The Skin Of Cheese: A Bacterial Haven

Cheese is not just a tasty snack, it's an ecosystem. The food is made from four ingredients: milk, salt, rennet (or some other coagulant), and microbes. The latter is what makes cheese distinctive and delicious.

Cheese teems with bacteria, yeasts, and molds. More than 100 different microbial species can easily be found in a single cheese type. In other words, cheese isn't just a snack, it's an ecosystem. Every slice contains billions of microbes, and they are what makes cheeses distinctive and delicious.

Cheese is fundamentally about decomposition. Like microbes on a rotten log in the woods, the bacteria and fungi in cheese break down their environment (in this case, the milk fats and proteins). This makes cheeses creamy and gives them flavor.

The microbes that colonize cheese come from many places. Some are intentionally added to the milk, while others drift there from the environment and from the cheesemakers themselves. Details of temperature, salt, acidity, and other variables determine which of the colonists survive and dominate as the cheese matures.

Lactic Acid Bacteria

LAB are added to the milk very early in the cheese-making process. The main LAB found in cheese are Lactococcus, Lactobacillus, Streptococcus, Leuconostoc, and Enterococcus.

The LAB in these cheeses may have health-promoting effects, and are generally regarded as safe.

Surface Ripened Bacteria

Surface-ripened cheeses are a large group of cheeses that ripen from the outside in. This group includes washed-rind cheeses, smear-ripened cheeses, and Brie and Camembert. The surface of these cheeses is inoculated with a mixture of yeast and bacteria, with Brevibacterium linens being the most common bacteria, and yeasts such as Geotrichum candidum and Debaryomyces hansenii.

The yeast metabolises the lactate to CO2 and H2O, increasing the pH so that the bacteria can flourish. The bacteria then feed on the cheese and produce compounds that flavour the cheese.

The smear is usually applied by washing the surface of the cheese with a saline solution containing the desired bacteria and yeast. This is called the 'old-young' method, and can be a source of pathogens, particularly Listeria monocytogenes. Commercial ripening cultures are often used to smear young cheeses.

The ripening of surface-ripened cheeses is characterised by the development of a viscous, red-orange surface smear on their rind. The surface smear microbiota is mainly inhabited by Gram-positive bacteria and, to a lesser extent, by yeast and mould. The typical surface microbiota is usually dominated by salt-tolerant yeast and 'coryneform bacteria' such as Corynebacterium spp., Brevibacterium spp., Arthrobacter spp., Glutamicibacter spp., and Microbacterium spp., together with Staphylococcus spp. and some Gram-negative bacteria, including Halomonas spp. and Psychrobacter spp.

The role of bacteria in cheese flavour

Cheese is a milk-derived food product that comes in a great variety of forms, sizes, textures, aromas, and tastes. The sensorial properties of cheese depend on the milk type used, the feed given to the providing animal, the manufacturing practices involved, the ripening environment, the duration of ripening, and the type, numbers, and activity of the microorganisms in the forming product.

The magic that is cheese only really needs four ingredients to happen: milk, salt, rennet (or some other coagulant), and microbes. The bacteria, molds, yeasts, etc. that find their way into cheese can be added intentionally by the cheesemaker or affineur. And by intentionally, I mean a person made a judgement call and chose which organism to add to the cheese.

Many modern cheeses are made with preselected cultures, consisting of only a few types of microbe, but many traditional cheeses are inoculated using whey or other products from previous batches, meaning that they can be made with dozens of types of microbe, some highly unusual. This microbial wealth is among the many reasons that traditional cheeses can be so much more complex than modern, controlled-inoculation cheeses.

Lactic acid bacteria are often called "starter cultures", as they play the main role in converting the basic milk sugar, lactose, into lactic acid, a step that lowers cheese pH and makes the cheese inhospitable to many spoilage organisms and is the first step towards deliciousness.

Propionic acid bacteria, and specifically Propionibacter shermanii, are able to digest acetic acid and convert it to sharp, sweaty-smelling propionic acid and carbon dioxide. The carbon dioxide is what gives Emmental and other Swiss cheeses their characteristic "holes", and the propionic acid contributes to their complex, especially sharp bouquet. Interestingly, several species of propionibacteria inhabit human skin, and help produce "unwashed" odors.

Molds love cheese. Leave any cheese in a fridge without protection and it will quickly be colonized by a fuzzy carpet of interestingly colored spores. Most of the molds that grow on cheese are species of Penicillium, but some cheeses, like St. Nectaire, develop all kinds of crazy surface molds, most of which are not harmful and in fact contribute to that cheese's unique flavor. Only two species of blue mold, P. roqueforti and P. glaucum, give rise to the unique flavor and texture of the hundreds of blue cheese which are revered throughout the world. These blue molds are capable of growing in remarkably low-oxygen environments, which makes them perfectly suited for the small cracks in the interior of a ripening cheese. In fact, cheesemakers regularly pierce and inoculate channels through to-be-blued cheeses in order to encourage the growth of these molds. Blues, like Roquefort, Stilton, Gorgonzola, and Cabrales are some of the most famous cheeses in the world, but blue molds also frequently grow on the surface of goat cheeses, adding color and flavor, but without growing in the interior, as is the case of the unsurpassable Monte Enebro.

White molds, which are found on the outside of all types of soft-ripened cheeses, are subspecies of P. camembertii (also called P. candidum). These white molds produce enzymes that break down the milk proteins of the curds, leading to the characteristic ripened layer surrounding a firm interior in many of these cheeses. The flavor compounds that are produced by this enzymatic breakdown are generally garlicky or earthy; unfortunately, ammonia is also a by-product of this breakdown, and so it is advisable to let these cheeses breathe (sit uncovered) in order to allow the highly volatile ammonia to dissipate.

Last but perhaps most notorious are the smear bacteria, which are responsible for the room-clearing ability of Epoisses, Münster, and Limburger. These smear bacteria are officially known as Brevibacter linens. They can't live in acidic or deoxygenated environments, and so cannot survive in the interior of cheeses. Since they need salty (up to 15%), moist environments to grow, they must be encouraged to do so by continual washing or wiping of the cheese surfaces, a process which results in the development of the characteristic red "smear" surface of so-called "washed-rind" cheeses. B. linens excels at breaking down proteins into, well, stinky odor compounds, producing oniony or garlicky, fishy, and sweaty aromas. Some cheeses, like the great alpine cheeses, are wiped only during part of their ripening, producing a muted complexity of flavor, rather than the nasal assault that accompanies many cheeses that play host to B. linens. The aroma of the washed-rind cheeses is often compared to smelly feet by both fans and detractors, a fact which is easily understood, as brevibacter are ubiquitous on human skin, and grow especially well without interference from personal hygiene.

So, next time you pick up a piece of cheese, think of the huge microbial society that's gone to the trouble of producing the masterpiece in front of you. Then you can pretend you're Godzilla or something when you eat the cheese, with thousands of screaming microbes fleeing for the escape capsules.

The role of bacteria in cheese texture

Bacteria play a key role in the texture and flavour of cheese. The most common bacteria found in cheese are lactic acid bacteria, which are often called "starter cultures" as they play the main role in converting the basic milk sugar, lactose, into lactic acid. This lowers the cheese's pH, making it inhospitable to many spoilage organisms and is the first step towards deliciousness.

Lactic acid bacteria are either lactococci (sphere-shaped, lactic-acid producing bacteria) or 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.

Propionic acid bacteria, specifically Propionibacter shermanii, are able to digest acetic acid and convert it to sharp, sweaty-smelling propionic acid and carbon dioxide. The carbon dioxide is what gives Emmental and other Swiss cheeses their characteristic "holes", and the propionic acid contributes to their complex, especially sharp bouquet.

Moulds, specifically Penicillium roqueforti and Penicillium glaucum, are the big players in the blue mould world. These are what give bleus the blues. The pigments are created by the moulds as well as the unique flavour and distinctive texture. As mentioned in the opening paragraph of the post, these moulds are living breathing organisms. Starving them of oxygen will change their metabolism and they will change colour as well as produce off-flavours. For this reason it’s best not to vacuum-package blue cheeses (or any mould-containing cheese for that matter).

Penicillium camemberti is the most popular mould species that is responsible for the nice white lawn on the surfaces of cheeses like Camembert and Brie. The metabolism of this mould is responsible for some of the characteristic aromas associated with white mould cheeses (mushroom, ammonia, etc.) as well as the texture (soupy goodness).

Last but perhaps most notorious are the smear bacteria, which are responsible for the room-clearing ability of Epoisses, Münster, and Limburger. These smear bacteria are officially known as Brevibacter linens. They can't live in acidic or deoxygenated environments, and so cannot survive in the interior of cheeses. Since they need salty (up to 15%), moist environments to grow, they must be encouraged to do so by continual washing or wiping of the cheese surfaces, a process which results in the development of the characteristic red "smear" surface of so-called "washed-rind" cheeses. B. linens excels at breaking down proteins into, well, stinky odour compounds, producing oniony or garlicky, fishy, and sweaty aromas.

So, next time you pick up a piece of cheese, think of the huge microbial society that's gone to the trouble of producing the masterpiece in front of you.

The role of bacteria in cheese preservation

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 key components of milk for cheese-making are the sugar lactose, fat, and the milk proteins known as caseins. In their native state, milk proteins repulse one another, keeping them suspended in liquid. To create cheese, cheese-makers generate conditions that overcome these forces, leading the proteins to coalesce and the milk to form a semi-solid. This conversion step requires the milk to become more acidic, that is, for its pH to be lowered through the process of fermentation, followed by the coagulation of milk proteins to form a gel.

The acid produced during fermentation helps to form curds, also called a gel, and contributes to syneresis of the curd, removal of water held within the milk proteins.

The acidification of milk is achieved through the addition of a starter culture, a selectively characterised group of bacteria that are intentionally added to the collected milk. Their primary purpose is to convert lactose in milk into lactic acid. In addition, the starter culture contributes directly to flavour development through the production of enzymes and metabolites.

The starter culture changes the cheese microenvironment, affecting a variety of factors, including pH, redox potential, levels of organic acids such as lactate and acetate, and other nutrients. As some of the lactic acid bacteria (LAB) within the starter culture begin to die, the cells release enzymes that further break down milk proteins, mainly casein, to small peptides and amino acids.

The added starter cultures dominate the cheese microbiota, establishing conditions that select for the next microorganisms that will be capable of thriving in the changing cheese matrix. This process is referred to as microbial succession and an example is the development of the characteristic appearance of Swiss cheese. The starter culture used in Swiss cheese-making reduces the pH to a target value that is on the high end of the pH range for rennet coagulated cheeses. The salt level for such cheese is also deliberately kept low. These conditions (high pH and low salt) and adjustments made by the cheese-maker during ripening set up a milieu ideal for the addition of a secondary culture containing the specific bacterial species, Propionibacterium freundenreichii. In this environment, the P. freundenreichii bacteria convert some of the lactic acid produced by the starter culture into propionic acid, which contributes to the unique flavour of Swiss-type cheeses, along with acetic acid, and carbon dioxide gas (CO2). This gas collects at weak spots in the cheese matrix, building up pressure until holes form.

The metabolic activities of the cheese microbiota change conditions (e.g. microbial nutrients and concentration of lactate) within the cheese matrix. These changes allow other microorganisms to thrive and, eventually, to dominate the cheese microbiota. This process is referred to as microbial succession and an example is the development of the characteristic appearance of Swiss cheese.

The distinct aromas of cheese, detected in the nose, are due mainly to a special set of volatile organic compounds, or VOCs, that result from microbial metabolic activities and enzymatic reactions during cheese-making and ripening. For example, the distinctive aroma of Cheddar cheese is due to more than 500 different compounds produced by the microbes that are used in producing this cheese.

Cheese was traditionally ripened in caves or cellars because they provided consistently cool and humid environments suitable for ripening. In modern cheese-making plants, special ripening rooms are designed where temperature and humidity can be even more tightly controlled, leading to greater consistency in product quality. The labor-intensive process of "natural" ripening can include rubbing the cheese periodically with salt, wine, beer, fruit juices, or liqueurs to enhance flavour. Depending on the cheese variety, the ripening process is allowed to continue for extended periods, referred to as the aging process. Young cheeses that are not aged are known as fresh cheeses, some of which are spreadable. Hard cheeses are typically aged, at minimum, for 6 months to one year. The longer a cheese is aged, the sharper its flavour becomes. For example, there are mild, sharp, and extra sharp Cheddars. Mild Cheddar is aged for 2-3 months; sharp Cheddar is aged for approximately 8-12 months; and extra sharp Cheddar is aged anywhere from 18 months up to 5 years.

The overall flavour is produced as a result of the combined metabolic activities of these microbial species.

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