Reading your starter: when is it actually ripe?

A starter at peak is one with the maximum population of active yeast and bacteria for that feeding cycle. Used at peak, it ferments your dough fastest, with the cleanest flavor profile, and the most predictable timing. Used past peak, it's in decline: fewer active cells, more acid, weaker rise. Used before peak, it hasn't built enough population yet and your dough will under-ferment.

The good news is that reading peak is one of the easiest things in sourdough once you know what to look for. The bad news is that the most-cited test (the float test) only works some of the time. The reliable signals are different.

What 'peak' actually looks like

After a feeding, your starter goes through three phases.

1. 1.Lag phase (first 1–4 hours after feeding). The microbes are eating, multiplying, but not yet producing enough gas to inflate the starter much. Volume changes slowly. Surface looks the same as right after feeding.
2. 2.Active phase (peak ascent). The starter rises visibly. Bubbles appear at the surface and through the sides if you're using a clear jar. The volume climbs, sometimes dramatically. The smell becomes more pleasant: yogurt-like, mildly sour, slightly bready.
3. 3.Decline (past peak). The starter rises to its maximum, holds briefly, then begins to fall. The surface goes from domed to flat to slightly concave. Bubbles thin out. The smell sharpens: more acetic, more vinegary.

Peak is the moment between active phase and decline: when the starter has risen as high as it's going to rise, just before it starts falling back. For most starters at room temperature, this window is short (maybe 30–60 minutes) before the descent starts.

The three reliable signals

1. Volume: at least doubled from the post-feeding mark

This is the most reliable single signal. Mark the level of the starter right after each feeding (a rubber band around the jar, or a piece of tape). Peak is reached when the starter has at least doubled in height from that mark; for many starters, it triples.

If your starter never doubles, something is off, usually too low a feeding ratio, too cold a kitchen, or a starter that's still establishing. A healthy mature starter should reliably more than double after each feeding.

2. Surface: domed, then flat

While ascending, a healthy starter has a slightly domed surface (gas pushing up from underneath). At peak, the dome flattens. As it begins to decline, the centre often drops below the edges, forming a shallow concave shape.

Catch it at flat-but-not-yet-sunken and you've caught peak.

3. Smell: pleasant, not sharp

A starter at peak smells mildly sour, often like yogurt or mild beer or apple cider, never sharply vinegary. If it smells like nail polish remover (acetone) or strong vinegar, it's past peak. If it smells like raw flour with no complexity, it's not there yet.

Smell is harder to learn than the other two signals, but once you've done it for a few weeks you'll know peak by smell from across the kitchen.

Why the float test isn't enough

The float test (drop a spoonful of starter into water and see if it floats) works because gas-rich starter is less dense than water. But the test only confirms one thing: there's enough trapped gas to float. It doesn't distinguish between a starter at peak and one that's already declined.

A starter that's two hours past peak can still pass the float test. So can a starter that's been sitting hungry for a day with hooch on top. The test gives you a yes/no on "has any gas in it," not on "is at peak right now."

Use the float test as a quick pre-bake confirmation if you want, but don't use it as your primary peak signal. The volume and surface checks are far more reliable.

The one experiment that calibrates everything

Spend one weekend doing this once and you'll understand your starter for the rest of the year.

1. 1.Feed your starter at a known ratio (1:5:5 is a good choice). Note the time and the kitchen temperature.
2. 2.Mark the post-feeding level with a rubber band.
3. 3.Check it every hour. Take a quick photo each time so you can compare later.
4. 4.Note the time when it doubles. Note the time when it triples (if it does). Note when it peaks (flat dome, just before decline).
5. 5.Note the smell at peak.

From this single experiment you'll know roughly how long your starter takes to peak at your kitchen temperature, with your feeding ratio, with your flour. From there, the Q10 rule predicts cooler/warmer days. A 5 °C / 9 °F cooler kitchen ≈ peak takes ~30% longer; warmer ≈ ~30% shorter.

What if it never seems to peak?

Three causes, in order of likelihood:

1. 1.Too cold. Below 18 °C / 64 °F the rise is slow enough that you can't tell it's rising. Move it somewhere warmer for a feeding cycle and watch what happens.
2. 2.Feeding ratio too low. A 1:1:1 starter at warm room temperature can peak in 3–4 hours and you might miss it. Try 1:5:5 and watch over a longer window.
3. 3.Genuinely weak culture. If you've ruled out temperature and ratio, the culture itself may be underdeveloped. A few days of twice-daily 1:5:5 feedings at warm temperature usually fixes this.

Using past-peak starter

If you miss peak by a couple of hours, the starter is still useful, just slightly less powerful. You can compensate by using a bit more in the dough (10–15% more), or by pushing your bulk fermentation slightly longer.

If you miss peak by more than 6–8 hours and there's hooch on top (a layer of greyish liquid), the starter is hungry and acidic. You can use it, but for serious bread it's better to give it a fresh feeding and use it at the next peak. The flavor and rise will both be better.

The shortcut

Volume doubles, surface flat, smell mild: your starter is at peak. Use it now. The float test is optional.