Mesophilic Fermentation: The Science Behind Kajmak and Soured Milk
How Central European home cooks used mesophilic fermentation to preserve dairy — the living science behind kajmak and traditionally soured milk.
Two Products, One Process
Kajmak and soured milk appear to be very different things. One is a rich, layered cream skimmed from heated milk and salted over several days. The other is whole milk left to thicken and acidify. But the biological process driving both is identical: mesophilic fermentation, carried out by lactic acid bacteria that work slowly, quietly, and at the temperatures that a cool Central European kitchen or cellar naturally provided.
Early 20th century home cooks did not use that term. They knew that a spoonful of yesterday’s soured milk would reliably turn today’s fresh milk into the same thing. They knew that kajmak left long enough in the right conditions would develop a sharpness and complexity that young kajmak did not have. They were managing fermentation — they simply described it in practical terms rather than biological ones.
Understanding what is actually happening in both products makes you a better maker of both.
What Mesophilic Fermentation Is
Fermentation, in the dairy context, means that bacteria are converting lactose — the sugar naturally present in milk — into lactic acid. The lactic acid does two things: it lowers the pH of the milk, which inhibits the growth of most spoilage organisms, and it causes the milk proteins to coagulate, which changes the texture from liquid to thick or solid.
Mesophilic means moderate temperature. The bacteria responsible for traditional dairy fermentation — primarily species of Lactococcus and Leuconostoc — are most active between 20°C and 30°C. They work slowly at these temperatures, which is precisely what produces the mild, complex flavour profile of traditionally fermented dairy. Push the temperature higher and you shift to thermophilic fermentation — the process used for commercial yogurt, which works at 40–45°C, produces results in a few hours, and gives a sharper, more uniform tang.
The slow pace of mesophilic fermentation is not a disadvantage. It is what gives kajmak and traditionally soured milk their character.
The Starter: What Maja Actually Does
In traditional Central European dairy practice, a new batch of soured milk was always started with a spoonful of the previous batch. This spoonful is called maja — a word that translates approximately as “leaven” or “starter,” carrying the same implication as the sourdough starter kept alive from one baking to the next.
Maja works because lactic acid bacteria, once established in a fermented product, remain viable for a considerable time. A tablespoon of well-made soured milk contains millions of active bacteria. Added to fresh warm milk, they have a significant numerical advantage over any competing organisms present, and they begin producing lactic acid immediately. Within 12 to 24 hours at room temperature, the fresh milk has been colonized and transformed.
The continuous passing of maja from batch to batch is a form of biological inheritance. Over generations of use in a single household, the microbial community in the starter would have reflected the specific organisms present in that kitchen, that cellar, that milk. This is one reason why traditionally made soured dairy products vary so noticeably between households and regions even when the method appears identical — the starter is not a standardized product, it is a living culture shaped by its environment.
Raw milk contains its own native bacterial population and will sour naturally without any starter. However, it also carries a higher risk of harmful organisms — including Listeria, Salmonella, and pathogenic E. coli — that pasteurization eliminates. Raw milk fermented dairy is not recommended for pregnant women, children under 18, elderly persons, or immunocompromised individuals.
Kajmak: Fermentation Layered Over Time
Kajmak is often described as a cream product, which is accurate but incomplete. Young kajmak, consumed within a few days of making, has had very little time to ferment — its character comes primarily from the fat concentration of the skimmed cream and the salt added to each layer. The mild tanginess present even in young kajmak is early lactic acid fermentation beginning in the cream layers.
Aged kajmak is a genuinely fermented product. Over weeks and months in a cool environment, the lactic acid bacteria present in the cream — from the raw milk, from the salt, from the wooden vessel in traditional practice — continue their work. Proteins break down, acidity increases, moisture drains away through the perforated base of the traditional vessel, and the texture transforms from soft and spreadable to dense and crumbly. The flavour moves from mild and creamy to sharp, complex, and unmistakably fermented.
The butter seal applied to the surface of the finished container serves a dual purpose in this process: it excludes oxygen, preventing the growth of surface molds and aerobic spoilage organisms, while the anaerobic environment beneath allows lactic acid bacteria — which do not require oxygen — to continue working undisturbed. This is not accidental. It is a preservation logic developed over centuries of practical observation.
Traditionally Soured Milk: The Simpler Version
Where kajmak is built up layer by layer over several days, traditionally soured milk is a single-batch process. Milk is heated — often for longer than a simple pasteurization would require, which reduces the water content slightly and concentrates the sugars and proteins — then cooled to room temperature, inoculated with maja, covered, and left undisturbed.
The reduction of water through extended heating has a practical effect: less free moisture means a thicker, more stable product once fermentation is complete. This is an empirical technique that anticipates what dairy technologists would later formalize as water activity management.
The result, after 12 to 24 hours, is a thick, mildly acidic, softly set dairy product — closer in texture and character to crème fraîche or a mild labneh than to commercial yogurt, though it is none of those things exactly. It is its own product, shaped by the specific bacteria in the maja, the fat content of the milk, and the temperature of the room in which it fermented.
Authentic vs. Commercial: What Changes in Production
Commercial versions of both kajmak and soured milk exist and are widely sold. They are not fraudulent products, but they are different ones.
Commercial kajmak is typically produced from standardized cream rather than from whole milk skimmed daily. The fat content is controlled, the salting is uniform, and the fermentation — where it occurs at all — is abbreviated and carried out with specific industrial starter cultures rather than the open microbial community of traditional practice. The result is consistent, mild, and shelf-stable in a way that traditionally made kajmak is not.
Commercial soured milk similarly uses standardized thermophilic or mesophilic starter cultures selected for predictable acidification rates and flavour profiles. The fermentation is completed in a controlled environment in a matter of hours, then the product is chilled to arrest further development. What you buy is a snapshot of a single fermentation moment, not a living product that continues to develop.
Neither is wrong. But they are the product of industrial logic — consistency and shelf life — rather than traditional logic, which valued complexity and the accumulation of time.
The practical consequence for anyone making these products at home is that the commercial version cannot reliably serve as maja for a traditional batch. The bacteria present in a commercial product are often selected strains that may not propagate well at room temperature over multiple generations. For a reliable starter, either use a previous homemade batch or source a traditional mesophilic starter culture from a specialist dairy supplier.
Practical Takeaways
Mesophilic fermentation works at room temperature. You do not need special equipment, a precise thermostat, or commercial starter cultures to make traditionally soured dairy. You need good milk, a clean vessel, and a reliable starter — either from a previous batch or from a trusted source.
The starter is the most critical variable. A spoonful of good maja from a well-made batch will outperform any commercial substitute. Once you have a reliable culture established, maintain it by reserving a spoonful of each batch before consuming it.
Commercial products cannot substitute for traditional maja without risk of inconsistent results. The bacteria in commercial soured dairy are often selected for industrial conditions, not for room-temperature home fermentation.
Temperature matters, but not precisely. Anywhere between 20°C and 28°C is suitable for mesophilic fermentation. A warm kitchen corner in summer, a cool room in autumn — both work. Avoid placing the vessel in direct sunlight or near a heat source, which can push temperatures into the thermophilic range and alter the result.
Time is the ingredient that cannot be substituted. The complexity of traditionally fermented dairy comes from slow biological activity over hours, days, or months. Speeding the process shortens that complexity.
Attic Recipes — digitizing and adapting Central European home cooking from the early twentieth century.
Frequently Asked Questions
01What is mesophilic fermentation in dairy?▶
Mesophilic fermentation is the process by which lactic acid bacteria convert milk sugars into lactic acid at moderate temperatures — typically between 20–30°C. It is the mechanism behind kajmak, traditionally soured milk, crème fraîche, and many aged cheeses. No heat is required; the bacteria work slowly at room or cellar temperature.
02What is the difference between traditionally soured milk and commercial yogurt?▶
Commercial yogurt is made with thermophilic bacteria that work at high temperatures (40–45°C), producing a tangy, firm result quickly. Traditionally soured milk uses mesophilic bacteria that work at room temperature over many hours, producing a milder, more complex flavour. The texture is softer and the acidity more gentle.
03What is maja and how does it work as a starter?▶
Maja is a small quantity of previously soured milk used to inoculate a fresh batch — the same principle as a sourdough starter. It introduces a population of active lactic acid bacteria into the new milk, giving them a head start over competing organisms. A tablespoon of good maja from a previous batch is sufficient for a litre of milk.
04Can I make soured dairy products with pasteurized milk?▶
Yes. Pasteurized milk has had harmful bacteria killed by heat, but it can still be fermented by adding a starter culture — either commercial or a spoonful of traditionally soured milk. Raw milk carries its own native bacteria and will sour naturally without a starter, but this also carries higher food safety risk.
05Why does traditionally made kajmak taste different from commercial versions?▶
Commercial kajmak is produced under standardized conditions designed for consistency and shelf stability — the fermentation is controlled and often abbreviated. Traditionally made kajmak develops its character through slow, uncontrolled mesophilic fermentation over weeks or months, during which a more complex community of bacteria produces a wider range of flavour compounds. The difference is similar to the one between industrial bread and a long-fermented sourdough.