Kumis (also spelled koumiss) is a lightly fizzy, sour, mildly alcoholic drink made by fermenting mare’s milk with a combination of lactic acid bacteria and yeast. The traditional version uses raw mare’s milk, but you can make a close approximation at home using cow’s milk with a few adjustments. The finished product typically contains about 2% alcohol and has a tangy, slightly effervescent quality unlike any other fermented dairy.
What Makes Kumis Different From Other Fermented Milks
Most fermented dairy products, like yogurt or kefir, rely primarily on bacteria. Kumis depends on a partnership between lactic acid bacteria and yeast working together. The bacteria produce lactic acid, giving kumis its sour tang, while the yeast converts sugars into alcohol and carbon dioxide, creating the fizz. A 1:1 ratio of bacteria to yeast produces the best balance of flavor and texture.
This dual fermentation is what gives kumis its distinctive character: thinner than yogurt, more sour than milk, gently carbonated, and with a mild boozy warmth. Traditional producers in Central Asia and Mongolia have been making it this way for over 2,500 years, originally as a way to preserve the nutrients in mare’s milk during nomadic life.
Starting With the Right Milk
Mare’s milk is the traditional base, but it’s nearly impossible to find in most countries. The key difference between mare’s milk and cow’s milk is lactose content. Mare’s milk contains significantly more sugar than cow’s milk, and that extra sugar is what feeds the yeast and produces kumis’s carbonation and alcohol. Mare’s milk is also lower in fat and protein.
To approximate mare’s milk using cow’s milk, you’ll need to make two adjustments. First, use low-fat or skim milk to get closer to mare’s milk’s thin consistency. Second, add a source of sugar to boost the lactose level. Honey works well for this, as does plain white sugar. Adding about one to two tablespoons of honey per liter of milk brings the sugar content closer to what yeast needs to produce proper carbonation and the characteristic 2% alcohol level.
What You Need
- Milk: 1 liter of low-fat or skim milk (pasteurized is fine)
- Starter culture: Kefir grains or a few tablespoons of active plain kefir (this provides both the lactic acid bacteria and yeast you need)
- Sugar source: 1 to 2 tablespoons of honey or sugar
- Fermentation vessel: A clean glass jar with a loose-fitting lid or cloth cover
- Airtight bottles: Swing-top glass bottles or plastic soda bottles for the carbonation stage
Kefir grains are the most accessible starter for home kumis because they naturally contain both lactic acid bacteria and yeast species. The dominant bacteria in traditional kumis include several Lactobacillus and Lactococcus species, while the yeast side includes species closely related to those found in kefir grains. You won’t get an identical microbial profile, but the fermentation dynamic is the same: bacteria souring, yeast fizzing.
Step-by-Step Fermentation
Warm your milk to room temperature, roughly 20 to 25°C (68 to 77°F). Cold milk slows fermentation significantly, and milk that’s too hot will kill the microorganisms. Dissolve your honey or sugar into the warm milk and stir well.
Add your starter culture. If you’re using kefir grains, add about two tablespoons per liter. If you’re using liquid kefir as a starter, use three to four tablespoons per liter. Stir gently to distribute the culture throughout the milk.
Cover the jar loosely. You want air exchange but not contamination, so a cloth secured with a rubber band or a lid set on top without sealing works well. Place the jar somewhere at a stable room temperature, out of direct sunlight.
Here’s where kumis diverges from simple kefir: agitation matters. Traditional kumis makers stirred or churned the fermenting milk regularly, sometimes hundreds of times per day. You don’t need to be that dedicated, but stirring vigorously every few hours encourages yeast activity and keeps the fermentation even. This agitation introduces small amounts of oxygen that help the yeast thrive alongside the bacteria.
Fermentation peaks at around 12 hours, when nutrient levels and microbial activity are at their highest. For a mild, lightly sour kumis (sometimes called “light” kumis), ferment for 12 to 18 hours. For a stronger, more acidic version, go 24 to 48 hours. Light kumis has a higher pH (around 4.5 to 5.0), while strong kumis drops to a pH of 3.3 to 3.6, making it noticeably more sour and slightly higher in alcohol.
Building Carbonation
Once your primary fermentation is done, strain out any kefir grains if you used them. The liquid should look thinner than yogurt, slightly foamy, and smell sour but clean. Now transfer the liquid into airtight bottles, leaving about two inches of headspace at the top.
Seal the bottles and leave them at room temperature for another 12 to 24 hours. This secondary fermentation traps the carbon dioxide produced by the yeast, carbonating the drink. If you’re using plastic bottles, you can squeeze them periodically to check pressure. When the bottle feels firm and doesn’t give under pressure, carbonation is ready.
Move the bottles to the refrigerator to slow fermentation and chill the kumis. Be cautious when opening, as the carbonation can be vigorous. “Burping” the bottles (briefly cracking the seal) once or twice during secondary fermentation reduces the risk of overflow or, in extreme cases, a burst bottle.
Storage and Shelf Life
Refrigerated kumis keeps well for about 15 to 20 days at 4°C (39°F). The flavor will continue to develop slowly in the fridge, becoming more sour over time as bacteria keep working at a reduced pace. The best drinking window is typically within the first week, when the balance of tang, fizz, and mild sweetness is most pleasant.
Shake or stir before serving. Kumis naturally separates during storage, with heavier proteins settling to the bottom. This is normal and not a sign of spoilage.
Signs of a Good Batch vs. a Bad One
A successful kumis is uniformly white or slightly off-white, smells cleanly sour (like yogurt with a hint of yeastiness), and tastes tangy with gentle effervescence. The texture should be thin and pourable, closer to buttermilk than yogurt.
Spontaneous fermentation without well-defined starter cultures, or fermentation under poorly controlled conditions, can allow harmful bacteria to grow. Watch for these warning signs: pink, green, or black discoloration; a foul, rotten, or strongly “off” smell distinct from the expected sourness; visible mold on the surface; or a slimy, ropy texture. Pathogens like E. coli, Salmonella, and Staphylococcus have been detected in poorly made fermented dairy products. If anything looks or smells wrong, discard the entire batch. Clean all equipment thoroughly with hot water before starting again.
Using a reliable starter culture rather than relying on wild fermentation is the single most important safety step. It ensures the beneficial microorganisms dominate the milk quickly, dropping the pH fast enough to inhibit harmful bacteria before they can establish themselves.
Improving Your Kumis Over Time
Your first batch will likely taste more like fizzy kefir than authentic kumis. That’s expected. Traditional kumis cultures have been maintained and passed down for generations, developing a specific microbial community tuned to mare’s milk. With repeated batches using the same starter, your culture adapts and the flavor profile deepens.
Experiment with fermentation time to find your preferred balance. A shorter ferment gives a milder, sweeter drink. A longer ferment pushes the sourness and alcohol content higher. Adjusting the amount of added honey also shifts the final character, since more sugar means more fuel for the yeast and a more carbonated, slightly boozier result. Temperature matters too: warmer conditions speed everything up, while cooler temperatures slow fermentation and can produce a smoother, less aggressively sour product.
In Central Asian tradition, kumis is valued not just as a drink but as a health food. It’s considered supportive of digestion and overall vitality, which aligns with what we now understand about probiotic-rich fermented foods and their effects on gut health. The living bacteria and yeast in a properly made batch are the same types of organisms found in other well-regarded fermented foods.