microorganisms, including bacteria, yeasts, and moulds. These microbes may be intentionally added by the cheesemaker or introduced inadvertently through the production environment and equipment. The unique combinations of these microbes contribute to the distinct characteristics of different cheese varieties.

Biofilms: A source of spoilage enzymes in dairy products

Biofilms are a source of spoilage enzymes in dairy products. They are formed by the growth of bacteria, yeasts, and filamentous fungi (moulds) on the surface of cheese. These microorganisms are introduced into the cheese at every stage of the cheese-making process and can be added intentionally or unintentionally. While some of these microbes are desirable, others are undesirable and can lead to the development of off-flavours, poor texture, unappetising appearance, or pose health risks.

The formation of biofilms by undesirable microbes can be controlled through various means, including sanitation, the use of heat or antimicrobial treatments, and the creation of unfavourable conditions for their growth, such as controlling temperature and salt concentration.

The presence of biofilms can affect the quality and safety of cheese. For example, the formation of biofilms by Listeria monocytogenes, a pathogenic bacterium, can contaminate cheese and pose a health risk to consumers, especially those who are immunocompromised or pregnant. On the other hand, the formation of biofilms by desirable microbes, such as lactic acid bacteria, can contribute to the development of flavour, aroma, texture, and colour in cheese.

The microbial communities that form biofilms play a crucial role in the unique characteristics of different types of cheese. For instance, the presence of propionic bacteria in specific types of cheese, such as Swiss cheese, leads to the production of carbon dioxide, which creates the characteristic holes in the cheese.

In conclusion, biofilms can be a significant source of spoilage enzymes in dairy products, and their formation and control are essential aspects of cheese-making and preservation.

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