I came up with this sourdough quick bread by heavily tweaking one of my favorite “regular” banana bread recipes. Why the adaptation? It could be that I believe that my changes — incorporating a hefty amount of sourdough starter, significantly reducing the amount of sweetener and fat, and replacing the butter with olive oil — transform what is essentially cake into something falling somewhere between turnip greens and quinoa on the healthy food scale. Or it could be that sourdough just makes everything better, and that’s reason enough.
The idea in adaptations like this is to substitute all or a portion of the flour in the original recipe with the flour in the sourdough starter. But one challenge in adapting pastry recipes is that the starter must be fairly liquid (around 100% or more hydration) in order to incorporate easily with the other ingredients without having to work it very much, which would produce gluten development that is generally undesirable in pastries. How can you bring all that water along without making the batter too wet?
If the original recipe calls for water as an ingredient, the amount can be reduced to account for the water in the starter. Otherwise, it’s a little tricker. If there are other liquids, such as milk or egg whites, you might substitute a powdered form of that ingredient, such as milk powder, and let the starter water stand in for the liquid component of the ingredient. Reducing the amount of sweetener can also help make a batter less “wet” (as well as, of course, less sweet, which I generally find to be a good thing).