Bubbles in the starter show that it's active. Sourdough starter rises and deflates in between feedings. How often it's fed can determine how active it is.
The float test is a way of checking if your sourdough starter is light and airy enough to make your bread rise.
Sourdough starter must be used when it is DEFINITELY ACTIVE. That is actually the only real prerequisite to making sure it will ferment your dough!
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