Definitive Explanation of Dairy Kefir (U.S. Context)

History and Traditional Production of Kefir

Kefir is an ancient fermented milk beverage believed to have originated in the Caucasus Mountains thousands of years ago (Sequencing-Based Analysis of the Bacterial and Fungal Composition of Kefir Grains and Milks from Multiple Sources | PLOS One). Traditionally, kefir was made by adding kefir grains to animal milk (cow, goat, or sheep) in a skin bag or clay pot and allowing it to ferment at ambient temperatures. These kefir grains look like small cauliflower-like clusters and were passed down through generations as a family treasure. Over time, the microbes in the grains ferment the lactose in milk into lactic acid (souring and preserving the milk) and produce small amounts of carbon dioxide (giving a slight natural effervescence) and trace alcohol (Sequencing-Based Analysis of the Bacterial and Fungal Composition of Kefir Grains and Milks from Multiple Sources | PLOS One). The result is a tart, tangy, slightly fizzy drink often called the “Champagne of Dairy.” Early records from Eastern Europe and Russia describe kefir’s health benefits and its unique preparation method using these grains. In summary, traditional kefir is defined by its use of living kefir grains to ferment milk, distinguishing it from other fermented dairy like yogurt which use only specific bacterial starters.

Kefir Grains and Microbial Composition

(File:Kefir junto a moneda.jpg - Wikimedia Commons) Kefir grains (white, clumped masses) next to a coin for scale. These soft, gelatinous “grains” are not true grains like wheat, but a living colony of bacteria and yeast held together by a polysaccharide-protein matrix (Sequencing-Based Analysis of the Bacterial and Fungal Composition of Kefir Grains and Milks from Multiple Sources | PLOS One). They are reused to ferment fresh milk in traditional kefir making.
Kefir grains are the natural starter culture for kefir. They are rubbery, irregular masses composed of a matrix called kefiran (a polysaccharide) produced by microbes during fermentation ( The Microbiota and Health Promoting Characteristics of the Fermented Beverage Kefir - PMC ). Within this matrix lives a symbiotic community of lactic acid bacteria, yeasts, and acetic acid bacteria. In fact, kefir grains harbor dozens of different microbial species – more than 50 species have been identified across various grains ( Quantification of Major Bacteria and Yeast Species in Kefir Consortia by Multiplex TaqMan qPCR - PMC ). Common bacteria in kefir grains include Lactobacillus (e.g. Lactobacillus kefiranofaciens, L. kefiri), Lactococcus, Leuconostoc, and some Streptococcus species ( The Microbiota and Health Promoting Characteristics of the Fermented Beverage Kefir - PMC ). These lactic acid bacteria typically dominate the grain (around 10^8 CFU per gram) and are responsible for most of the lactose fermentation (Unit 5a. Lactic Cultures in Yogurt and Kefir – CMC1 – Introduction to Cheesemaking). Uniquely, kefir also contains live yeasts (about 10^7 CFU/g), such as Saccharomyces cerevisiae, Saccharomyces unisporus, Kluyveromyces marxianus, and Candida kefyr, which ferment sugars to ethanol and CO₂ (Unit 5a. Lactic Cultures in Yogurt and Kefir – CMC1 – Introduction to Cheesemaking). A smaller fraction of acetic acid bacteria (Acetobacter spp.) is also present (around 10^5 CFU/g) (Unit 5a. Lactic Cultures in Yogurt and Kefir – CMC1 – Introduction to Cheesemaking), contributing to flavor and preservative organic acids.

This rich microbial diversity sets kefir apart from other cultured dairy products. For example, yogurt is typically fermented by just two bacteria (Lactobacillus bulgaricus and Streptococcus thermophilus), and even “probiotic” yogurts or drinks usually have only a handful of added strains. Kefir, by contrast, is a complex ecosystem: bacteria and yeasts coexist and mutually support each other (yeast growth can stimulate kefiran production by bacteria, and bacteria create conditions favorable to yeast) (Unit 5a. Lactic Cultures in Yogurt and Kefir – CMC1 – Introduction to Cheesemaking). The presence of yeasts in kefir means it undergoes a mixed lactic-alcoholic fermentation, whereas most other fermented milks (yogurt, cultured buttermilk) are purely lactic fermentations. In terms of probiotic content, a serving of traditional kefir can deliver a broader spectrum of microbes than typical single-strain supplements or yogurts – kefir’s Lactobacillus species are joined by many other genera with potential health benefits (Sequencing-Based Analysis of the Bacterial and Fungal Composition of Kefir Grains and Milks from Multiple Sources | PLOS One). Studies have shown kefir isolates are generally robust, surviving transit through stomach acid and producing antimicrobial compounds (organic acids, bacteriocins) that can inhibit pathogens (untitled). Thus, dairy kefir is essentially a self-propagating probiotic culture drink, thanks to the diverse microbial consortium living in the grains.

”Active Cultures” vs. “Kefir Grains” in Commercial Products

(File:LifewayKefir.jpg - Wikimedia Commons) Bottled kefir products in a U.S. store. The label (Lifeway brand) prominently uses the word “Kefir” and notes it is a cultured milk beverage. Commercial kefir brands advertise live probiotic cultures, but they typically do not include actual kefir grains in the package.
In U.S. grocery stores, kefir is sold as a refrigerated cultured milk smoothie. You’ll often see labels touting “Live and Active Cultures” or listing specific probiotic strains, but you will not find actual kefir grains floating in the bottle – nor are grains usually mentioned as an ingredient. The term “kefir grains” refers to the traditional starter lumps used on a small scale; in industrial production the process is different. Large-scale kefir manufacturers rarely ferment with physical kefir grains on the factory floor ( The Microbiota and Health Promoting Characteristics of the Fermented Beverage Kefir - PMC ). Instead, they rely on standardized starter cultures that were originally isolated from kefir grains. These cultures (sometimes freeze-dried preparations) contain a selected mix of bacteria and yeasts that can produce a kefir-like fermentation. Using a defined starter gives more consistent results in flavor and texture and is easier to manage than handling gelatinous grains in thousands of gallons of milk. For instance, dairy culture suppliers offer direct-to-vat inoculation packets derived from kefir grain microbiota (Unit 5a. Lactic Cultures in Yogurt and Kefir – CMC1 – Introduction to Cheesemaking).

Because of this, U.S. kefir brands typically label their product simply as “cultured milk” with live cultures, rather than mentioning kefir grains. The phrase “active cultures” on a label indicates the product contains live microbes, but it doesn’t specify their origin. Some smaller or artisanal producers may use the phrase “fermented with authentic kefir grains” in marketing if they do ferment in the traditional way, but this is niche. The major U.S. brands (e.g. Lifeway Kefir) use a laboratory-grown blend of cultures to ferment pasteurized milk – the end product is still called kefir because it has similar taste and microbial profile, but it’s produced without constantly transferring the gelatinous grains. In practice, some manufacturers maintain a mother culture that was started with real grains and use a portion of that fermented milk to inoculate each new batch (this preserves more of the traditional consortium over time), whereas others use a fixed cocktail of strains added to each batch (Commercial Milk Kefir vs Home | Yemoos Nourishing Cultures). Either way, the consumer sees “kefir” on the bottle and “contains live probiotic cultures” on the nutrition panel. The term “kefir grain” is mostly absent from ingredient lists in mass-produced kefir. The important point is that the product still delivers live cultures, even if produced by modern methods. However, it’s worth noting that the diversity of microbes in commercially cultured kefir may be somewhat reduced compared to true grain-fermented kefir – for example, a brand might include 10–12 strains of lactic acid bacteria and a couple of yeasts, whereas researchers have identified 30–50 strains in traditional grain-fermented kefir (Commercial Milk Kefir vs Home | Yemoos Nourishing Cultures). Nonetheless, both traditional and commercial kefir are considered probiotic-rich foods.

Fermentation Process: In-Vat vs. In-Bottle

The heart of kefir production is the fermentation step, where milk is cultured by the kefir microbes. Traditionally, kefir fermentation happened in the same vessel that would be used for storage or serving (like a goatskin bag or glass jar); this could be seen as “in-bottle” fermentation in a rustic sense. In modern commercial operations, fermentation is typically done in large stainless steel vats rather than in the retail bottles. A typical process flow is: fresh milk is pasteurized, cooled to fermentation temperature (often around 20–25 °C for mesophilic kefir cultures), inoculated with the kefir culture, and then kept in a fermentation tank for a set time (e.g. 12–20 hours) (Frequently Asked Questions). During this period, the bacteria and yeasts consume lactose and produce acids and flavor compounds. The endpoint is determined by acidity (pH or titratable acidity) and flavor – for example, Lifeway Foods (a leading U.S. kefir producer) ferments their milk for about 14–18 hours to achieve the desired tartness and microbial count (Frequently Asked Questions).

Once fermentation is complete, the kefir is usually cooled to stop further activity and then filled into bottles under cold, sanitary conditions. This in-vat method ensures uniform fermentation and allows the manufacturer to control variables like temperature and agitation. Notably, most commercial kefir is not fully fermented under sealed conditions, so it is only mildly carbonated. In fact, companies take steps to avoid excessive yeast fermentation and carbonation in the package (Commercial Milk Kefir vs Home | Yemoos Nourishing Cultures). If kefir were fermented completely in a sealed bottle, the continued CO₂ produced by yeasts could build up pressure – risking bulging or exploding bottles on the shelf. To prevent this, producers might ferment in vented tanks (so gas can escape) or ferment to a point where most fermentable sugars are depleted, then chill the product. As a result, store-bought kefir typically has a light effervescence at most, and is not as fizzy as homemade kefir that undergoes a secondary fermentation in a closed jar. According to fermentation experts, commercial kefir often has a “milder” culture activity because yeast growth is kept in check for safety and consistency (Commercial Milk Kefir vs Home | Yemoos Nourishing Cultures).

It’s worth noting that some smaller dairy farms or brands may use a partial in-bottle fermentation: for example, they might fill bottles with inoculated milk and incubate them, then refrigerate when done. This can happen especially in cases like Eastern European kefir sold in local markets, where a slight natural carbonation is traditional. But mainstream U.S. brands generally do not ferment individually in each consumer bottle. They also often homogenize the product by gentle stirring before bottling, to distribute the whey and curds evenly (kefir can separate after fermentation). The fermentation process is what creates kefir’s distinct sour taste, thickness, and probiotic content – unlike some “probiotic milk drinks” that are made by just mixing bacteria into yogurt or milk, real kefir must undergo fermentation. The live cultures produce lactic acid (which gives tartness and a pH typically around 4.5 or lower) and also exopolysaccharides like kefiran that contribute to the creamy, slightly viscous texture of kefir ( The Microbiota and Health Promoting Characteristics of the Fermented Beverage Kefir - PMC ). Additionally, fermentation generates flavor compounds like diacetyl (buttery note), acetaldehyde (sharp yogurt-like note), and in true grain fermentation, a trace of ethanol – though most U.S. commercial kefir has negligible alcohol (<0.1%) because fermentation is controlled to avoid it. In summary, commercial kefir is usually fermented in bulk vats and then packaged, rather than fermenting in the package, to ensure safety and consistency, with the trade-off of less carbonation than traditional methods.

Pasteurization and Live Cultures

A key aspect of producing kefir (and any fermented dairy for retail) is managing pasteurization so that the final product still contains live beneficial microbes. The general rule in dairy fermentation is: pasteurize the raw milk before adding the cultures, and do not pasteurize (or reheat) after fermentation if you want to maintain live cultures (Frequently Asked Questions). Kefir manufacturers follow this rule. Milk is first pasteurized to eliminate any unwanted bacteria or pathogens naturally present in raw milk; this creates a clean slate so that the introduced kefir culture can dominate the fermentation. After pasteurization and cooling, the kefir culture is added and fermentation proceeds. The kefir you buy in the store is not pasteurized after fermentation – doing so would indeed kill the very probiotics that are the selling point of kefir. In fact, U.S. kefir brands emphasize this in their marketing: for example, Lifeway states that all the milk is pasteurized before culturing, ensuring the kefir cultures are alive when consumed (Frequently Asked Questions). They explicitly note that some other dairy products or yogurts are heat-treated post-culturing (thus have few live bacteria), but authentic kefir is not; it’s delivered as a living product with “billions of CFUs” (colony forming units) of probiotics (What’s In My Kefir?).

To clarify, by law in the U.S., Grade “A” dairy products like fluid milk must be pasteurized. Kefir uses pasteurized milk as the base. Once the kefir culture has fermented the milk, the finished kefir is bottled cold and kept refrigerated. It is not pasteurized after fermentation, which is why the label can claim for instance “Contains 12 Live & Active Cultures” or “Contains billions of probiotics.” Those microbes are indeed present and viable at the time of sale. If the product were heated or sterilized after fermenting, it would have to be labeled differently (in some jurisdictions, a fermented milk that’s been heat-treated cannot legally be called “kefir” or must say “heat-treated” on the label). For example, international standards state that if a yogurt or kefir is pasteurized after fermentation, the label should disclose “pasteurized after fermentation” because the microorganisms are no longer active (Unit 5a. Lactic Cultures in Yogurt and Kefir – CMC1 – Introduction to Cheesemaking). In practice, virtually all kefir sold in North America is refrigerated and contains live cultures. The presence of live cultures also means kefir continues to age slowly in the bottle – over its shelf life it may get more sour as the bacteria produce more acid (though kept cold, their activity is minimal). Manufacturers print a sell-by date that factors in how the culture might continue fermenting slowly. The product is formulated to have a high viable cell count. For instance, one cup of commercial kefir can contain on the order of 10^9 (a billion) live bacteria at time of manufacture (Lifeway Organic Whole Milk Kefir - 10g Protein, 12 Live Active …). As long as it’s kept cold and not overly aged, the vast majority of those microbes survive to provide a probiotic benefit.

It’s important to dispel confusion: kefir milk is pasteurized (for safety), but kefir itself (the fermented drink) is not pasteurized after culturing. This is how it retains live bacteria and yeast. Some consumers wonder if kefir might be shelf-stable or sold unrefrigerated – a truly shelf-stable kefir would have to be sterilized or dried, which would kill or inactivate the culture. Such products would no longer be probiotic (unless companies added back probiotic strains in capsules, which would be a different kind of product). The kefir found in U.S. stores is always in the dairy cooler, similar to yogurt, indicating it’s perishable and contains living organisms. In summary, pasteurization is done before fermentation in kefir-making, and not afterwards, which is why kefir can be labeled with “billions of live cultures” – those microbes are alive because they were never subjected to a post-fermentation kill step (Frequently Asked Questions). Consumers should store kefir chilled to keep those cultures alive and healthy up to consumption.

Definition and Standards of Identity for “Kefir”

In the United States, the term “kefir” does not (as of 2025) have its own unique FDA Standard of Identity in the Code of Federal Regulations, the way “yogurt” or “buttermilk” do. Legally, kefir is generally sold under the umbrella of “cultured milk” as defined in 21 CFR §131.112. This dated standard lumps together various fermented milk drinks. For example, one requirement of the U.S. cultured milk standard is that products be named with the term “cultured” or “fermented” milk along with a qualifier (unless there is a separate standard of identity) ( ). In practice, however, U.S. companies market the product simply as “Kefir” and have obtained informal permission to do so, even though strictly speaking “kefir cultured milk” would satisfy the standard naming convention. The FDA has recognized “kefir” as a class of cultured dairy product, but the regulations haven’t been modernized in decades. This led to some regulatory quirks – for instance, a few years ago the FDA warned certain producers that just calling the product “Kefir” on the label was not in line with the cultured milk regulation, which technically would call it “kefir cultured milk” ( ). Major U.S. kefir producer Lifeway Foods petitioned the FDA to update the standard, pointing out that kefir is a traditional name used worldwide and consumers understand it as a dairy product, analogous to how “yogurt” is an accepted name without saying “cultured milk yogurt” ( ) ( ). As of now, no specific new standard has been published, but in practical terms FDA allows the sale of kefir under its common or usual name. So, in the U.S., what “counts” as kefir is generally up to the manufacturer following the loose model of traditional kefir: a fermented milk with a mixed culture of lactic acid bacteria and yeasts. There is no strict legal requirement for which microbes must be present (unlike yogurt, which by standard must contain L. bulgaricus and S. thermophilus). However, reputable kefir brands align with international norms by including multiple bacteria (often labeled as probiotic strains) and some yeast, and by ensuring a high viable cell count at sale (often 10^7 CFU/g or higher).

Internationally, definitions of kefir are more clearly outlined. The Codex Alimentarius (an international food standards body) has a standard for Fermented Milks (Codex Stan 243-2003) that includes specific criteria for kefir. According to Codex, “kefir” is a fermented milk obtained by the action of kefir grains and the resulting microflora – including lactic acid bacteria and yeasts – must be viable and abundant in the final product. The Codex standard specifies a minimum of 10^7 CFU per mL total live microorganisms in kefir, and a minimum of 10^4 CFU/mL yeasts present in the product (untitled). It also sets basic chemical composition requirements: for example, kefir should contain at least 0.6% titratable acidity (as lactic acid) and at least 2.7% protein (since it’s made from milk) (untitled). There is no maximum alcohol content fixed by Codex, but in practice authentic kefir usually has up to around 0.1–0.5% alcohol from yeast fermentation. These standards ensure that anything called “kefir” internationally has the characteristics of the traditional product: a mixed LAB and yeast fermentation and a significant level of live cultures. Similarly, in Canada, kefir is recognized as a distinct fermented milk product. Canadian regulations (Food and Drug Regulations and Safe Food for Canadians Regulations) don’t spell out “kefir” in detail like yogurt, but they treat it akin to yogurt for certain exemptions and fortification rules. For instance, Health Canada recently allowed vitamin D fortification in kefir, listing it alongside yogurt – implying that kefir is defined sufficiently as a cultured dairy food to be covered by dairy fortification policies (Marketing Authorization for Vitamin D in Yogurt and Kefir) (Labelling requirements for dairy products - inspection.canada.ca). The Canadian Food Inspection Agency also groups kefir with yogurt and buttermilk as dairy products that inherently contain significant live cultures, and thus these products are exempt from some new labeling requirements for sugar and saturated fat (because they are considered “healthful” dairy choices in their plain form). In essence, Canada’s view of kefir aligns with Codex: it is “the product made by the lactic acid and alcoholic fermentation of milk by kefir grains or the corresponding starter cultures.” As a result, Canadian consumers see products plainly labeled “kefir” (often with a description like “fermented milk”) and those products do have to meet the expectation of containing live cultures at consumption.

Beyond Codex and North America, many countries have their own traditional terminology for fermented milks but often refer to Codex for guidance if needed. In Eastern Europe, where kefir has been consumed for centuries, standards exist – for example, Russian and Polish standards define kefir by specific microbiological content and acidity. The European Union doesn’t have an EU-wide law specifically for kefir, but producers there also adhere to similar norms (live cultures, mixed fermentation). In summary, what “counts” as kefir is generally consistent globally: it must be a fermented milk with a symbiotic culture of lactic bacteria and yeasts, resulting in a sour, thickened beverage with live microbes. The U.S. has no strict legal microbiological criteria for kefir, but producers follow the customary definition. International standards (Codex) and Canadian guidelines emphasize minimum live culture counts and the presence of yeast as key distinguishing features of kefir versus other cultured dairy. Consumers should thus expect that any product called “kefir” (in the U.S. or elsewhere) will be a live-cultured dairy drink made with a complex culture, as opposed to a simple milk or yogurt. That complexity – rooted in the traditional kefir grains – is what defines kefir and gives it its probiotic reputation.

Sources:

1. Marsh et al. (2013) – PLoS One: Origin and description of kefir and grains (Sequencing-Based Analysis of the Bacterial and Fungal Composition of Kefir Grains and Milks from Multiple Sources | PLOS One).
2. Nejati et al. (2020) – Front. Microbiol.: Kefir grain microbiome diversity (50+ species) ( Quantification of Major Bacteria and Yeast Species in Kefir Consortia by Multiplex TaqMan qPCR - PMC ).
3. Bourrie et al. (2016) – Front. Microbiol.: Review of kefir’s microbes and health aspects ( The Microbiota and Health Promoting Characteristics of the Fermented Beverage Kefir - PMC ) ( The Microbiota and Health Promoting Characteristics of the Fermented Beverage Kefir - PMC ).
4. U. of Guelph, Dairy Science Text: Composition of kefir grains and commercial starter use (Unit 5a. Lactic Cultures in Yogurt and Kefir – CMC1 – Introduction to Cheesemaking) (Unit 5a. Lactic Cultures in Yogurt and Kefir – CMC1 – Introduction to Cheesemaking).
5. Yemoos Kefir Guide: Differences in commercial vs. traditional kefir cultures (Commercial Milk Kefir vs Home | Yemoos Nourishing Cultures) (Commercial Milk Kefir vs Home | Yemoos Nourishing Cultures).
6. Lifeway Foods – FAQs and “What’s in my Kefir?”: Details on fermentation, live cultures (12 strains, 25–30 billion CFU), and pasteurization practice (Frequently Asked Questions) (Frequently Asked Questions) (What’s In My Kefir?).
7. Codex Alimentarius Standard for Fermented Milks (Codex Stan 243-2003): Definitions and required microbe counts for kefir (untitled).
8. FDA Lifeway Petition (2019): Discussion of U.S. standard of identity issues for kefir (cultured milk category) ( ) ( ).