Take home message

• Yoghurt is the only milk-based drink for which a health claim has been authorised (EFSA).
• If you compare yoghurt with kefir, kefir has health benefits over yoghurt because of the yeasts and mesophilic cultures.
• If you compare raw-milk kefir with ‘ordinary’, pasteurised kefir, the raw-milk variety has advantages in terms of immunology.
• Not all fermented milk products are the same.

Spot the differences

‘Real’ kefir contains both lactic acid bacteria and yeasts. Kefir differs fundamentally from yoghurt in two respects. Firstly, the bacterial starter culture in yoghurt is thermophilic, whilst that in kefir is mesophilic. Secondly, kefir contains yeasts, which cause a second fermentation: the breakdown of the lactic acid formed by the bacteria into carbon dioxide and by-products. Yoghurt must be free from yeasts. There may also be a third difference, namely heating. Kefir is also available as raw-milk kefir rather than kefir made from heated milk, and this has the advantage that the protective properties of raw milk (against allergies) are partially retained, as the whey proteins remain intact.

Mesophilic versus thermophilic: vitamin K2

Gouda, Cheddar, quark and kefir are made at low temperatures. Kefir is made at 24–28 ^oC, as is quark, whilst Gouda is made at a temperature no higher than body temperature. Mesophilic lactic acid bacteria are used for this. In contrast, there are mountain cheeses, such as Emmental, Gruyère and the very large Parmesan cheeses, as well as fresh cheeses like Mozzarella, which are made at much higher temperatures, namely between 50–58 ^oC. In German, this is referred to as ‘brennen’ (burning) the cheese curd. This requires different types of bacteria, known as thermophilic lactic acid bacteria. The same applies to yoghurt, which is made after the milk has been boiled above 90 ^oC before inoculation at 42–45 ^oC.

We have previously written about the formation of vitamin K2 during lactic acid fermentation. There is a key difference in the amount of K2 between mesophilic and thermophilic cultures. Yoghurt (thermophilic) contains virtually no K2, whilst quark (mesophilic) has very high K2 levels. The same trend can be seen between Gouda (mesophilic) and mountain cheeses (thermophilic).

Vitamin K2 is credited with various positive health effects. K2 appears to help maintain the elasticity of artery walls, thereby lowering blood pressure and reducing the risk of cardiovascular disease. K2 also ensures the ‘correct’ deposition of calcium, namely in the bones. It helps to reduce the risk of bone fractures in older age. Researchers summarise this effect of K2 as ‘ensuring strong bones but flexible blood vessels’ (Buchanan et al., 2016). Research into vitamin K2 has been conducted at Maastricht University (Nl) for many years. A recently published study (Bittner et al., 2026) draws attention to another aspect of vitamin K2, namely the regulation of fat storage, visceral fat and associated enzyme levels. Such university studies typically involve purified forms of vitamin K2, administered in tablet form.

The participants were given a daily dose of menaquinone-7 (MK-7), a form of vitamin K2 commonly found in fermented dairy products based on mesophilic cultures (quark, kefir, Gouda cheese). The study was double-blind, and the groups were followed up for one year. The abstract of the article states: “After one year, there was a small but significant reduction in BMI (−0.66%; p = 0.041). Based on the differences between responders and non-responders, responders showed improvements in fat mass (−1.93%, p = 0.039), waist-to-hip ratio (−1.43%, p = 0.012), BMI (−1.37% ± 3.21, p = 0.004) and weight (−1.25%, p = 0.008). Women, particularly those who had not yet reached the menopause, showed greater reductions in BMI, fat mass and weight.”

When these results are compared with other studies examining the consumption of fermented milk products in relation to weight and cardiovascular disease, it is often demonstrated that such products contribute to various aspects of our health and the maintenance of a ‘healthy weight’. You donot get fat from full-fat cheese. Vitamin K2 may play a role in this.

Yeasts alongside lactic acid bacteria: carbon dioxide and cognitive ability

The yoghurt is ready once the thermophilic bacteria have done their work. The result is a mildly sour product, in which the presence of yeasts is regarded as contamination. Yeasts must not be present. When making good kefir, both bacteria and yeasts are present in the starter culture. If you use kefir grains, there are often several types of yeast present. Kefir varies slightly from one origin to the next and differs from region to region. Yeasts take over where the lactic acid bacteria left off: the lactic acid that has formed is broken down, producing carbon dioxide, alcohol (in small quantities) and flavourings, as well as bioactive substances. In some countries, yeasts are not tolerated in kefir. This may be for reasons of principle or religious belief. For example, it is felt that alcohol does not belong in a dairy product, in which case the yeasts must be omitted. Such a product should not be called kefir, but rather a type of yoghurt (based on mesophilic cultures). ‘Kefir’ as a product name is not protected or registered, and anyone can label their fermented milk drink as kefir drink. Nevertheless, yeasts are essential for drinks such as kefir, as well as kombucha.

Recently, a group of Italian researchers (Ullah et al., 2026) investigated the impact of two yeast extracts that may also be present in kefir: Kluyveromyces fragilis and Saccharomyces cerevisiae. The genus Kluyveromyces is found in kefir made from SCOBY. In such cases, it often involves other species within the genus. The yeast S. cerevisiae is known as baker’s or brewer’s yeast. In kefir, one is more likely to include the yeast Debaryomyces (for example, D. hansenii) in freeze-dried kefir starter cultures. The two species are related. Although species and strains may have different effects, there are also many things they have in common: the breakdown of lactic acid and the formation of, amongst other things, carbon dioxide and alcohol.

There were three groups of older adults (average age 73.5 years). Over a period of six months, participants were given either a dummy pill (a placebo containing no yeast) or one containing yeast extract from K. fragilis or S. cerevisiae. Cognitive function was assessed using two different questionnaires (MoCA and MMSE scores, each out of a maximum of 30 points). The tests are designed to assess cognitive decline and early-stage dementia (MoCA), or orientation and memory skills (MMSE). The results are described in their summary: “After 180 days, MoCA scores had increased by 4.4 points in the K. fragilis group and by 3.9 points in the S. cerevisiae group, compared with 0.6 points in the placebo group (Fig. 1). The MMSE scores increased by 1.6 and 3.1 points, respectively, in the K. fragilis and S. cerevisiae groups, compared with 0.3 points in the placebo group (Fig. 2).”

Fig. 1. MoCa score as a measure of orientation and memory prior to the study (T0), after 3 months (T1) and 6 months (T2) in three groups of older adults: no yeast (placebo), or the yeasts K. fragilis or S. cerevisiae (adapted from Ullah et al., 2026).

Fig. 2. MMSE score for cognitive decline prior to treatment (T0), after 3 months (T1) and 6 months (T2) in three groups of older adults: no yeast (placebo), or the yeasts K. fragilis or S. cerevisiae (adapted from Ullah et al., 2026).

All in all, this shows that cognitive functions improve as a result of the yeast extracts, which were taken in tablet form. The authors attribute the effect primarily to the quantity of nucleotides (such as uridine and choline) produced by the yeasts. There are likely to be other mechanisms and explanations as well when the yeasts (and their metabolic products and constituents) are consumed as a whole. Numerous other substances are produced, such as B vitamins and antioxidants.

Raw milk versus heated kefir: immunity

Finally, the impact of heating the milk before making kefir. Research into both raw milk and kefir made from raw milk shows that the immunological properties of the end product deteriorate when the milk has been heated. In her thesis, Suzanne Abbring examined the various fractions in cow’s milk in detail and concluded that heating whole milk (4% fat) or skimmed milk (0% fat), or whey from raw milk (0% fat, no longer containing casein), invariably leads to an immunological or allergic reaction (Abbring et al., 2020). Raw products offer protection. Further research into kefir made from raw or heated cow’s milk yielded the same results; after fermentation, the same differences are present as between raw milk and heated milk (Baars, Van Esch et al., 2023). This may be explained by the fact that a whey protein such as beta-lactoglobulin (BLG) passes through the stomach unchanged and exerts its effect in the upper part of the small intestine. Both raw milk and kefir made from raw milk have a calming effect on the immune system, in conjunction with the unheated BLG.

Conclusion

Yoghurt is a tasty fermented product with a neutral flavour. Yoghurt is a product recognised by the EFSA with a health claim, primarily due to the reduced lactose content compared to milk. Wild yeasts are regarded as contaminants. Genuine kefir is a tangy-tasting, fermented product, in which the yeasts not only produce carbon dioxide (bubbles) but also yield other by-products that influence health and cognitive functions. Furthermore, unlike thermophilic yoghurt, mesophilic kefir has a high concentration of vitamin K2. Vitamin K2 is not only associated with combating osteoporosis and preventing cardiovascular disease, but also with obesity. Finally, kefir made from raw milk provides additional immunological protection.

Thus, fermented dairy products differ in terms of bacterial cultures, yeasts and/or heat treatment, with implications for various aspects of our health.

Literature

• Abbring, S., Xiong, L., Diks, M. A., Baars, T., Garssen, J., Hettinga, K., & van Esch, B. C. (2020). Loss of allergy-protective capacity of raw cow’s milk after heat treatment coincides with loss of immunologically active whey proteins. Food & function, 11(6), 4982-4993.
• Baars, T., van Esch, B., van Ooijen, L., Zhang, Z., Dekker, P., Boeren, S., … & Kort, R. (2023). Raw milk kefir: microbiota, bioactive peptides, and immune modulation. Food & Function, 14(3), 1648-1661.
• Bittner, R., de Vries, F., Machuron, F., Maresz, K., Gåserød, O., & Schurgers, L. (2026). Sex-Specific Effects of Menaquinone-7 (MK-7) Supplementation on Body Composition and Adiposity Markers. Nutrients, 18(11), 1699.
• Buchanan, M.D., Grant S., Melvin, T., Merritt, B., Bishop, C., Shuler, F.D. (2016). Vitamin K2 (menaquinone) Supplementation and its Benefits in Cardiovascular Disease, Osteoporosis, and Cancer. Marshall Journal of Medicine, 2 (3), 8.
• Ullah, H., Cordara, M., Morone, M. V., Piccinocchi, R., De Lellis, L. F., Cerqua, A., … & Daglia, M. (2026). Effects of Nucleotide-Rich Kluyveromyces fragilis and Saccharomyces cerevisiae Yeast Extracts on Cognitive Function in Older Adults with Mild Cognitive Impairment: A Randomized Placebo-Controlled Trial. Nutrients, 18(12), 1869.