Ah, poolish. It's thought that poolish actually originated in Poland and migrated through Austria to France. Poolish is the preferment of choice among French bakers for the symbol of French bread, the baguette.
Poolish is a mixture of 100 percent flour and 100 percent water, with maybe about .1 percent yeast. That means that there will be the same amount of water and flour mixed together, and just a very small amount of yeast is part of the mix. Poolish should be a wetter mixture, not unlike a thick pancake batter. You'll know that this preferment is ready to use when the surface is covered in tiny bubbles and it looks like it has expanded and then flattened out. If the poolish has fallen at all, then it is overdeveloped and probably not good.
A poolish should be left in a cool spot overnight, but not refrigerated unless you'll need more than 12 to 16 hours. If you're making it in the afternoon for the next afternoons use, then I'd go ahead and refrigerate it. Just be sure to allow it to come back to room temperature before you use it.
Poolish, being a more hydrated mixture, lends itself particularly well to the development of lactic acid. If you taste a mature poolish, you should be able to taste yogurt or milk. The moister environment is more attractive to bacteria that produce the lactic acid.
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