Poolish is the French word for a pre-ferment, or starter. A starter usually contains flour, water and a leavening agent (usually yeast or sourdough culture), and is added to a a dough instead of using yeast directly.
Using a starter can lead to a totally different texture in the bread you're making. I started researching Poolish while looking at how to create Parisian Baguettes, and a poolish is a key part of this recipe as you want the inside of the bread to be soft, chewy and airy.
The main thing you need for a pre-ferment is time. Time allows the yeast to work longer on the starch and proteins in the dough.
Due to the not-so-consistent weather in the UK I've dedicated my loft as my "proofing room" for the poolish. It's consistently warmer up there and I find the dried yeast reactivates well up there.
The article I used as a reference outlines the ratios of dried yeast to use against equal parts flour and water (both measured in weight).
|8 hours in advance||0.23% – 0.33%|
|12 hours in advance||0.1% – 0.2%|
|16 hours in advance||0.03% – 0.08%|
There's a great article on Weekend Bakery that discusses Poolish (French) and Biga (Italian) pre-ferments, including more detail on the ratios to be using when making them. Wikipedia has some great information on pre-ferment too.