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Fermented milk

Ingredients: milk
Taste: creamy, tart
Popular in: Eastern Europe
Time: 24+ hours
Dominant microbes: L. kefiranofaciens and L. kefiri
Tools: jar, coffee filter cover




Basic Kefir


kefir grains starter

coffee filter
rubber band
tea strainer

  1. For 1L of pasteurized milk in a clean glass jar, add 60g tbsp of kefir grains (or follow the instuctions provided)
  2. Cover with a coffee filter or cloth, and tighten with a rubber band to prevent bugs from getting in, but allowing the kefir surface to be exposed to air
  3. Let ferment at room temperature (68°-75°F)
  4. After about 36 hours, check your kefir. It should be thick and have a creamy or yeasty smell; if it looks a little clumpy and there is separation of the kefir and a yellowish water, that’s ok! Just mix it in. It means next time you need to add more milk, or ferment for less time.
  5. Once the kefir is ready, use a tea strainer to filter out the kefir grain; you can also scoop them out with a spoon. Accidentally eating a grain is completely safe. Just an extra dose of probiotics and EPS.
  6. Mix the kefir grains with a fresh batch of milk, or store them for future use.
kefir grains
kefir grains look like small jelly clumps, made of a polysaccharide matrix synthesized by kefir microorganisms and also acting like a reservoir for the various bacteria and fungi to inoculate the milk and ferment it into kefir. You can find kefir grains from various online retailed (you want milk kefir grain-- water kefir grains are different!)

Kefir grains will grow as you continue passaging them, so you’ll eventually be able to make larger batches or share with friends!

kefir microbiology

what are kefir grains?
Kefir grains small gelatinous granules made of EPSs called kefirans, which house a mixture of lactic acid bacteria, yeast, and acetic acid bacteria. 

How are kefir grains formed?
In the beginning, there were the bacteria Lactobacillus kefiranofaciens and the yeast Saccharomyces turicensis, sitting in milk. Together, the began to produce kefirans and aggregate, and forming small granules. At the same time, another bacteria, L. kefiri, and the yeast K. marxianus HY1 and P. fermentans HY3, begin to adhere to the surface of the L. kefiranofaciens and S. turicensis granules, producing biofilm, and enhancing the aggregation and growth of the lactic acid bacteria and yeasts, making the granules larger and producing visible kefir grains (Prado et al. 2015)

or extracellular polymeric substances are polymers made sugars and proteins, and sometimes including DNA, lipids, and other substances. Exopolysaccharides (often referred to as EPSs) are the sugar  components of EPS, which form large complexes of many sugars (polysaccharides)

How do kefir grains work?
In a 2021 Nature Microbiology paper presented a “basecamp lifestyle” of kefir fermentation (Blasche and Kim et al. 2021). Acting as a reservoir, microbes in the kefir grains will move from the grain to the milk, drive the milk to kefir fermentation process, and continue to form new kefir grains. The composition of the kefir grains themselves does not change over the course of fermentation, only grow in number. However, the microbiome of the milk itself changes rapidly over the fermentation period creating a thick and tart drink. 

First, L. kefiranofaciens is the dominant strain in the milk. Then L. lactis and L. mesenteroides begin to rapidly grow. Towards the end, most microbial species have reached their maximum growth and slow down, but A. fabarum and L. kefiri continue to grown. By the end of the fermentation period, L. kefiri is the dominant strain (~80% of bacteria in kefir).

What microbes are found in kefir and kefir grains?

additional reading

The basecamp lifestyle
Blasche, Sonja, et al. "Metabolic cooperation and spatiotemporal niche partitioning in a kefir microbial community." Nature Microbiology 6.2 (2021): 196-208.

Basic kefir microbiology
Prado, Maria R., et al. "Milk kefir: composition, microbial cultures, biological activities, and related products." Frontiers in microbiology 6 (2015): 1177.

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