Your sourdough starter is a living culture, and like any living thing, it has good days and bad days. Some days it doubles in size predictably; other days it does nothing. Understanding why your starter behaves the way it does is the first step toward managing it consistently. Most starter problems fall into a handful of categories, and most are fixable without starting over.
My Starter Isn't Rising
The most common starter complaint. The starter was active last week, but now it just sits there, barely bubbling. The most likely cause: temperature. Below 20°C, yeast activity slows dramatically. Below 15°C, it's barely active. Move your jar to a warmer spot — above the refrigerator, in a sunny kitchen corner, or in a cooler with a warm water bottle.
Second most likely: your flour has run out of nutrients. A starter fed only white flour eventually depletes the nutrients available to the yeast. Try feeding with whole wheat or rye flour for a few days — the bran and germ contain more nutrients and wild yeast that can revive a sluggish starter.
Third: the starter is too acidic. If you've been feeding consistently without discarding, the accumulation of acid can inhibit yeast activity. Try a heavy discard and feed (discard 80% of the starter, then feed with equal parts flour and water) for 2-3 days to dilute the acid.
It Smells Terrible
Some smells during the first week of a new starter are normal. But a healthy established starter should smell pleasantly sour — like yogurt, tangy bread, or淡淡 wine. An ammonia smell means the starter is starving and needs feeding. A rotting garbage smell means something has gone wrong. A pink or orange color at any point means contamination — discard and start over.
The acetone/nail polish remover smell (ethanol) is normal in small amounts — it means the yeast is actively fermenting. If it persists after feeding, the starter is over-fermented before feeding and needs more frequent feedings or a smaller feeding ratio.
Mold on the Surface
Any visible mold — green, blue, black, white fuzzy spots — means discard immediately and start over. A thin white film on the surface that isn't fuzzy is usually just a skin from the surface drying out. Skim it off, feed normally, and the starter will recover.
To prevent surface mold: keep the starter covered with a loose lid or cloth (not airtight, which prevents gas escape but can trap moisture), feed regularly, and don't let the starter sit at room temperature for more than 2-3 days without feeding.
It Rose Too Much and Collapsed
A starter that rises aggressively and then falls is normal if it was fed recently. But if it rises and stays risen for days, then suddenly collapses, the yeast may have run out of food and died. This is the classic sign of an unfed starter: the yeast consumed everything and the bacterial acids have made the environment inhospitable.
The fix: discard most of the starter and feed with fresh flour and water. If this happens repeatedly, try feeding more frequently (every 12 hours instead of every 24) or increase the feeding ratio (more flour, same starter).
Separated into Layers
A starter that separates into a dark liquid on top and a dense mass on the bottom has "hoached" — it's not a disaster, but it needs attention. The liquid is called "hooch" and is a mixture of alcohol and acetic acid. It means the starter is hungry and needs feeding.
Pour off the hooch, give the starter a thorough stir, and feed normally. The starter will recover within 2-3 feedings. Some bakers actually prefer a slightly hoached starter for bread — the alcohol evaporates during baking and the acidity can add complexity to the flavor.