Kefir is made by adding small, rubbery clusters called kefir grains to milk and letting the mixture ferment at room temperature for roughly 24 hours. The grains are then strained out and reused in a fresh batch, a cycle that can repeat indefinitely. What makes this process unusual is the grains themselves: they aren’t a single organism but a living colony of dozens of bacteria and yeasts bound together in a soft, gel-like structure.
What Kefir Grains Actually Are
Kefir grains look like small, white, cauliflower-shaped lumps, typically ranging from the size of a grain of rice to a walnut. They’re held together by a substance called kefiran, a polysaccharide made of glucose and galactose units that acts as scaffolding. This gel-like matrix protects the microbes inside from drying out, from nutrient shortages, and even from bacteriophages (viruses that attack bacteria).
The dominant bacterium inside the grains is Lactobacillus kefiranofaciens, which is also the primary producer of kefiran itself. Alongside it live other lactic acid bacteria, acetic acid bacteria, and various yeasts. A study analyzing grains from different Italian regions found the yeast Dekkera anomala as the most common, with bacteria like Lactococcus lactis and Streptococcus thermophilus present in smaller numbers. This diversity is what separates kefir from yogurt, which relies on just two or three bacterial strains.
You can’t manufacture kefir grains from scratch. They’re passed from person to person or purchased from suppliers who maintain live cultures. As long as the grains are fed fresh milk regularly, they grow slowly over time, and excess grains can be shared or stored.
The Basic Fermentation Process
The traditional method is straightforward. You place kefir grains in a jar, pour milk over them (typically at a ratio of about one tablespoon of grains per cup of milk), cover loosely, and leave the jar at room temperature. At around 20 to 25°C (68 to 77°F), fermentation takes approximately 24 hours. In a hot room, this can drop to 12 hours. The goal is to find the balance of grain-to-milk ratio and room temperature that produces a finished ferment right at the 24-hour mark.
During this time, the bacteria consume lactose (milk sugar) and convert it into lactic acid and acetic acid, which drop the pH to roughly 3.6 to 4.0. This is what gives kefir its characteristic tangy, slightly sour taste. The yeasts, meanwhile, produce small amounts of ethanol and carbon dioxide, which contribute a mild effervescence and a faintly yeasty aroma. The result is thicker than milk but thinner than yogurt, with a complex, slightly fizzy flavor.
Once the milk has thickened and tastes tangy, you strain the grains out using a plastic or stainless steel sieve, transfer the finished kefir to a bottle, and place the grains into fresh milk to start the next batch. The grains need fresh milk every 24 to 48 hours to stay healthy.
What Happens to the Milk During Fermentation
The microbes in kefir grains transform the milk in several ways simultaneously. The most obvious change is acidification: lactic acid bacteria break down lactose into lactic acid, which thickens the milk proteins (casein) and creates that sour flavor. Research on homemade kefir found that lactose decreases by about 20 to 25% over a standard 24-hour fermentation. This partial breakdown is why many people with mild lactose intolerance find kefir easier to digest than regular milk, though it doesn’t eliminate lactose entirely.
The yeasts in the grain carry out a separate, parallel fermentation. They produce ethanol and carbon dioxide as byproducts. In milk kefir, the alcohol content stays very low, generally well under 0.5%. The carbon dioxide is what gives freshly strained kefir a gentle fizz, especially if you cap the bottle and let it sit in the fridge for a few hours after straining.
Certain lactic acid bacteria in the grains can also synthesize B-group vitamins, particularly riboflavin (B2) and folate (B9), during fermentation. This means the finished kefir can contain slightly higher levels of these vitamins than the milk it was made from.
The low pH of the finished product also plays a protective role. Kefir’s acidity, combined with antimicrobial compounds the bacteria secrete during fermentation, inhibits the growth of common foodborne pathogens. Research has shown that kefir fermented to a pH between 3.6 and 4.0 can suppress organisms like Staphylococcus aureus, Salmonella, and Listeria, and the inhibition increases the longer fermentation continues.
Commercial Kefir vs. Homemade
The kefir you buy in a grocery store is made differently from the traditional grain-based method. Commercial producers use defined starter cultures: carefully selected, freeze-dried mixtures of specific bacterial strains. They do this because real kefir grains are unpredictable. The microbial balance shifts from batch to batch, which makes it difficult to produce a consistent product at scale. Using grains also limits shelf life, since the complex microbial community keeps fermenting slowly even under refrigeration.
The trade-off is significant. A comparison study found that none of the bacteria most characteristic of traditional kefir, specifically Lactobacillus kefiranofaciens and Lactobacillus kefiri (which make up the majority of grain-based cultures), were present in commercially produced kefirs. Commercial versions still contain beneficial bacteria and have a similar tangy taste, but they lack the full microbial diversity that traditional kefir is known for. If the probiotic complexity matters to you, homemade grain-fermented kefir is a meaningfully different product from what’s on store shelves.
Water Kefir: A Non-Dairy Alternative
Water kefir uses a different type of grain, sometimes called tibicos, to ferment sugar water instead of milk. The grains are smaller and translucent, and the base liquid is typically water with dissolved sugar, sometimes with dried fruit or a slice of lemon added for minerals and flavor. The bacterial communities in water kefir grains overlap with milk kefir (both are dominated by lactic acid bacteria), but the yeast populations differ. Milk kefir grains contain primarily non-Saccharomyces yeasts, while water kefir grains harbor a different yeast community.
The process is similar: grains go into the sugar solution, ferment for 24 to 48 hours at room temperature, and are then strained out. Many people follow this with a secondary fermentation, where the strained liquid is bottled with added fruit juice, tea, or herbs and sealed in an airtight container. This second stage happens without the grains and produces more carbon dioxide, resulting in a noticeably fizzy, effervescent drink. Water kefir from this process typically contains between 0.14% and 1.70% alcohol by volume, though products sold in stores must stay below 0.5% ABV in the United States.
Tips for Consistent Results at Home
Temperature is the single biggest variable. In winter, fermentation slows down and may take a full 24 hours or longer. In summer, the same grain-to-milk ratio might over-ferment in 12 hours, producing a very sour, separated kefir with visible whey. If your kefir is too sour, try using more milk, fewer grains, or moving the jar to a cooler spot. If it’s still thin and sweet after 24 hours, reduce the milk or move to a warmer location.
Use non-reactive utensils (plastic, wood, or stainless steel) when handling grains, and avoid rinsing them with tap water, which can introduce chlorine. The grains thrive on the milk itself. Whole milk generally produces a creamier, milder kefir than skim, because the fat buffers the acidity somewhat. Goat milk, sheep milk, and cow milk all work. If you need to take a break from brewing, grains can be stored in a small amount of fresh milk in the refrigerator for up to two weeks, though they may take a batch or two to return to full activity afterward.