A "peaked" starter is a starter at the precise moment when yeast and lactic acid bacteria have maximized gas production, the culture is biologically active, and the resulting dough will have the best balance of rise and flavor. Catching that moment is one of the hardest skills in sourdough — and the one most worth learning.
A sourdough starter is a symbiotic culture of wild yeast (mainly Saccharomyces cerevisiae and Candida humilis) and lactic acid bacteria (Lactobacillus sanfranciscensis, L. brevis, and friends). After a feed, the yeast population grows exponentially as it consumes the maltose and glucose released from the flour. CO₂ production peaks when the yeast population reaches its maximum density — typically when the culture has tripled in volume from its starting point.
At peak, three things are simultaneously true. The yeast population has not yet started declining. Gas production rate is at its maximum. Acidity has not yet collapsed the gluten-forming proteins in the dough. After peak, all three reverse: acid keeps accumulating, gas production slows, and yeast cells begin to die or sporulate. A starter used past peak still leavens bread — but more slowly, with more sour flavor, and with reduced rise.
The most reliable single sign is volume. A healthy starter at peak has tripled from its post-feed level. Mark the starting volume with a rubber band on the jar. Peak is when the culture reaches 3× and the dome at the top has just stopped rising — or has begun to flatten very slightly. The exact multiplier depends on your starter strain and feeding ratio: a 1:1:1 feed peaks at 4–5× volume, while a 1:5:5 feed peaks at 2.5–3×.
A starter at peak has a smooth, domed surface. As it crosses peak, the dome flattens. A minute or two past peak, you see a slight depression form in the center as gas escapes faster than it's produced. If you see a deep crater in the surface or visible "cliffs" where the dome has collapsed against the jar walls, you are 30 minutes to 2 hours past peak.
At peak, the side walls of the jar show a dense network of small to medium bubbles. The structure looks "alive" — open and webby but still contained. Past peak, the bubble walls collapse, leaving large irregular voids and a slick, glossy surface on the sides where gas has escaped.
A peaked starter smells primarily of bread — yeasty, slightly sweet, with a clean lactic tang underneath. There should be no acetone, no vinegar punch, and no nail-polish-remover sharpness. If the aroma is dominantly acetic (sharp vinegar) or has acetone notes, the culture is significantly past peak and acid-dominant.
Press your finger lightly into the surface. At peak, the indentation springs back about halfway and the texture feels light and slightly resistant. Pre-peak, the starter feels dense and resists the press. Past peak, the indentation stays — the culture has lost gas-holding capacity and behaves more like a liquid than a foam.
Bring the jar near your ear. At peak, you can often hear soft, irregular pops as bubbles break the surface. This is the most subjective sign but the most fun to observe. Past peak, the bubbles get bigger, slower, and the pops change character — fewer but louder.
The float test — drop a spoonful of starter into water and see if it floats — is the most widely cited test in home sourdough, and the most misunderstood. It works on a simple physical principle: if the starter is full of trapped gas, it floats. If gas has escaped, it sinks.
The problem is timing. The float test passes throughout a wide window — from about 70% rise all the way through peak and 1–2 hours past peak. So a starter that floats might be at peak, near peak, or already past peak. The float test gives a binary result (ready / not ready) where you actually want a precise window.
Worse, the test can fail at peak under certain conditions. Whole-grain or rye starters at high hydration sometimes don't float at peak because the gas bubbles are distributed differently. Low-hydration stiff starters (50% water) almost never float regardless of activity.
Many recipes say "feed your starter 6–8 hours before use." This works some of the time. It fails when:
This is the core problem S.D Timer was built to solve. By taking your target bake time, ambient temperature, inoculation ratio, and flour type as inputs, the app calculates the feed time backward — so peak lands exactly when you need it. Visual signs are still the final check, but the schedule gets you within a 30-minute window without trial and error.
📅 Plan Your Next Bake with S.D TIMERTo catch peak reliably, set up two reference points. First, take a photo of your starter immediately after feeding and mark the level with a rubber band. Second, take a photo at the projected peak time and compare to the rubber band line. If the culture has tripled and the dome is still rising, you're 30–60 minutes pre-peak. If it has tripled and the dome is just stable or flattening, you are at peak. Use the starter within the next hour for best results.
This works even better when paired with a quantitative schedule. Most home bakers underestimate how much temperature affects peak time. A 4°C rise from 22°C to 26°C cuts peak time by roughly 30%. A 4°C drop from 22°C to 18°C extends it by 50%. Without compensating for this, your peak will drift by 2–4 hours between summer and winter feeds.
If you catch your starter pre-peak (say, only 80% risen), you have two options. Wait — typically another 30–90 minutes at room temperature gets you to peak. Or refrigerate it: cold slows the population dramatically, and you can effectively pause progress for 12–18 hours. When you bring it back to room temperature, peak resumes within 1–2 hours.
If you catch your starter past peak, you can still bake with it, but expect slower rise and more sour flavor. Two recovery options: feed it again at the same ratio and wait 2–4 hours for the next peak; or use it as-is and add 10–20% commercial yeast as a safety net. Past-peak starter that has fully collapsed (surface crater, sharp vinegar smell) is generally too acidic for a same-day bake — feed it twice over 24 hours to bring acidity back down and yeast population back up.
Every starter has a personal rhythm. Track peak time over a week of consistent feeds (same ratio, same flour, same ambient temperature) and you'll find your starter's natural cadence. Most home starters fall into one of three rough patterns:
Once you know your starter's pattern, S.D Timer's reverse-calculation can match your schedule to it. You input your target time and the app computes which inoculation ratio and feed time produces peak at the right moment. The combination of knowing your starter and letting the math do the timing is what turns sourdough from guesswork into a planned process.
Peak is a 30–60 minute window where your sourdough starter is at its biological best. Catching it consistently is the single biggest leap in sourdough quality. Three sources of information work together: the visual signs above (volume, surface, bubbles, aroma, texture, sound); your starter's personal peak pattern (built over a few weeks of observation); and a quantitative schedule that accounts for inoculation ratio, temperature, and flour type. Pair all three and you'll catch peak almost every time.