The sourdough float test: what it actually tells you

The sourdough float test tells you whether a spoonful of starter has trapped enough gas to be less dense than water. That measures gas retention, not yeast health. Vigorous starters sink all the time, and spent ones float.

What the sourdough float test actually measures

Water sits at about 1 g/mL. A flour-water paste starts denser, and only floats once fermentation drops it below that. Two things control it: how much carbon dioxide the culture produces, and how well the flour-and-water matrix holds gas instead of venting it.

The sourdough starter float test collapses both into one yes-or-no answer and never says which one moved. A sinking spoonful might mean a sluggish culture, or a thriving one in a batter that leaks.

Sampling skews it further. A spoonful skimmed off the domed top carries the biggest bubbles and floats easily. Stir first, take the same volume from the middle, and it can sink. Same starter, same minute, opposite verdicts.

Why a healthy starter sinks and a spent one floats

Stiff starters — roughly 45–55% hydration, the style kept as lievito madre — start denser and hold less gas per volume at any stage, so a stiff levain at full strength routinely sinks. Rye is the other classic false negative: it forms almost no gluten network, so bubbles migrate and escape instead of staying put. A rye starter can double on schedule and still drop like a stone.

The false positive runs the other way. A slack white-flour starter at 100% hydration or higher whips in air while you stir it, floats well before it peaks, and often keeps floating for hours after the culture has burned through the flour — the foam outlasts the gas production. The test can pass an hour early and again three hours late.

The two numbers that predict readiness

Peak rise multiple

Mark the level right after feeding, in a straight-sided jar — a tapered one exaggerates the rise near the top. A white-flour starter on a 1:1:1 feed usually tops out around 2.5–3x its post-feed volume. Consistently under 2x usually means too cold, too much flour per feed, or a culture that needs rebuilding.

That multiple shifts with ratio and flour, so treat it as a personal benchmark, not a universal one. A 1:5:5 feed takes longer and often rises further; a stiff starter rises less by definition. Compare a starter to its own last three feeds. If you're changing ratios, settle how to feed sourdough starter before you trust any rise figure.

Hours to peak

Time the gap between feeding and maximum height. In a 75–78°F kitchen, a mature starter on a 1:1:1 feed commonly peaks in four to eight hours; near 65°F the same starter can want ten to fourteen. Stability matters more than the absolute figure. Once hours-to-peak repeats within about half an hour across three feeds, you can schedule mixing around it instead of hovering.

Sourdough Tracker, a sourdough baking companion app for iPhone, is one place to park those two timestamps. Marking feed time and peak time across a few consecutive feeds turns "is it ready yet?" into a number of hours you can plan a bake around.

What does sourdough starter look like at peak

Watch the surface, not the bubbles. On the way up the top is domed and slightly convex. At the sourdough starter peak that dome flattens and the rise stalls. Soon after, the center dips, the surface turns wrinkled and receding, and a dried tide line shows on the glass above it. The flattening — not the collapse — is the top.

Inside, expect bubbles of mixed sizes through the body and a webby pull when you scoop. It should smell yogurt-tangy, faintly beery. Sharp acetone means the flour is gone. Many bakers mix just before peak, while the levain still has food left for the dough; just-past-peak costs strength.

Readiness signals compared

Signal	What it measures	Where it fails	Verdict
Float test	Gas retention of one spoonful	Stiff and rye starters sink while active; slack ones float long past peak	Confirmation only
Peak rise multiple	Total gas produced per feed	Needs a straight-sided jar and a marked start line	Reliable
Hours to peak	Fermentation speed at your temperature	Moves with room temperature, ratio, and flour	Reliable, and schedulable
Surface shape	Where you are on the curve right now	Easy to miss if you check every four hours	Best single glance
Smell	Acid balance, whether flour remains	Subjective, drifts with your own nose	Cross-check

Dormant or dead: how to tell if sourdough starter is dead

Very little actually kills a starter. Work down this list in order.

Fuzzy mold of any color, or pink and orange streaks. That is the genuine end point. Bin the whole jar rather than scraping the top, then start over with how to make sourdough starter.

Cooked. Sustained heat well above normal proofing temperatures kills yeast — a jar left in a hot car, or a proofing box set wrong.

Everything else is dormant. Dark hooch, an acetone smell, gray liquid, a dry crust, weeks forgotten at the fridge's back: that's hunger, not death. Asking "is my sourdough starter dead" after a fridge stint almost always ends with a revived culture.

To revive, discard down to a tablespoon, feed 1:1:1 by weight with half whole rye or whole wheat and half white flour, use unchlorinated water, and hold it near 78–82°F. Feed every 12 hours for five to seven days. Smell shifts first — sour instead of solvent — then fine bubbles, then rise.

FAQ

Is the sourdough starter float test reliable?

As confirmation, sometimes. As a decision, no. It reports only whether one spoonful traps enough gas to float, which depends heavily on hydration and flour type. Use it alongside rise and timing, never instead of them.

When is sourdough starter ready to use?

When it has risen to about 2.5–3x its post-feed line and the domed surface has just flattened, on the same schedule it hit the last few feeds. For most white-flour starters at room temperature that lands four to eight hours after a 1:1:1 feed.

What does sourdough starter look like when it has gone too far?

Sunken in the middle, wrinkled or flat on top, with a tide line above it and often a layer of hooch. Bubbles look coarse and popped rather than fine. It will still raise bread, just slower and more sourly.

How can I tell if my sourdough starter is dead?

Fuzzy mold or pink and orange streaks mean discard. Anything else, including hooch and a solvent smell, is a hungry starter. Feed it twice daily somewhere warm for a week before writing it off.

Can I bake with a starter that sank?

Yes, if the rise and timing check out. Stiff and rye starters frequently fail the float test at exactly the moment they are most active.