How to make milk kefir at home

TL;DR

You need live milk kefir grains to start; there is no powder substitute that produces ongoing kefir. Once you have grains:

Put 1 tbsp of grains in a clean quart jar. Pour in 2 cups of whole milk. Cover with a tea towel and rubber band. Leave at room temperature, 65 to 75°F, for about 24 hours. The milk thickens, develops a tangy yeasty smell, and turns into kefir.

Pour through a plastic strainer over a bowl. The thick liquid that drains through is finished kefir; the grains stay in the strainer. Put the grains back in a clean jar and add fresh milk to start the next cycle. The kefir in the bowl is ready to drink, or seal it in a jar at room temperature for another 12 to 24 hours to “second-ferment” for carbonation and more complex flavor.

Repeat daily. The grains multiply over time.

What milk kefir actually is

Milk kefir is a fermented dairy beverage made by inoculating milk with kefir grains: small, soft, cauliflower-like clusters of bacteria and yeasts bound together in a polysaccharide matrix. The grains are a true symbiotic culture, hosting dozens of microbial species (notably Lactobacillus kefiri, Lactococcus, Leuconostoc, and yeasts including Saccharomyces and Kluyveromyces) living together in stable proportions.

When grains are added to milk, the bacteria consume lactose and produce lactic acid; the yeasts consume sugars and produce small amounts of ethanol and CO2. Over 24 hours, the milk thickens, develops a tangy yeasty flavor, and becomes finished kefir. The grains themselves do not dissolve; they are reusable indefinitely and multiply over weeks and months of regular feeding.

Kefir is microbially much more diverse than yogurt, which uses only a few thermophilic species. Some research suggests this diversity is why kefir is often better tolerated by people with mild dairy sensitivities; the broader culture appears to break down more of the milk components during fermentation.

The conditions

Three variables matter for milk kefir:

1. Live grains. This cannot be substituted. Starter powders sold as “kefir culture” produce a kefir-like drink for a few batches but do not sustain themselves the way real grains do. Real grains are visibly grain-like (clusters of soft cauliflower-shaped curds), are off-white to cream colored, and smell mildly dairy-tangy.
2. Whole milk, not UHT. Whole cow’s milk is the standard. Pasteurized is fine; ultra-pasteurized (UHT, the kind that sits unrefrigerated in shelf-stable cartons) is heat-damaged enough that the grains often struggle to ferment it properly. Raw milk works well. Goat and sheep milk work; soy and coconut work for occasional batches but not long-term grain maintenance.
3. Temperature: 65 to 75°F. Below 60°F the ferment slows significantly and may not finish in 24 hours. Above 80°F it ferments fast and over-sours quickly. Most kitchens are in the right range year-round.

The ratio of grains to milk determines how fast the batch ferments. The default of 1 tbsp grains per 2 cups milk finishes in about 24 hours at room temperature. More grains means a faster ferment; fewer grains means a slower one.

Step by step

1. Set up the first jar

Wash a quart glass jar in hot water; rinse well to remove any soap residue. Soap residue can damage the grains. Air dry.

Place 1 tbsp of live kefir grains in the jar. If your grains arrived in a small amount of milk, just pour the whole packet in; the residual milk does not need to be drained.

Pour 2 cups (475 ml) of whole milk over the grains. Stir once with a wooden spoon (brief metal contact is fine if that is what you have; just don’t store metal in the jar).

2. Cover and place

Cover the mouth of the jar with a tea towel, a coffee filter, or a square of cloth. Secure with a rubber band. The cover lets CO2 out and keeps fruit flies and dust out, while allowing the slight aerobic respiration the yeasts perform.

Place the jar somewhere stable at room temperature, out of direct sunlight. A countertop, open shelf, or pantry works well. Avoid the top of the refrigerator (too warm) and unheated rooms in winter (too cold).

3. Wait about 24 hours

The ferment progresses gradually. Hour 0 looks like fresh milk. By hour 12, you may see slight thickening near the top and faint bubbles. By hour 18 to 24, the milk has noticeably thickened, the consistency has shifted from pourable to soft yogurt-like, and the smell has developed into a pleasant tangy-yeasty aroma.

Visual cue for done: tip the jar gently to one side. If the contents move as a single thick mass rather than splashing like fresh milk, the ferment is ready. A bit of clear yellowish whey separating at the edges is normal and a sign of fermentation; significant pooled whey at the bottom means it has gone a bit long but is still fine.

4. Strain

Place a plastic strainer (fine mesh, large enough to hold the contents) over a clean glass bowl or large jar. Pour the contents of the jar through the strainer. The kefir liquid drains through into the bowl; the grains stay in the strainer.

The kefir may need gentle nudging through the mesh with a wooden spoon, especially toward the end. Do not press hard; just stir lightly. After most of the liquid has drained, you will have a small mass of kefir grains in the strainer, looking essentially the same as when they went in (off-white clusters, mildly stickier than before).

5. Restart the next batch immediately

Rinse the original jar briefly with cold (unchlorinated) water. Place the grains from the strainer back into the jar. Pour 2 cups of fresh milk over. Cover, return to room temperature. Cycle continues.

Grains live by being fed. They should not be stored in plain water, in air, or in the strainer. Move from one milk to the next promptly. The total time between strain-and-refill should be measured in minutes, not hours.

6. Drink, or second-ferment

The finished kefir in the bowl is ready to drink immediately. It will have a tang similar to a thinned yogurt, with a slight effervescence and faint yeasty aroma.

Optional second ferment: pour the strained kefir into a clean jar, seal the lid (this is one of the only times you do this), and leave at room temperature for another 12 to 24 hours. The yeasts continue working in the absence of grains, building CO2 and developing more complex flavor. After second ferment, chill in the refrigerator before opening, and open slowly over a sink; pressure can be significant.

Second fermenting also allows flavor additions: a slice of fresh lemon, a handful of crushed berries, a piece of fresh ginger, or a sprig of mint placed in the jar during the second ferment infuses well.

What you might see along the way

A whey layer separating during fermentation. Normal as the ferment progresses. A small layer of yellowish whey at the bottom or edges by hour 24 is fine. Large pools of whey with curd at the top means the batch over-fermented; still safe, just tart.

Grains floating to the top of the jar. Common during active fermentation. CO2 produced by the yeasts attaches to the grains and lifts them. Stir occasionally if you want them more distributed, or just let them be.

Grains growing visibly larger over weeks. Healthy grains multiply. Expect 20 to 50% growth per month. When you have noticeably more grains than you started with, either keep them as backup, share with someone, or shift to a slightly larger milk volume to maintain the right ratio.

A pleasant yeasty bread-dough aroma. Normal and a good sign. The yeasts in the culture produce small amounts of ethanol and aromatics during fermentation. The smell should never be sharp, vinegary, or rotten.

Kefir that tastes slightly fizzy on the tongue. A first-ferment kefir typically has very mild carbonation, especially if fermented warm. Second-fermented kefir is more pronouncedly fizzy.

The grains feeling firmer or softer than usual. Grain texture varies with milk fat content, temperature, and ferment duration. Soft grains in a warm summer kitchen and firmer grains in a cool fall kitchen is normal. Slimy grains or grains that feel like jelly indicate stress.

Common mistakes

Using ultra-pasteurized (UHT) milk. The shelf-stable cartons of milk that don’t need refrigeration before opening have been heat-treated above 280°F. Proteins are denatured enough that the grains often cannot ferment it properly. The first batch may work; subsequent batches weaken. Use regularly pasteurized or raw milk.

Storing grains in chlorinated water during a break. If you need to pause kefir-making for a few days, store the grains in fresh milk in the refrigerator (slows fermentation, keeps grains fed). Do not store in plain water; chlorinated tap water especially will damage them.

Letting the grains dry out or sit empty. Grains die without fresh milk within 2 to 3 weeks. Cold storage extends that window to 4 to 6 weeks. Beyond that, grains usually do not recover.

Sealing the jar during the first ferment. Pressure builds. The first ferment needs to off-gas, hence the breathable cover. Sealed jars during the first ferment risk popping open or cracking.

Washing grains with soap or hot water. Both kill grains. If you absolutely need to rinse them (rare; usually not needed), use cool unchlorinated water and a brief swirl. Most fermenters never rinse their grains.

Using too many grains for a small milk volume. The classic “my kefir is always too sour” issue. Grain volume should be no more than 1 tbsp per 2 cups of milk for a 24-hour cycle. More grains shorten the cycle drastically and over-sour any standard batch.

Treating finished kefir as shelf-stable. Once strained, finished kefir keeps in the refrigerator for 2 to 3 weeks but is past its peak after a week. The grains, not the finished kefir, are the long-term keepers.

Troubleshooting

Milk does not thicken after 24 hours. Most likely UHT milk. Try a different brand of pasteurized whole milk. Other causes: very cold kitchen (move to warmer spot), too few grains for the milk volume, or grains that have weakened from prolonged storage.

Kefir tastes acetone or solvent-like. Yeast stress. Usually from too-warm fermentation or too-long fermentation. Shorten the cycle to 18 hours and move to a cooler spot. The off-flavor should fade within 2 to 3 batches.

Grains stopped multiplying. Slowed growth often means the milk is too low in fat (skim and 1% milk produce weaker growth) or the grains are slightly stressed from temperature swings or chlorinated water. Switch to whole milk, give grains a stable warm spot, and growth usually resumes within 2 weeks.

Grains turned slimy and the kefir is stringy. Stress response to over-acidic conditions or to certain milk proteins. Strain the grains, rinse briefly in fresh whole milk, and start a new batch with extra milk volume (less grains per milk). Usually resolves within 3 to 5 batches.

A pinkish or yellowish discoloration on grains or in the milk. Contamination. Discard the batch. If you have backup grains, switch to them. If not, rinse the grains in fresh milk and move to a clean jar; if discoloration recurs in the next batch, the grains are compromised and need to be replaced.

Fuzzy growth in the jar. Mold. Discard the entire batch. Most often happens when grains have been left at room temperature in finished kefir for too many days; the surface dries enough for mold to take hold. Maintain the daily cycle.

For variations once you have a reliable routine going (water kefir, kefir cheese, kefir-second-fermented sodas), see dairy ferments. For the microbiology shared with other live cultures, see the fundamentals. For surface diagnostics, see kahm yeast vs mold.