Because poolish has a relatively high percentage of yeast and no salt, it can over ferment quite easily. For this reason, it's best to store poolish in a refrigerator, or a very cold room, as soon as the fermentation process has begun (usually after 1 hour or so). The best poolish is cold fermented for 16 to 24 hours before use to ensure the best flavor and texture without the risk of over fermenting.