Can I substitute a biga or poolish in recipes that call for a sourdough starter?
It depends on whether the recipe relies only on the sourdough starter for leavening, or if it also includes commercial yeast. If it relies on the starter for leavening, you cannot substitute a biga or poolish, as they will not provide sufficient leavening power (even if the biga contains a small amount of yeast). If, however, you find a recipe that uses both a starter and commercial yeast, you can substitute a biga or poolish with a similar hydration level for the starter. I'd recommend a biga with a touch of yeast to provide leavening like the starter would provide.