Hi Leora! We're a little confused since our Montreal Bagels recipe doesn't call for sourdough starter. Did you mean you'd like to substitute sourdough starter for the yeast? If so, you certainly can, it's just a bit of an experiment and requires some math. You need to make calculations so that the flour in your starter winds up replacing a portion of the flour in your recipe. For white flour recipes, the flour in your starter needs to make up about 30% of the flour in the recipe. For whole wheat recipes, 35%-40%. And for rye, 45%. Since starters are typically equal parts of flour and water by weight, you'll be removing an equal portion of flour and water from the recipe and replacing them with starter, the amount will just vary depending on the main type of flour used in the recipe. (White flour, whole wheat, or rye.)

Here's an example using our No-Knead Crusty White Bread which uses white flour. Since this recipe calls for 907g flour, and 30% of that is roughly 272g, and your starter is equal parts flour and water by weight, you'll need to add 544g ripe bubbly sourdough starter to this recipe and remove the yeast. So you're removing 272g flour and 272g water from the recipe itself and replacing them with 544g ripe starter. The rise time might be longer and it may never rise as high as the loaves in the photo. Happy baking!

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