Overview
Milk kefir is made by fermenting milk with “kefir grains” — not a grain at all, but a symbiotic culture of bacteria and yeasts held together in a polysaccharide matrix called kefiran. The grains are reusable: they are strained out of finished kefir and can be added to a fresh batch of milk repeatedly. Kefir traces its origins to the North Caucasus mountains, where it has been made for centuries.
Microbial diversity
Kefir grains typically host a wider range of microorganisms than yogurt cultures — commonly including several Lactobacillus species, Leuconostoc, Acetobacter, and yeasts such as Saccharomyces and Kluyveromyces — compared with the two required bacterial species in standard yogurt. This diversity is one reason kefir is often marketed as a broader-spectrum probiotic food, though the exact strain composition varies by batch and source.
Fermentation and composition
Milk ferments with kefir grains over roughly 12–48 hours at room temperature, producing lactic acid, a small amount of alcohol (typically well under 2%), and carbon dioxide, which gives finished kefir its characteristic tang and light effervescence. The fermentation process breaks down a portion of the milk’s lactose, and many people with mild lactose intolerance tolerate kefir better than plain milk, though it is not lactose-free.
Nutritional profile
Milk kefir provides protein, calcium, and B vitamins (including B12) similar to the milk it’s made from, along with live bacterial cultures. As with other fermented dairy, exact nutrient values vary by brand and fat content, so checking the label is the most reliable way to compare products.
Safety notes
Because kefir contains trace alcohol and live microbial cultures, people who are pregnant, immunocompromised, or advised to avoid fermented foods for a specific medical reason should check with a clinician before adding it to their diet. Kefir made from cow’s milk is not appropriate for those with a dairy allergy, and lactose content — while reduced — is not eliminated, so severely lactose-intolerant individuals may still react.
Sources
- NIH Office of Dietary Supplements — probiotics and fermented food background
- USDA FoodData Central — fdc.nal.usda.gov
Clinically reviewed by Dr. Shilpa Thakur. Last reviewed 2026-07-20. Educational information, not medical advice.