Kefir: Microbiological Characteristics and Commercial Production

Introduction

Kefir is a fermented dairy beverage with a rich microbiological profile and a long history of traditional production. Unlike simpler fermented milks (e.g. yogurt), kefir is produced by a complex symbiotic community of microorganisms that yield a tart, lightly effervescent drink containing a mix of organic acids, mild alcohol, and carbon dioxide (Kefir - Wikipedia) (Kefir as a Functional Beverage Gaining Momentum towards Its Health Promoting Attributes). This report provides an in-depth look at commercial dairy kefir, focusing on its microbiology and production process. Key topics include the definition of kefir from a microbiological standpoint, the nature of kefir grains and their microbial communities, authoritative standards defining kefir, how kefir grains differ from single-strain cultures, methods of commercial kefir fermentation (with or without grains), the stage at which fermentation occurs in production, the role of pasteurization and how products maintain “live cultures,” and labeling standards for terms like kefir and live cultures. Each section draws on authoritative microbiology and food science sources to present a clear explanation.

1. Microbiological Definition of Kefir

Kefir is an acidic, mildly alcoholic fermented milk drink produced through the action of a mixed microbial culture (bacteria and yeast) on milk lactose. Microbiologically, authentic kefir is defined by a symbiotic fermenting community that includes lactic acid bacteria (LAB) (such as Lactobacillus, Lactococcus, Leuconostoc species), acetic acid bacteria, and yeasts () ( Milk kefir: composition, microbial cultures, biological activities, and related products - PMC ). These microbes act together to ferment lactose into lactic acid (souring the milk) and produce ethanol and CO₂ (via yeast fermentation), as well as small amounts of acetic acid (from oxidation of ethanol by acetic bacteria) (Kefir - Wikipedia) (Kefir as a Functional Beverage Gaining Momentum towards Its Health Promoting Attributes). The result is a tart, effervescent beverage with a consistency similar to drinkable yogurt. Typical kefir has a pH of ~4.5 or lower and contains a low level of alcohol (generally 0.1–0.5% v/v) along with dissolved CO₂ that gives a light “fizz” (Kefir - Wikipedia).

Microbiologically, what distinguishes kefir is the diversity and synergy of its culture. In the traditional process, kefir is fermented by kefir grains – these are gelatinous biological particles harboring the robust community of microbes (detailed in the next section). This complex starter leads to a high load of viable organisms in the final product: kefir typically contains on the order of 10^7–10^8 CFU/mL of LAB and 10^4–10^6 CFU/mL of yeasts by the end of fermentation ( Global Regulatory Frameworks for Fermented Foods: A Review - PMC ). The Codex Alimentarius (an international food standards body) defines kefir as a fermented milk produced with a “starter culture prepared from kefir grains” that includes Lactobacillus kefiri as well as bacteria of genera Leuconostoc, Lactococcus, and Acetobacter, in symbiotic association with yeasts (both lactose-fermenting, e.g. Kluyveromyces marxianus, and non-lactose-fermenting, e.g. Saccharomyces unisporus, S. cerevisiae, S. exiguus) (). In other words, from a microbiological perspective, kefir is defined by the presence of multiple genera of microbes working in concert. These microorganisms must remain viable and active in the final product up to the point of consumption (unless the product is heat-treated post-fermentation) (). Together, they produce the lactic acid (for tartness), polysaccharides, and flavor compounds that characterize kefir, as well as potential probiotic benefits due to the live cultures.

(Kefir as a Functional Beverage Gaining Momentum towards Its Health Promoting Attributes) Kefir fermentation involves multiple microbial pathways: lactic acid bacteria ferment lactose to lactic acid (souring the milk), yeasts carry out alcoholic fermentation producing ethanol and CO₂ (creating slight alcohol content and carbonation), and acetic acid bacteria oxidize ethanol to acetic acid. This symbiotic fermentation results in kefir’s tart flavor, mild effervescence, and preservation of the milk (Kefir - Wikipedia) ( Microbiological, technological and therapeutic properties of kefir: a natural probiotic beverage - PMC ).

2. Kefir Grains and Their Microbial Communities

Kefir grains are the authentic starter culture for traditional kefir fermentation. They are gelatinous, irregular granules (whitish to yellow in color, 0.3–3 cm in size) that resemble small pieces of cauliflower ( Microbiological, technological and therapeutic properties of kefir: a natural probiotic beverage - PMC ) (see figure below). Microbiologically, a kefir grain is a natural biofilm matrix densely packed with a symbiotic consortium of bacteria and yeasts. The grain’s structure is composed of proteins from milk (casein) and a complex exopolysaccharide called kefiran, which is produced by certain bacteria within the grain ( Milk kefir: composition, microbial cultures, biological activities, and related products - PMC ). Kefiran is a heteropolysaccharide made of roughly equal glucose and galactose, and it serves as the “glue” that holds the community together in a three-dimensional matrix ( Milk kefir: composition, microbial cultures, biological activities, and related products - PMC ). Lactobacillus kefiranofaciens (a prominent grain bacterium) is known to synthesize kefiran, contributing to the grain’s growth and texture ( Milk kefir: composition, microbial cultures, biological activities, and related products - PMC ). Within this polysaccharide-protein scaffold, numerous microbial species coexist in a stable symbiotic association, each contributing to the fermentation and to the integrity of the grain ( Milk kefir: composition, microbial cultures, biological activities, and related products - PMC ) ( Microbiological, technological and therapeutic properties of kefir: a natural probiotic beverage - PMC ).

( Microbiological, technological and therapeutic properties of kefir: a natural probiotic beverage - PMC ) A typical kefir grain (cauliflower-like in appearance). This grain is a concentrated mass of proteins and the polysaccharide kefiran, inhabited by billions of microorganisms (lactic acid bacteria, yeasts, and some acetic acid bacteria). The ruler scale (cm) shows the grain’s size ( Microbiological, technological and therapeutic properties of kefir: a natural probiotic beverage - PMC ).

Microbial composition of kefir grains: Kefir grains harbor a rich community of lactic acid bacteria (LAB) along with yeasts and minor proportions of acetic acid bacteria (AAB). Some of the key bacterial and yeast species commonly found in kefir grains and their roles include:

Bacterial species in kefir grains:

Lactobacillus kefiranofaciens - Dominant in grains; produces kefiran polysaccharide for grain matrix; ferments lactose producing lactic acid; contributes to viscosity and mouthfeel.
Lactobacillus kefiri - Predominant Lactobacillus in the final kefir drink; ferments lactose to lactic acid and mild amounts of ethanol/CO₂; contributes to flavor and acidity.
Lactobacillus paracasei, L. plantarum - Present in grains in smaller numbers; rapidly produce lactic acid from lactose; some may contribute to flavor.
Lactococcus lactis - Ferments lactose to lactic acid; aroma production (diacetyl, giving buttery notes) by certain strains; helps coagulate milk proteins.
Leuconostoc mesenteroides - Produces lactic acid, CO₂, and flavor compounds from lactose and citrate; contributes to mild carbonation and aroma.
Acetobacter spp. - Oxidize ethanol to acetic acid; add to the acidic flavor complexity; help regulate yeast growth by consuming ethanol.

Yeasts in kefir grains:

Kluyveromyces marxianus - Lactose-fermenting yeast; produces ethanol and CO₂, contributing to kefir’s slight alcohol content and natural carbonation; produces aromatic compounds.
Saccharomyces cerevisiae - Non-lactose-fermenting yeast; ferments other sugars to ethanol and CO₂; contributes to yeasty aroma and carbonation.
Saccharomyces unisporus - Common in traditional kefir grains; produces ethanol/CO₂ from residual sugars.
Kazachstania spp. - Present in some kefir cultures; contributes to fermentation of sugars and flavor complexity.

Overall, kefir grains represent a self-sustaining probiotic ecosystem. The bacteria and yeasts adhere to the grain’s polysaccharide surface, forming a stable community that can be reused indefinitely to ferment fresh milk. This community is so robust and interdependent that many isolates from kefir grains do not grow as well alone – when separated, they may lose some functionality or vigor in milk. The grain’s unique microbiology is what gives kefir its broad spectrum of metabolites (lactic acid, ethanol, CO₂, diacetyl, acetaldehyde, etc.) and its reputation for health-promoting properties.

3. Defining Kefir: Authorities and Standards

Given kefir’s traditional origins and complex microbiology, food authorities and standard-setting bodies have established definitions to distinguish authentic kefir and its culture. The most widely cited authority is the Codex Alimentarius Commission, which sets international food standards. In the Codex Standard for Fermented Milks (CXS 243-2003), kefir is defined by its starter culture and microbial composition: “Kefir: Starter culture prepared from kefir grains, Lactobacillus kefiri, species of the genera Leuconostoc, Lactococcus and Acetobacter growing in a strong specific relationship. Kefir grains constitute both lactose fermenting yeasts (Kluyveromyces marxianus) and non-lactose-fermenting yeasts (Saccharomyces unisporus, S. cerevisiae, S. exiguus).” (). In simpler terms, Codex requires that something sold as “kefir” be made with a culture originating from kefir grains and containing that mix of LAB and yeasts in symbiosis. The Codex standard also specifies that yeasts should be present in significant amount – traditionally, a minimum of 10^4 colony-forming units per gram (CFU/g) of yeast in the final product, and a total of 10^7 CFU/g of viable bacteria from the starter culture ( Global Regulatory Frameworks for Fermented Foods: A Review - PMC ). These criteria reflect the microbial richness expected of kefir, distinguishing it from simpler fermented milk like yogurt (which by Codex is defined only by two bacteria and no yeast).

In addition to Codex, national and regional food standards often define kefir in their jurisdictions. For example, some countries of Eastern Europe – where kefir has a long history – have traditional standards of identity. The Russian standard (formerly GOST) and others in that region require that kefir be produced with kefir grain fermentation and set allowable ranges for acidity and ethanol.

In the United States, kefir does not yet have a federal Standard of Identity (unlike yogurt, which does). It is generally sold under the FDA’s classification of “cultured milk” products. However, industry leaders are advocating for formal recognition. The CEO of Lifeway Foods (a major US kefir producer) noted in 2018 that the company has worked with the FDA to begin “clarifying the proper use of dairy terms, including kefir,” and is pushing for “formalizing a separate standard of identity for kefir.” One concern has been misuse of the term “kefir” on non-dairy or non-authentic products. For instance, the FDA has flagged the use of “kefir” on plant-based products as misleading, since kefir is a dairy term with an established meaning involving fermentation of milk. In absence of a formal U.S. standard, most reputable manufacturers adhere to the international definitions and emphasize that their kefir is made with live cultures from kefir grains.

In summary, the authorities defining kefir – from Codex Alimentarius to national food standards and industry guidelines – concur that true kefir is a cultured milk produced with a symbiotic LAB/yeast fermentation derived from kefir grains. The presence of both bacteria and yeasts (in specific combinations and viability levels) is the hallmark that any product called “kefir” should have. These definitions ensure that consumers get an authentic product with the live microbial communities and resulting metabolites that kefir is known for.

4. Kefir Grains vs. Single-Strain Cultures (Yogurt Starters and Others)

Kefir grains represent a natural, multi-species consortium, whereas many other fermented dairy products use only one or a few isolated strains of microbes. This fundamental difference has several implications:

Complexity of Microbes: Kefir grains contain dozens of microbial species simultaneously (as detailed above), including multiple LAB (both homofermentative and heterofermentative types) and yeasts. In contrast, a typical yogurt uses just two bacterial strains in symbiosis (Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus) (), and no yeast. Similarly, other fermented milks or probiotic drinks might add a single species (e.g. Lactobacillus acidophilus milk or a bifidobacteria-fortified yogurt). These single or dual cultures perform a much more limited fermentation (mainly lactic acid production) compared to the multi-pathway fermentation in kefir (which produces lactic acid, ethanol, CO₂, and various flavors concurrently). The metabolic interplay in kefir grains (LAB creating conditions for yeasts, yeasts producing substrates for AAB, etc.) leads to a broader range of end-products and a different flavor and texture profile that one or two strains alone cannot replicate (Kefir as a Functional Beverage Gaining Momentum towards Its Health Promoting Attributes) ( Microbiological, technological and therapeutic properties of kefir: a natural probiotic beverage - PMC ).
Symbiotic Dependency: Organisms in kefir grains often depend on each other for optimal growth. Studies have shown that when individual bacteria are isolated from kefir grains and grown alone, they may not thrive or ferment milk effectively on their own ( Microbiological, technological and therapeutic properties of kefir: a natural probiotic beverage - PMC ). For example, certain Lactobacillus or Acetobacter species from kefir might grow weakly in pure culture but do very well in the grain, likely because the grain community provides essential growth factors or removes inhibitory substances. The grain functions as a mini-ecosystem: yeasts supply vitamins and amino acids that lactobacilli need, lactococci reduce oxygen and produce acid that favors lactobacilli and yeasts, etc. ( Microbiological, technological and therapeutic properties of kefir: a natural probiotic beverage - PMC ) (Kefir as a Functional Beverage Gaining Momentum towards Its Health Promoting Attributes). In a single-strain starter (like a pure Lactobacillus culture), these synergistic interactions are absent, and so the fermentation may not be as robust or may yield a different product. This is why simply mixing a few separate strains in milk does not easily recreate a “kefir grain” – the grain’s structure and stability emerge from prolonged co-evolution of its microbes.
Formation of a Matrix (SCOBY): Kefir grains are essentially a form of SCOBY (Symbiotic Culture of Bacteria and Yeast), analogous to the mother of vinegar or kombucha scoby, but in a dairy environment. The microbes in the grain actively produce a polysaccharide-protein matrix (kefiran) that binds them together ( Milk kefir: composition, microbial cultures, biological activities, and related products - PMC ). Individual strains like a pure Lactobacillus do not form such macroscopic structures in milk. Yogurt bacteria, for instance, can coagulate milk protein but do not produce a resilient polysaccharide that they all live in; yogurt is typically a dispersed fermentation (unless one adds gums or uses strains that produce exopolysaccharides for texture, but still nothing like a grain). The kefir grain’s existence means it can be filtered out and reused, something not possible with typical single-strain cultures that are homogenously mixed in the product.
Diversity of Bioactive Compounds: Because of its mixed culture, kefir produces not only lactic acid but also other compounds like ethanol, carbon dioxide, acetate, diacetyl, acetaldehyde, and peptides that contribute to flavor and potential health effects. A single-strain culture (e.g. Lactobacillus acidophilus added to milk) will primarily produce lactic acid and maybe some mild flavor compounds, but it won’t produce alcohol or CO₂. The presence of yeast in kefir grains is what leads to its slight carbonation and alcohol – a unique feature among dairy drinks. Other fermented milks (yogurt, cultured buttermilk) have essentially zero alcohol and no effervescence because they use no yeast. Similarly, kefir grains produce the polymer kefiran, which has been studied for health benefits (e.g. antimicrobial, anti-inflammatory properties); a single bacterial strain would not produce this spectrum of metabolites.
Consistency vs. Authenticity: Single-strain or defined limited-strain cultures are easier to standardize for industrial production – they give more uniform results batch to batch. Kefir grains, being living communities, can have natural variation and require maintenance. In commercial practice, some products marketed as “kefir” actually use defined mixtures of a few known strains (isolated from kefir) to simplify production. These can ferment the milk and produce a kefir-like beverage, but they may lack some of the less common microbes found in true grains (for example, a powder culture might include two lactococci, a couple of lactobacilli, and one yeast). Such products may not have the full diversity or may have a milder yeast presence compared to grain-fermented kefir (Kefir as a Functional Beverage Gaining Momentum towards Its Health Promoting Attributes) (Kefir as a Functional Beverage Gaining Momentum towards Its Health Promoting Attributes). Traditionalists often argue that kefir made from real grains has unique qualities that a pure-strain culture can’t entirely duplicate.

In summary, kefir grains vs individual strains is the difference between a natural consortium and an isolated monoculture. Kefir grains provide a template where multiple species co-ferment and support each other, resulting in a complex probiotic food. Individual strains (like those used in yogurt or probiotic supplements) can be highly useful for specific purposes, but they operate in isolation. They do not create the same broad metabolic environment or stable biofilm as kefir grains. In fact, the success of kefir as a fermentation is a classic example of how microbial synergy can produce a result (the grain and the fermented kefir) that no single organism could achieve alone.

5. Commercial Kefir Production Methods (Grains vs. Direct Cultures)

In modern dairy manufacturing, kefir is produced using one of two general methods: traditional grain fermentation (often called the “Russian method”) or cultures of isolated strains (pure culture method).

Traditional (Grain) Method: This method uses actual kefir grains as the starter, much like in home-style preparation but adapted to larger scale. In industrial practice, a series of fermentations is often employed. A small amount of grains may be used to ferment milk in a “starter tank,” creating a highly active fermented liquid (sometimes called stock kefir). The grains are then removed (via filtration), and the resulting cultured liquid – rich in kefir microbes – is used to inoculate a larger batch of milk (this is sometimes termed a “percolate” or mother culture approach). This stepwise propagation can be done in multiple stages to build up volume. The final large batch ferments without grains present (only the microorganisms carried over), but because the culture originated from grains, it contains the full spectrum of organisms. This Russian-style method was historically used in Eastern Europe/Russia for factory production: they would maintain large quantities of grains in fermentation jars, then use the fermented milk (after grain removal) to seed production vats of kefir. The grains themselves can be rinsed and reused indefinitely. This approach ensures authentic culture makeup, but it is labor-intensive – grains need to be handled and kept healthy. Controlling consistency can be challenging, as grains are living and may change activity over time.
Pure Culture (Direct Inoculation) Method: Many commercial producers today use freeze-dried or concentrated cultures that contain known strains isolated from kefir grains. These are often referred to as DVI/DVS (Direct Vat Inoculation/Set) cultures. A culture pack might contain a mixture such as: Lactobacillus kefiri, Lactococcus lactis, Leuconostoc mesenteroides (or related heterofermentative LAB), and one or more yeasts like Kluyveromyces marxianus and Saccharomyces (plus sometimes Acetobacter). The dairy manufacturer can add this culture directly to pasteurized milk in the production tank, without having to maintain grains. The microbes grow and ferment the milk to produce kefir. Using defined cultures gives more control and uniformity – the strain ratios are known and can be replicated each batch, and there are no physical grains to filter out later. However, pure cultures might not include every minor organism found in traditional grains (for example, they might leave out some species that are hard to grow or stabilize industrially). Some companies also add probiotic adjuncts that were not originally in kefir grains, to boost health claims – for instance, adding Bifidobacterium strains or Lactobacillus acidophilus, or even a probiotic yeast like Saccharomyces boulardii, alongside the core kefir culture. These adjuncts are intended to increase the probiotic cell count or functional benefits, though they are not part of the classic kefir consortium.

In practice, large dairy companies often prefer the direct-culture method for convenience. Firms such as Chr. Hansen or Danisco supply “kefir culture” packets to dairies, which when added to milk will produce a kefir-like product. Nonetheless, some smaller or specialty dairies continue to use real kefir grains in production to differentiate their product (they may advertise “authentic grain-fermented kefir”). Both methods are valid, and regulatory definitions (like Codex) do not insist that physical grains be present in every batch – using isolates from grains is acceptable as long as the characteristic mix of microbes is achieved.

From a fermentation perspective, both approaches should yield kefir that contains LAB and yeast and has the proper taste and microbial counts. However, differences can occur. Research comparing grain-fermented vs. commercially cultured kefir finds that grain-fermented kefir often has higher yeast counts and sometimes more species diversity (especially among yeasts) than kefir made from a limited strain cocktail. Commercial cultures might be optimized to produce a consistent flavor (not too yeasty) and therefore could have fewer yeast strains.

To summarize, commercial kefir fermentation is achieved either by (a) using actual kefir grains in a controlled propagation process, or (b) inoculating milk with a selected blend of kefir microbes. The first method is the direct continuation of the ancient practice, while the second is a modern adaptation for scalability.

6. Fermentation Process in Commercial Production

The fermentation of kefir is a distinct step in the production process, occurring after milk preparation (standardization and pasteurization) and before packaging. In commercial dairy plants, the process flow for kefir typically looks like this:

Milk Preparation: Raw milk (cow, goat, etc.) is standardized to the desired fat and solids content for the kefir (e.g. adjust fat to 1-3% for low-fat or whole milk kefir). The milk is then pasteurized (commonly at high temperature for a short time, e.g. ~90 °C for 2–5 minutes). Pasteurization is done prior to fermentation to eliminate pathogens and reduce background microbes, ensuring that the kefir culture will dominate and that the final product is safe. Some producers also homogenize the milk before pasteurization to improve texture (homogenization breaks fat into small globules which can give a smoother, stable kefir).
Cooling and Inoculation: After pasteurization, the milk is cooled to the fermentation temperature, typically around 18–25 °C for kefir. (This temperature is cooler than yogurt fermentation, which is ~40–45 °C; kefir’s mixed culture, especially yeasts, perform better at moderate temperatures.) Once the milk reaches the target temperature, the kefir culture is added. In grain-based systems, this might mean adding kefir grains (usually at about 2–10% of the volume; a common inoculum is ~5% w/v of grains). In direct culture systems, a measured dose of liquid or powdered starter is stirred into the milk. Good mixing is important to distribute the microbes. The inoculated milk is then kept in a fermentation vessel (tank or vat).
Fermentation (Incubation): The inoculated milk is allowed to ferment at the set temperature for a specified time. Commercial kefir fermentation typically lasts on the order of 14–24 hours depending on the culture activity and desired flavor profile. For example, Lifeway (a commercial producer) notes that their kefir is fermented for about 14–18 hours after inoculation. During this period, the bacteria and yeasts multiply and convert lactose and other nutrients: lactic acid bacteria acidify the milk (pH drops to ~4.2–4.6 by end of fermentation), causing proteins to coagulate (the milk thickens slightly, forming a soft curd), and yeasts produce ethanol and CO₂ (creating tiny bubbles and a mild yeast aroma). If grains are used, they usually float or can be stirred gently to contact fresh milk; agitation is generally minimal or gentle, to allow CO₂ to develop and not shear the grains (some processes ferment without agitation, others use intermittent stirring – agitation can favor certain microbes over others). The end-point of fermentation is determined by pH and taste – once titratable acidity around 0.6–0.8% lactic acid is reached (pH in mid-4s), and a distinct kefir aroma is present, the fermentation is stopped.
Removal of Grains (if applicable): In processes using actual kefir grains, the grains must be separated from the fermented liquid. This is usually done by straining or filtration. Large stainless steel strainers or mesh filters catch the grains as the kefir is pumped out of the fermentation tank. The grains are then collected, washed if necessary, and kept for re-use in the next batch (often they are rinsed with cool sterile water or milk and stored in a nutrient medium at cold temp until reuse). In pure culture methods, this step isn’t needed since no physical grains are present – the microbes remain dissolved/suspended in the kefir.
Cooling and Maturation: Once fermentation has reached the target acidity, the kefir is rapidly cooled to refrigeration temperatures (around 4 °C) to slow down microbial activity and stop further acidification. At this stage, the kefir is a mildly thick, drinkable yogurt-like liquid. Some manufacturers allow a short “maturation” period at low temperature to let flavors develop (e.g. a day or two at 4 °C, during which a small amount of additional fermentation by surviving yeasts can occur, enhancing fizz and flavor). However, most often the product is cooled and immediately prepared for packaging.
Packaging: The finished kefir is filled into bottles or cartons under sanitary conditions. Because kefir is intended to contain live cultures, it is not pasteurized after fermentation. The filling operation is usually done cold, and the product is kept refrigerated throughout the supply chain to maintain the live bacteria/yeasts and to prevent over-fermentation or spoilage. Modern bottling lines may flush bottles with CO₂ or nitrogen to reduce oxygen (as excess oxygen could stimulate acetic acid bacteria or mold), but generally normal hygienic filling is sufficient since the low pH (~4.5) of kefir already inhibits most contaminants.
Storage and Distribution: Kefir is stored and shipped at 4 °C. The shelf life is typically a few weeks (up to ~4 weeks) under refrigeration. Over time, the cultures in the kefir may continue to ferment slowly even at fridge temperature, so older kefir might become more sour or carbonated (some gas buildup can occur in sealed bottles if stored for long, due to slow yeast activity – this is normal, and bottles are designed to handle a bit of pressure). Manufacturers test the product during shelf life to ensure the yeast and bacteria counts remain high and undesirable microbes (molds, etc.) do not grow.

It’s important to note that fermentation is completed before packaging for commercial kefir. Unlike certain bottle-conditioned beverages (e.g. some beers or kombucha which ferment in the bottle to carbonate), kefir is usually fermented in bulk and then bottled cold. This is because ongoing fermentation in a closed bottle could lead to too much CO₂ or alcohol beyond desired levels. That said, kefir is often sold with a slight natural carbonation, which is essentially the CO₂ produced during fermentation that remains dissolved in the liquid when chilled. Some brands advertise a “light fizz” – evidence that the product still has live yeast at work (Lifeway’s FAQ notes that the probiotic activity produces kefir’s signature fizz).

In summary, the fermentation step in kefir production is conducted in a controlled tank environment after milk pasteurization and cooling, and it ends when the desired acidity and microbial growth are reached, prior to final cooling and packaging. This ensures consistency and safety. The timing of fermentation is a critical control point: too short and the kefir may be overly mild with low cell counts; too long and it could become overly sour or high in alcohol. Manufacturers find the sweet spot (often around 18–20 hours at ~20 °C, depending on inoculum) that yields a balance of flavor and a high viable cell count (commonly, kefir will contain >10^7 CFU/mL of lactic acid bacteria and ~10^5 CFU/mL yeast at time of packaging).

7. Role of Pasteurization and Live Cultures in Kefir Production

Pasteurization plays a crucial role in kefir production, but its timing is key: the milk is pasteurized before fermentation, not after. The reasons and effects are as follows:

Pre-Fermentation Pasteurization: All commercial dairy kefir begins with pasteurized milk (unless it’s an artisanal raw milk kefir, which is rare and not sold widely due to safety concerns). Heating the milk (commonly to 85–95 °C for a few minutes) serves multiple purposes: it kills pathogenic bacteria and wild spoilage microbes, making the substrate safe and essentially sterile for the starter culture. It also denatures milk proteins slightly, which improves the texture of fermented products (denatured whey proteins can hold water and contribute to a creamy consistency). Pasteurization ensures that the kefir culture (whether grains or added starter) won’t have to compete with unwanted bacteria in the milk, and that the resulting product’s microbial content is exclusively the intended bacteria and yeast. As one industry source puts it, pasteurization of milk for kefir is “an incontrovertible microbial safeguard” that is now standard in industrial kefir making. Traditional methods in the Caucasus sometimes fermented raw milk, but from a commercial perspective, raw milk fermentation is generally avoided for safety and consistency.
No Post-Fermentation Pasteurization: Kefir is distinct from some fermented foods in that it is typically not heat-treated after fermentation. The goal is to deliver a drink with live cultures to the consumer. If the kefir were pasteurized after fermenting, it would kill the very bacteria and yeasts that define the product (as well as halt any further flavor development). Such a product would still be fermented milk (with the acid and alcohol produced), but it would no longer have living probiotics. Most regulations recognize this; for example, Codex standards note that if a fermented milk is heat-treated after fermentation, the requirement for viable microorganisms no longer applies. In other words, you can call it “fermented milk” but not claim it has active cultures if you pasteurize it at the end.

In the case of yogurt, there are products on the market that are heat-treated for shelf stability (they must then label “heat treated after culturing” and cannot advertise live cultures). For kefir, this practice is uncommon because kefir’s identity is so tied to probiotics. Commercial kefir is almost always sold as a live culture product, kept chilled, rather than a shelf-stable sterilized drink. Pasteurizing kefir after fermentation would also alter its taste (removing the fresh yeasty notes) and texture. Thus, producers avoid it. Lifeway Foods emphasizes that they pasteurize the milk before fermentation so that “our kefir cultures are alive and active when consumed,” implicitly affirming that there is no second pasteurization. The live microbes also contribute to kefir’s preservation, producing antimicrobial compounds that help prevent spoilage.
If Kefir Were Pasteurized After Fermentation: In scenarios where a “kefir drink” is made shelf-stable, the manufacturer would have to either forego claiming live cultures or would need to reintroduce some cultures afterwards. For example, a company could theoretically ferment milk with kefir culture, then heat-treat it to stop fermentation (killing the original culture), and then add a specific probiotic strain back into the bottled product to claim some live bacteria. However, this is not standard for dairy kefir. It would essentially no longer be traditional kefir, just a pasteurized fermented milk with added bacteria. There are some kefir-based products (like certain flavored smoothies or extended-life drinks) that undergo additional processing. If they are pasteurized or aseptically packaged, they typically do not claim “live and active cultures” on the label, or they use terms like “cultured milk product” instead of kefir. Both FDA and international guidelines would consider it misleading to claim probiotic benefits if the cultures have been destroyed. In yogurt regulations, for instance, the FDA now requires that if yogurt is heat-treated after culturing, the label must state “does not contain live and active cultures.” While there isn’t a specific kefir regulation, the same principle of truth-in-labeling applies.
Pasteurization and Culture Viability: By pasteurizing the milk first and not pasteurizing later, manufacturers ensure that kefir can be sold with high counts of live microorganisms. It is not uncommon for a cup (8 oz ~ 240 mL) of commercial kefir to contain tens of billions of CFUs of bacteria when fresh. These numbers will gradually drop over shelf life, but products are formulated to still have a beneficial quantity by the expiration date. The combination of low pH and refrigeration keeps the culture mostly dormant after packaging, so they survive longer. Because kefir contains yeast, some activity can continue (hence the slight increase in fizz over time), but typically the acidity self-limits further growth.

In summary, pasteurization is used to prepare the milk for fermentation, but not applied to the finished kefir, in order to maintain the integrity of the live culture. This approach allows kefir brands to legitimately market their products as containing live probiotic cultures. If you encounter a product labeled as kefir that is shelf-stable at room temperature, it has likely been pasteurized post-fermentation or ultra-heat-treated, meaning it no longer has live cultures (and should indicate as such on the label, or else simply not make probiotic claims). The vast majority of dairy kefir in the market, however, is sold as a refrigerated, living product. This is a key difference from some other fermented foods – for example, sauerkraut or kimchi might be pasteurized and canned (losing live microbes), but kefir is almost always kept perishable to preserve its microbial benefits.

8. Labeling Standards and Claims (“Kefir”, “Live Cultures”, “Kefir Grains”)

Labeling of kefir and its culture content can sometimes be confusing. Here we address how dairy products use terms like “kefir,” “live cultures,” and “kefir grains” on labels, and what standards or guidelines govern these terms:

Use of the Term “Kefir”: The word kefir on a dairy product implies a specific kind of fermented milk. Although not every country has a published legal definition, there is a general industry and international understanding (as outlined by Codex and others) of what kefir is: it should be a milk fermented with a kefir-specific culture (including LAB and yeasts). In jurisdictions that have standards of identity or food codes, using the name “kefir” typically obligates the manufacturer to meet those criteria. In the European Union, there isn’t an EU-wide regulation solely for kefir, but many EU producers adhere to Codex or national guidelines. Misusing “kefir” (e.g. calling a product kefir when it only has yogurt cultures, or using it on non-dairy ferments without clarification) could be seen as misbranding. The FDA in the U.S. has indicated concern over plant-based beverages using the term (since consumers might expect kefir to be a dairy probiotic drink). In practice, legitimate dairy kefir products label themselves clearly as “kefir” and often mention they are cultured milk. If a product deviates (say a “water kefir” made from sugary water and kefir-like grains), it usually specifies the base (since “kefir” alone default implies milk). So, while no single global law exists, the term “kefir” generally signals a fermented dairy product with the characteristic kefir culture. Producers using the name are expected to include the requisite mix of microbes (including yeast), which differentiates kefir from simpler “cultured milks.”
“Live and Active Cultures” / Probiotic Claims: Many kefir brands highlight that their product contains live cultures. This phrase is borrowed from the yogurt industry’s “Live and Active Cultures” labeling. There isn’t a legally mandated threshold in most places (except that if they are killed by heat, you shouldn’t call them live). In the U.S., the National Yogurt Association had guidelines for yogurt to use their “Live & Active” seal (e.g. >10^8 CFU/mL at manufacture), but for kefir there is no specific seal program. Nonetheless, kefir usually far exceeds those counts. For labeling, a kefir maker may list the specific strains or species in the ingredients (e.g. “Contains live and active cultures: Lactobacillus kefiri, Lactococcus lactis, Leuconostoc mesenteroides, Saccharomyces…, etc.”). This gives consumers information on what probiotics they’re ingesting. However, as research has found, labels are not always perfectly accurate in terms of microbial content. A 2021 study examining commercial kefir brands in the US found that some products overstated the number of bacteria (CFUs) or listed species that were not detected, and conversely found some unlisted species present. For example, all five tested products contained Streptococcus thermophilus (common in yogurt) even though it wasn’t on the label, and most had Lactobacillus paracasei which also wasn’t listed. This suggests that while companies strive to list their main intended cultures, the ecosystem can be more complicated (or perhaps some yogurt bacteria got in if the facility also makes yogurt). The key point is that “live cultures” means the product has not been post-pasteurized and still has living microbes. There is no strict rule on how many or which ones must be alive (other than general expectations of high counts for a probiotic product). If a kefir is heat-treated, it should not claim live cultures. Regulatory agencies can take action if a product claims to have live probiotics but in reality is sterile – that would be considered a false claim. As quality control, most reputable brands test their CFUs and ensure a certain minimum until the “best by” date (for instance, a brand might guarantee at least 10^7 CFU/mL of combined cultures at expiry). In summary, “live cultures” on a kefir label is essentially a promise that the kefir was not pasteurized after fermentation and contains viable bacteria/yeast. Consumers often look for this phrasing to ensure they are getting probiotic benefits.
Mention of “Kefir Grains” on Labels: Some dairy products advertise that they are “made from real kefir grains” or “cultured using authentic kefir grains.” This is more of a marketing distinction than a regulated term. It is meant to signal that the traditional method or culture source was used, as opposed to just a lab-made culture. For example, a small artisan dairy might print “fermented with live kefir grains” to set their product apart. There are no specific labeling laws about using the word “grains” – but of course, it should be truthful (the company should indeed be using grains at some stage). Most large commercial brands do not use grains in production (they use pure cultures), but their cultures originated from grains, so some may use phrasing like “cultured with 12 live kefir cultures” instead of explicitly saying grains. If a label says “kefir grains,” it usually indicates a more traditional process. There is a bit of consumer perception at play: some kefir enthusiasts believe grain-fermented kefir is superior, so a brand might want to highlight grain use. However, because grains are not an “ingredient” that remains in the product (they’re removed), they might not be listed in the ingredient list. Instead, the label might describe the process on the side panel. No official standard dictates when you can or cannot mention kefir grains, beyond general truth-in-advertising. One thing to note: the term “kefir grain” only refers to the culture granules, never to cereal grains; so there’s little risk of misunderstanding except for those unfamiliar with kefir who might not know what grains are. Some regulatory bodies might object if the term confuses consumers, but generally in context it’s clear.
Labeling of Strains and Quantity: While not strictly required, many kefir producers list the microorganisms (at least generically). For example, Lifeway Kefir’s packaging and website mention it contains “12 Live and Active Probiotic Cultures” and names them (such as Lactobacillus kefiri, L. rhamnosus, L. casei, Leuconostoc cremoris, etc., and several yeasts like S. florentinus which is another name for S. unisporus) – these were likely part of their culture formula. They also often claim a ballpark figure for CFUs (e.g. ”### billion CFU per serving at time of manufacture” or similar). There aren’t set standards for kefir on minimum viable counts to call it probiotic, but generally delivering at least 10^9 total CFU per serving is considered beneficial. As the study above highlighted, actual counts can be lower than claimed in some cases, suggesting a need for better quality control. This falls under general food labeling law: labels have to be truthful and not misleading. So if a kefir claims “100 billion CFU per bottle” and actually has 100 million, that could be a legal issue if substantiated. However, such enforcement is rare due to the difficulty of testing and variability.
Standards for “Live” Terminology: International standards like Codex require that if a product is labeled as a fermented milk (like kefir) and not heat-treated, the microbes be viable and abundant up to the expiration. Codex even suggests a minimum of 10^7 CFU/g for the starter cultures in fermented milks. Additionally, some regulations (like in South Africa mentioned earlier) explicitly set the minimum yeast count for kefir at 10^4 CFU/g. These act as standards to ensure the product genuinely is kefir. However, many of these are guidelines unless incorporated into local law.

In practice, consumers should look for phrases like “contains live cultures” or a list of Latin names of bacteria on the label to know the product is probiotic. If a kefir were pasteurized after fermentation (thus with no live culture), it would likely omit those phrases and might instead emphasize the taste (or say “cultured dairy beverage” without mention of live probiotics). The industry by and large polices itself because the selling point of kefir is its live probiotics.

In summary, labeling standards for kefir are still developing but largely revolve around honesty about the culture. Kefir on the label implies the traditional microbial consortium (both bacteria and yeast), as per international definitions. “Live cultures” signals the product has not been heat-treated post-fermentation and contains viable microbes, which is expected for kefir (and omission of that in a dairy “kefir” would be unusual). Reference to kefir grains is more a qualitative claim indicating traditional fermentation – there’s no legal definition for that on a label, but it should be truthful. Regulatory authorities are increasingly interested in these definitions; for example, the FDA’s recent moves (in conjunction with industry input) aim to prevent misuse of the term kefir on non-dairy products and possibly to establish a standard of identity to protect what kefir means to consumers. As fermented foods gain popularity, we may see tighter guidelines to ensure consistency in labeling (much like how “yogurt” is now more strictly defined). For now, the combination of Codex standards, national guidelines, and industry best practices act to maintain the integrity of kefir labeling.

Kefir - Microbiological Characteristics and Commercial Production