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I tried asking a similar question: From regular fresh yeast to sourdough and didn't exactly get real answers, so here I am asking something easier.

Is there a formula to convert a sourdough recipe to a preferment recipe using either biga or poolish and vice versa?

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I would say no, because to me, it doesn't make sense for such a conversion to exist. You get conversion formulas when you can achieve the same result with different ingredients. Since you have no well-defined "same" here, you can't give a conversion either.

One function of sourdough is to leaven. You don't leaven with a preferment, so here you cannot have a conversion. Instead, you have to use yeast in the proper amount for your amount of flour and rising process. It doesn't matter that some of your flour will arrive as dry flour and some of it will be included in a preferment.

The other function of sourdough is to give you a specific taste (I am combining both flavor and texture in that word). For that, there is no specific amount of sourdough you have to add to the recipe - you can use more or less of it, depending on your taste (at least within a wide range, until you run into structural problems with the loaf). There is also no specific amount of preferment you have to add - you use as much or as little as you like, again such that you get a dough and not a glutenless paste or slurry.

You could say that you want an amount of preferment that makes your bread taste exactly the way it tasted with X percent sourdough, but that's not practicable. There are differences between one sourdough and the next, and between one preferment and the next, and if you are very exact, you can't achieve the same taste with a preferment that you can achieve with a sourdough anyway. Even for achieving something "similar", the variability is too high and you have to recreate the recipe for every specific case, there is no simple "plug it in" formula.

If all you want is a rough starting point, you can simply use the same baker's percentage of preferment as you used of sourdough and see how you like the result. I am suggesting this as a rule of thumb because it is the easiest to deal with, not because it is likely to produce the same taste though.

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