Kefir spoiling usually shows up as an overly sharp sourness, a moldy smell, and the container swelling or overflowing. Since kefir is already a fermented drink, a mild tang and some fizz are normal; spoilage starts once that normal character goes well past its usual range.
The clearest sign is colored mold on the surface or along the rim of the container; pink, orange or black spots should be taken even more seriously than white mold. The second sign is the container swelling far more than usual, almost to the point of bursting.
How do you tell normal sourness from spoilage?
Fresh kefir is mildly sour, faintly yeasty and lightly fizzy; that sourness naturally increases after a few days, and that alone isn't spoilage. But once the sourness turns sharp, almost biting, and the taste turns bitter, it shouldn't be drunk anymore.
Texture changes too: kefir that normally pours smoothly turns lumpy with separated liquid once it spoils. A little watery separation is normal and stirs back in easily; if the lumps won't blend back in with stirring, that's a spoilage sign.
When are mold and discoloration dangerous?
White, green, pink or orange fuzzy spots on the surface or along the inside of the container mean mold, and once you see it the whole batch should go, even the part at the bottom that still looks clean. In a liquid product, mold spreads much faster than it does in solid food.
Kefir turning yellowish or grayish is another warning sign; fresh kefir is white to cream-colored, and once that color shifts, the drink should be treated with suspicion even without visible mold.
What does excessive fizzing and swelling mean?
Kefir naturally produces a small amount of gas as it ferments, and in a sealed container that can build a bit of pressure. But if the container swells noticeably, the lid pops on its own, or it foams over when opened, fermentation has gone further than it should.
That level of fizzing usually means the kefir sat at room temperature too long; by that point it's both overly sour and less reliable to drink.
Why does homemade kefir spoil faster than store-bought?
Homemade kefir has a shorter shelf life than commercially made kefir, since pasteurization and standardized hygiene aren't applied the same way. Kefir grains or containers that aren't cleaned thoroughly enough give unwanted mold and bacteria room to grow.
Because of that, homemade kefir needs checking more often than store-bought and is usually best used within a few days; letting it sit longer hurts both taste and safety. Washing the spoon, strainer and jar used for fermenting in hot water each time also noticeably lowers the odds of unwanted bacteria getting into the batch.
How long does kefir keep?
Unopened store-bought kefir keeps safely in the fridge until the date printed on the package; once opened, that window generally drops to 5-7 days. Homemade kefir, on the other hand, loses quality fast if it isn't used within 3-4 days in the fridge.
Kefir should never sit at room temperature for hours; fermentation speeds up at room temperature, and the drink can turn overly sour and fizzy much sooner than expected. A tightly sealed jar kept on the coldest shelf of the fridge holds its freshness a little longer than one left in the door.