Sourdough Float Test: Is It 100% Reliable?

Do you wonder if the sourdough float test is reliable? Some bakers argue that it is not a dependable test, while others strongly believe in its importance. Learn how reliable it really is and what it means when the starter doesn't float.

The float test isn't 100% reliable, but it helps indicate the activity of the sourdough starter and the dough's rising potential during bulk fermentation.

What is the float test?

The float test is a simple way to check whether a sourdough starter is ready for baking. It involves dropping a small spoonful of unstirred starter into the water to see if it floats.

If it floats, the starter is full of air bubbles and active enough to help your dough rise. If it sinks, it may require additional time to ferment or need to be fed.

The science behind the float test

Wild yeasts in the sourdough starter feed on the sugars in the flour. As they eat, they produce carbon dioxide gas through the process of fermentation. The CO₂ gas is trapped inside the starter, causing it to become bubbly and rise.

But why does the starter float? The starter floats because the CO₂ gas from fermentation fills tiny bubbles within the mixture, making it less dense than water. This allows it to rise to the surface and stay afloat.

However, fermentation is a complex process, and not all active starters will pass the test.

During bulk fermentation, the wild yeasts of the dough consume the sugars in the flour, breaking them down through fermentation.

That CO₂ gets trapped inside the gluten network, a stretchy web of proteins in the dough, creating air pockets that cause the dough to rise and become less dense over time.

But why does the dough float? CO₂, trapped in the dough's gluten structure, lowers its overall density, less than that of water, so it floats.

In simple terms: More CO₂ bubbles = less dense dough = floats in water.

Is the float test reliable?

The float test can sometimes provide a good indication of your starter's activity, but it is not always 100% reliable.

The float test is reliable only when the starter is at peak with the most gas present. When the starter doesn't float, it means that the yeast hasn't produced enough gas to make it light and bubbly. This could happen for a couple of reasons:

• The starter hasn't reached its peak yet: It may not have developed enough gas bubbles and strength to float. In this case, it simply needs more time to reach its peak.

• The starter is past its peak: It may have lost its ability to trap gas, causing it to sink instead of float. It is likely time to feed it again or start with a fresh batch.

However, some starters may be strong enough to rise bread even if they don't float, while others may float but still not be at their peak for baking. The test is just one clue, not a perfect answer.

How to perform the float test

Wet a spoon with room-temperature water, then take a small scoop of your starter, making sure not to stir it first so you don't lose the bubbles. Gently drop the scoop into a cup or bowl of room-temperature water.

• If the starter floats, the test is positive, indicating your starter is ready for baking.
• If it sinks, the test is negative, meaning the starter is either past its peak stage or has not yet reached its peak.

When the float test works and when it doesn't

The float test works best with an established, active starter that has been recently fed and is at its peak. However, it may not work well if:

• The starter is too young: A new starter before 4 weeks old has not developed enough yeast to create the CO₂ bubbles needed for floating.

• It was stirred before testing: Stirring releases gas bubbles, which causes it to sink even if it is actually ready.

• The starter is too thick or dense: Even if active, a thicker starter may not float because it does not trap enough air to make it light enough to float.

• The bubbles were disrupted: Sometimes, handling the starter too much before the test can pop the bubbles, causing it to sink.

• The starter hits the lid: Once the starter hits the lid of the jar, it loses gas bubbles. The simple fix is to use a bigger jar.

• The type of flour used: Whole wheat or rye flours produce denser starters that may not float even when fully active. Remember this, especially when working with a 100% whole-grain starter.

• The starter is past its peak: If it has already peaked and started to collapse, it will not float, but it can still be strong enough to make your dough rise.

• Starter hydration: The high-hydration starter, which contains more water, will not float.

Float test during bulk fermentation

Some bakers rely on the float test to decide when to end bulk fermentation. If a small piece of dough floats in water, much like a sponge, it means the dough is holding carbon dioxide (CO₂) inside, making it lighter and less dense, and allowing it to float.

A positive float test is a good sign that the dough has strong rising potential. It gives you a rough idea that your bread dough is nearing the end of bulk fermentation.

At this stage, if you end bulk fermentation and move on, you will still get a decent, edible loaf of sourdough bread. It might be slightly underproofed, but the fact that it floats gives you a good starting point for adjusting bulk fermentation next time.

This means that the float test during bulk fermentation can be a helpful guide if you are not ready to get into all the science and techniques behind sourdough baking.

If you are ready to learn sourdough the right way, start with my Beginner's Sourdough Guide.

You will discover that the most accurate way to determine when bulk fermentation is done is by checking if the dough has reached its target percentage rise, taking into account the dough temperature.

Let's talk sourdough

Have you ever used the float test? Did it work for you, or leave you guessing? Share your experience in the comments.