Understanding the Composition and Types of Fermented Milks

Fermented milks are among the oldest and most widespread food products in human history. When beneficial microorganisms act on milk, they convert lactose into lactic acid and other compounds, producing dairy products with distinct flavours, textures, and enhanced nutritional profiles. From the thick, tangy yogurt you find on supermarket shelves to the fizzy, probiotic-rich kefir gaining popularity worldwide, fermented milks are as diverse as the cultures that produce them. Let’s break down their composition, explore the major types, and understand why fermentation makes milk more nutritious.

What are fermented milks? 🔗

According to the Codex Alimentarius Standard (CXS 243-2003), fermented milk is a product obtained by fermenting milk using suitable microorganisms, resulting in a reduction of pH with or without coagulation. The starter microorganisms must remain viable and active in the product until its expiry date. This definition encompasses a wide range of products – yogurt, kefir, acidophilus milk, kumys, and many regional varieties – each defined by its specific starter culture and fermentation conditions.

The fermentation process does more than just preserve milk. Lactic acid bacteria (LAB) and, in some cases, yeasts break down lactose (milk sugar) into lactic acid. This acidification thickens the milk, gives it a characteristic tangy flavour, and creates an environment hostile to spoilage organisms. The result is a product with a longer shelf life, a new sensory profile, and – as we’ll see – superior nutritional value.

Key factors that determine the composition of fermented milks 🔗

Starter cultures 🔗

The choice of starter culture is the single most important factor that defines a fermented milk product. Different microbial strains produce different acids, flavours, textures, and health benefits. For instance, yogurt requires the symbiotic action of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, while kefir relies on a complex community of bacteria and yeasts present in kefir grains. According to a review published in PMC, kefir grains can harbour over 30 different microbial species, including lactic acid bacteria (83-90% of total count) and yeasts (10-17%).

Fermentation conditions 🔗

Temperature, duration, and incubation method all shape the final product. Yogurt is typically fermented at a relatively high temperature (around 42-45°C) for 4-12 hours. Kefir, on the other hand, ferments at a lower temperature (18-25°C) for about 18-24 hours. A longer fermentation or higher temperature generally leads to a more acidic product with a sharper flavour and thicker texture. The pH, titratable acidity, and alcohol content (in the case of kefir and kumys) are all directly influenced by these parameters.

Type of milk used 🔗

The source of milk – whether from cows, buffaloes, goats, sheep, or camels – has a significant impact on the composition of the final fermented product. Buffalo milk, with its higher fat content, yields richer and creamier fermented products like the thick dahi popular across India. Goat milk, with its smaller fat globules, is easier to digest and is widely used for yogurt and kefir in many parts of the world. Sheep milk, rich in both protein and fat, produces especially thick and creamy fermented products common in Mediterranean countries.

Common types of fermented milks 🔗

Yogurt 🔗

Yogurt is the most widely consumed fermented milk product globally. It is produced by fermenting milk with a defined pair of bacteria: Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. These two organisms work synergistically – S. thermophilus initiates acid production, creating favourable conditions for L. bulgaricus, which in turn releases amino acids that stimulate the growth of S. thermophilus. This cooperative relationship produces yogurt’s smooth, semi-solid texture and mildly acidic flavour.

A concentrated form of yogurt, often called Greek yogurt or strained yogurt (known as Stragisto or Labneh in different regions), is made by removing whey after fermentation. This process increases the protein content to a minimum of 5.6%, as per Codex standards, and produces a thicker, creamier product. Yogurt is a rich source of calcium, phosphorus, potassium, vitamin A, vitamin B2, and vitamin B12, as noted in a review in the journal Foods. It also delivers high biological value proteins and essential fatty acids.

Kefir 🔗

Kefir is a fermented milk beverage that originated in the Caucasus region. What makes kefir unique is its starter: kefir grains. These are not actual grains but gelatinous clusters that house a symbiotic community of bacteria and yeasts. According to the University of Florida’s IFAS Extension, kefir grains contain lactic acid bacteria such as Lactobacillus, Streptococcus, and Lactococcus, along with acetic acid bacteria and various yeast species.

Because kefir undergoes both lactic acid fermentation and alcoholic fermentation (due to the yeasts), the final product has a slightly effervescent quality, a tangier taste than yogurt, and a small amount of alcohol (typically 0.5-2% by volume). Kefir has a thinner consistency than yogurt but offers a broader diversity of probiotic strains – potentially over 50 different species of beneficial bacteria and yeasts. The fermentation process also enriches kefir with B vitamins (especially B1, B12, and folic acid), calcium, and amino acids, as reported in a review in African Health Sciences.

Dahi 🔗

Dahi (also called curd) is a traditional fermented milk product deeply embedded in South Asian cuisine. While it resembles yogurt, dahi is distinct in its preparation and microbial composition. Traditional dahi is often made using indigenous bacterial cultures passed down from previous batches – a practice known as backslopping – rather than defined commercial starters. This gives dahi a flavour profile that varies from household to household and region to region.

Dahi is typically made from cow or buffalo milk, with buffalo milk dahi being particularly popular in India due to its thicker, creamier texture. The fermentation is carried out primarily by LAB, and the product tends to be slightly less acidic and smoother than Western-style yogurt. Dahi serves as the base for lassi, a popular yogurt-based drink that can be sweet or savoury, and raita, a cooling condiment for spicy foods. As noted in a review in Frontiers in Nutrition, dahi is a traditional LAB-fermented product made from yak, cow, or buffalo milk across the Indian subcontinent.

Acidophilus milk 🔗

Acidophilus milk is a therapeutic fermented milk product cultured specifically with Lactobacillus acidophilus, a bacterium that naturally resides in the human digestive tract. Unlike yogurt or kefir, acidophilus milk retains a liquid consistency similar to regular milk. It is available in two forms: fermented acidophilus milk (which has a tangy flavour) and sweet acidophilus milk (which tastes like regular milk because the bacteria are added without active fermentation at refrigerated temperatures).

According to ScienceDirect, L. acidophilus contains the enzyme beta-galactosidase (lactase), which can help hydrolyse lactose. This makes acidophilus milk potentially beneficial for people with lactose intolerance, particularly in its fermented form. The product is also commonly recommended for individuals recovering from antibiotic treatments, as L. acidophilus helps restore beneficial gut flora.

Kumys (koumiss) 🔗

Kumys is a traditional fermented milk beverage made from mare’s milk, popular in Central Asian countries. Its starter culture consists of Lactobacillus delbrueckii subsp. bulgaricus and the yeast Kluyveromyces marxianus. Like kefir, kumys undergoes both lactic and alcoholic fermentation, but its alcohol content is typically higher (around 0.7-2.5%). Mare’s milk is lower in fat and higher in vitamin C than cow’s milk, giving kumys a distinct nutritional profile. It contains all essential amino acids and is considered a functional food in the regions where it is traditionally consumed.

Nutritive value of fermented milks 🔗

Fermentation does not merely preserve milk – it enhances its nutritional value in measurable ways. The biochemical transformations carried out by starter cultures improve the digestibility, bioavailability, and health-promoting potential of the original milk.

Improved protein digestibility 🔗

During fermentation, bacterial enzymes (proteases) partially break down casein and whey proteins into smaller peptides and free amino acids. This pre-digestion makes the proteins more bioavailable – meaning your body can absorb and use them more efficiently. Some of these peptide fragments are bioactive, meaning they have functions beyond basic nutrition. Research has identified antimicrobial, antihypertensive, and immunomodulatory properties in peptides derived from fermented milk proteins.

Enhanced mineral absorption 🔗

The lactic acid produced during fermentation lowers the pH of the product, which increases the solubility and bioavailability of important minerals like calcium, magnesium, and zinc. This is particularly relevant for bone health. Some studies suggest that calcium from yogurt may be absorbed significantly more efficiently than calcium from unfermented milk. Additionally, research published in the Journal of Functional Foods indicates that short-chain fatty acids produced during fermentation can further enhance mineral absorption in the gut by modifying intestinal pH.

Reduced lactose content and better tolerance 🔗

One of the most significant benefits of fermentation is the partial breakdown of lactose. Since LAB convert lactose into lactic acid, fermented milk products contain substantially less lactose than regular milk. This makes products like yogurt and kefir more tolerable for individuals with lactose intolerance. The bacterial beta-galactosidase enzyme continues to work in the gut after consumption, further aiding lactose digestion. As noted in a review published in the Journal of Dairy Research, fermented dairy products serve as good sources of lactase and can be effective alternatives for lactose-intolerant individuals.

Probiotic benefits 🔗

Fermented milks are one of the primary dietary vehicles for probiotics – live beneficial microorganisms that, when consumed in adequate amounts, confer health benefits on the host. The probiotic bacteria in fermented milks support gut health in several ways: they compete with harmful bacteria for space and nutrients, produce antimicrobial substances, strengthen the intestinal barrier, and modulate the immune system.

According to a review of fermented milks and health, regular consumption of probiotic-rich fermented dairy has been associated with antimicrobial, antimutagenic, anticarcinogenic, and antihypertensive effects, as well as reduced cholesterol levels and improved management of type 2 diabetes. Probiotics in these products also help synthesise B-group vitamins (including B12 and folic acid) and vitamin K, adding further nutritional value.

Vitamin enrichment 🔗

The metabolic activity of fermenting bacteria increases the levels of several vitamins in the final product. B vitamins, in particular, are synthesised during fermentation. Kefir, for example, shows elevated levels of vitamins B1, B12, K, and folic acid compared to unfermented milk. Yogurt is recognised as a good source of riboflavin (B2) and cobalamin (B12). These vitamins play essential roles in energy metabolism, red blood cell production, and nervous system function.

Health benefits beyond basic nutrition 🔗

Gut health and immune function 🔗

The live cultures in fermented milks contribute to a balanced gut microbiome. They produce short-chain fatty acids (acetate, propionate, butyrate) that nourish intestinal cells, reduce inflammation, and support the integrity of the gut lining. A healthy gut microbiome, in turn, supports robust immune function. Multiple studies have linked regular consumption of fermented dairy products with reduced incidence of gastrointestinal infections and improved immune responses.

Cardiovascular health 🔗

Certain probiotic strains found in fermented milks can help lower LDL cholesterol levels through mechanisms such as binding bile acids and inhibiting cholesterol synthesis enzymes. Bioactive peptides released during fermentation may also help regulate blood pressure by inhibiting angiotensin-converting enzyme (ACE). These combined effects make fermented milks potentially beneficial for cardiovascular health when consumed as part of a balanced diet.

Bone health 🔗

Fermented milks are excellent sources of bioavailable calcium, which is critical for maintaining bone density throughout life. The combination of enhanced calcium absorption (due to lactic acid), phosphorus content, and probiotic activity makes these products particularly valuable for preventing osteoporosis, especially in ageing populations. As highlighted in research cited by a study in the Journal of Functional Foods, probiotic bacteria can boost calcium absorption and improve phosphorus bioavailability by breaking down phytate.

A quick comparison of major fermented milks 🔗

Here is a summary of how the four main types compare:

Yogurt uses S. thermophilus and L. bulgaricus as its starter, ferments at 42-45°C, has a semi-solid texture, and contains moderate probiotic diversity. Kefir uses kefir grains (bacteria + yeasts), ferments at 18-25°C, has a thin and effervescent texture, and offers high probiotic diversity with a small amount of alcohol. Dahi uses indigenous LAB cultures (often backslopped), ferments at ambient temperature, has a smooth and creamy texture, and varies in probiotic content based on preparation. Acidophilus milk uses L. acidophilus, may or may not undergo active fermentation, maintains a liquid consistency, and is specifically targeted for digestive health benefits.

What do you think? Have you noticed a difference in how your body responds to different fermented milks like yogurt versus kefir? And given the growing evidence for the health benefits of fermented dairy, do you think these traditional products deserve more attention in modern dietary guidelines?

References

1. https://www.fao.org/input/download/standards/400/CXS_243e.pdf
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC7071183/
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC9455928/
4. https://ask.ifas.ufl.edu/publication/FS457
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC11225442/
6. https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2025.1678150/full
7. https://www.sciencedirect.com/topics/food-science/acidophilus-milk
8. https://academic.oup.com/ijfst/article/60/2/vvaf227/8339879
9. https://www.sciencedirect.com/science/article/pii/S1756464620302838
10. https://www.cambridge.org/core/journals/journal-of-dairy-research/article/fermented-foods-and-probiotics-an-approach-to-lactose-intolerance/77AD1896FA744AA7453AE047FD85BD17
11. https://www.sciencedirect.com/science/article/pii/S1756464625004104