The preferment called a sponge is the quickest of all. It's generally ready in about three hours. If you're really in a hurry, or you forget to mix a preferment the night before and really want to have one, then the sponge is the way to go.
The sponge is generally a mixture of about half of the flour in a recipe, all of the water, and all of the yeast. That's why the sponge acts so quickly. Apart from its quick action, a sponge is similar to a poolish. Both are very wet, and the top surface of each will be covered in small bubbles when mature. Needless to say, a sponge should be left at room temperature for the whole fermentation period.
A sponge will also taste a little bit more like lactic acid, but the taste won't be as pronounced because it did not have as much time to develop. The yeast and organic acids will definitely be developed, just not as much as an overnight fermentation.
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