What & When: Sourdough

No, a poolish is a preferment made with commercial yeast.

If you have a starter that you know how to use, yeah, try substituting it instead of yeast. As far as "how much", I would just say as much by weight as you typically use for a batch of your sourdough bread. Assume the rise times will also need to be adjusted accordingly.

The easiest way I have found to adapt a recipe without altering the ingredients too much is to take all of the liquid from the recipe, stir in 2 Tbsp. starter, add the same amount of flour as the liquid. Let this sit, covered, overnight (room temp.) Next day, continue by adding the rest of the ingredients, remembering that you already used the liquid and part of the flour. If your recipe calls for milk rather than water, use water but then stir in some dry milk powder after the overnight proofing is complete and then mix your dough.

Yes, I have often converted poolish or biga recipes to a sourdough version and had good results. For example, I love Peter Reinhart's recipe for "raisin focaccia" in his book, Bread Baker's Apprentice done using sourdough "barm" in place of poolish. It works very well and has a great flavor from the sourdough, which seems to mix well with the raisins.

I've found that you can normally substitute a poolish with a starter that is created by adding 1:1 flour to water and get a reasonable result. The rise times will probably be slower with the sourdough version - maybe a lot slower, unless you also add a small amount of yeast when you make the final dough - usually called "hybrid" method of leavening. This is just another way of saying that if the ratio of water to flour in a poolish is 1:1, and you use a fully refreshed, ready to go 1:1 sourdough starter, you essentially have a vigorous yeast culture in either poolish or 1:1 starter and the right flour to water ratio, so it will be a good substitute.

A substitute for a biga can be made by creating a small dough out of a 1:1 starter by adding 1 part of 1:1 starter to a 1:2.25 ratio of water:flour to create a "firm starter". Let it rise and put it in the refrigerator overnight and use in place of a biga in your recipes that use a biga. In other words, take some starter, add flour and a little water to make dough, and let it rise, making a pre-ferment that is like a biga, which you can refrigerate overnight and use in the same way as a biga in your recipes.

All the ratios above are weight ratios, not volume ratios. Also, you probably will almost always have to adjust the water a little bit to get the right dough consistency, since the starter may not use the same flour, have the same acidity, and probably all kinds of other factors - and the poolish may not be exactly 1:1 flour:water for that matter.

I sometimes build up a spread sheet for a conversion from instant yeast to sourdough that summarizes essentially what SourdoLady describes, i.e. make the proportions of total  flour, water, salt, fat the same in the two recipes, no matter whether the contributions of those ingredients came from biga, poolish, sourdough starter, or ingredients added to the final dough.

I think you could do as follows, assuming you already have a reasonably active starter with approximately a 1:1 ratio by weight of flour:water:

• Sourdough starter - 3.2 oz
• flour - 9 oz
• water - 9 oz

Create the poolish substitute by mixing the starter, flour, and water. Let rise at about room temperature for several hours until it doubles in volume, and optionally leave it in the refrigerator overnight to have more flavor in this "poolish substitute". Use as you would use the poolish in your recipe. The rise times in the recipe may be longer with the sourdough poolish substitution, but you probably can reduce the rise times by adding a small amount of instant yeast to the dough recipe.

I would use the yeast in the recipe for the first try. Then you can experiment with it from there. It will probably take longer without the added yeast. You could even add a very small amount if you find it rises too slowly for your needs.