7 Signs Your Sourdough Starter Is Ready to Bake — Don't Miss #4

The foundational sign. Doubled means the top of the starter is at 2× the level it was right after feeding (the rubber band mark). Not 50% rise, not almost doubled — actually doubled. A starter that doubles in 4-8h at 22-24°C has the activity level needed to leaven bread reliably. If it doubles in 12-16h at that temperature, it works but is slower than optimal; a stronger feed (1:5:5) the day before a bake will bring it to peak faster.

At peak, the surface of a well-fed starter bulges upward in a dome shape — it's full of CO2 bubbles that haven't yet collapsed. A flat surface means it hasn't peaked yet or peaked and fell slightly. A sunken, liquid-looking surface means it's past peak and on its way down. Time your bake for when the dome is at its highest.

Look at the jar from the side. At peak, you'll see a network of irregular, stretchy-looking bubbles clinging to the glass — the gluten web the yeast CO2 is trapped in. Fine bubbles in an even grid mean low-gluten flour (rye or high whole-grain). Irregular, open, stretchy-looking bubbles mean good gluten development and a well-leavened starter. No bubbles at the sides, even at 'peak height', means under-developed fermentation.

This is the most important sign of readiness and the one that gets least attention. A single doubling tells you today's starter is active. Consistent doubling at the same time window, across five or more consecutive feeds at a stable temperature, tells you the colony is established and reliable. The difference: a starter that peaked once at 7h might peak at 11h next time and 5h the time after. A starter that reliably peaks at 7-8h for five days in a row will leaven your bread predictably every single bake. Consistency is the metric to optimise for, not a single impressive rise.

At peak, a healthy starter smells of lactic acid — mildly tangy, faintly dairy, alive. Not sharp vinegar (that's past peak), not acetone (that's hungry), not alcohol (that's starved). The smell is part of the readiness check. If the starter looks right but smells wrong, trust the smell — it's ahead of the visual.

At or near peak, just watching the jar for a minute you should see occasional small bubbles rising through the mass. This is CO2 being released by actively fermenting yeast. If the starter is completely still — no movement visible over a few minutes of observation — it's either pre-activity (just fed, too early) or post-peak (past its high point). Peak is when you can see the starter breathing.

The famous test: drop a small teaspoon of starter in a glass of water. If it floats, the starter is aerated enough for bread. If it sinks, it may not be ready. The caveat: false negatives are common — especially with rye. Rye contains compounds called pentosans that prevent it from forming a proper gluten network, so the CO2 produced during fermentation escapes instead of being trapped. Even a highly active rye starter won't pass the float test, not because it's unready but because rye physically can't hold the gas that makes it float. A starter fed with rye or high whole-grain flour reliably doubles but will sink every time — and still bakes excellent bread. Use the float test as one supportive signal alongside the other six, not as the sole readiness criterion. If signs 1-6 are all present, you don't need the float test.