This recipe uses the tannins (gallic acid, specifically) from walnuts, along with a tiny amount of iron from a vitamin tablet to give the crumb a vibrant purple color. (This will also work with pecans and—to a lesser degree—almonds.) In order to distribute the tannin “dye” throughout the crumb, we soak the walnuts to extract the tannins and use the soaking liquid as the water for the dough.
You’ll want to soak the nuts for at least 2 hours to extract the tannins, but overnight is best.
Tannins alone have a pale brown color; only after reacting with tiny amounts of iron sulfate (aka iron supplement tablets) does the color turn purple, after it forms ferrous tannate. There’s sometimes enough iron in flour and/or tap water for this reaction to occur spontaneously; adding iron sulfate just ensures it happens reliably.
Other types of iron supplements might work, but I’ve only tested this with iron sulfate, which you can find at most supermarkets and pharmacies.
Remember to scale out the water for the final dough after soaking the nuts, adding more water as needed to equal 340g total.
Because the initial proof of the dough takes from 12 to 14 hours to complete, it is best to start the dough either in the evening, or very early in the morning. (My usual approach is to start the dough in the evening, shape the loaf in the morning, and bake late in the day or the following morning.)
Any bread flour will work here, but my go-to “bread” flour is King Arthur all-purpose flour, with 11.9% protein.
You can use mature levain/sourdough starter that has been refreshed as described here within the previous 10 days; since the recipe uses such a small amount, the exact hydration of the levain is not important here.
If possible, proof the dough somewhere where ambient temperatures are between 75˚and 78˚F.
If that’s not possible and temperatures are very cold (below 68˚F), increase the amount of levain to 10%, or 80 grams.
If ambient temperatures are higher than 80˚F (as in the summer), reduce the amount of levain to 1%, or 9 grams (about 2 teaspoons).
To avoid overproofing, it is important to shape the loaf within an hour or so after the dough is ripe (i.e., when the it is “domed, lightly bubbly, and just doubled in volume”), especially when ambient temps are high. (In the winter, you’ll have way more wiggle room.)
If possible, use a glass bowl for mixing and proofing the dough, it will make knowing when it is ready to be shaped obvious (you will see a fine web of bubbles on the interior of the dough).
I usually cover the top of the banneton loosely with a clear plastic shower cap (or plastic wrap), but not the remainder of the banneton, to let it breathe.
The loaf can be retarded (refrigerated) in step 7 for 8 to 24 hours, depending upon whatever timing is most convenient for your schedule (it will get more slightly more sour in flavor the longer it proofs).
The loaf is baked cold, directly from the fridge.
Walnut Pain au Levain
Yield 1x 900g loaf
90% bread flour
10% rye flour
15% coarsely chopped walnuts
70g coarsely chopped walnuts
1 iron sulfate tablet, crushed or microplaned to a fine powder
410g bread flour
50g rye flour
340g soaker water (from above)
walnuts (from above)
FOR THE WALNUT SOAKER: Combine walnuts and water in small bowl and let sit for 2 to 16 hours.
FOR THE DOUGH: Decant or strain soaking liquid into second bowl. Add small amount of iron powder to soaking liquid and stir to dissolve. Continue adding iron as needed until liquid is a deep purple color (you shouldn’t need more than half a tablet).
Strain liquid through a fine meshed strainer to remove undissolved solids.
Place about 10 grams soaking liquid (~2 teaspoons) in small bowl and set aside. Combine remaining water and levain in large bowl and stir with wooden spoon until levain is mostly dissolved. Add flours and stir until no dry flour remains. Cover and let rest at room temperature for 20 minutes.
Add salt, walnuts, and reserved soaking liquid to top of dough, and knead gently in bowl until incorporated. Cover and let rest at room temperature for 30 minutes.
Using lightly wet hands, fold dough until tight and uniform, 6 to 8 folds.
Desired dough temperature: 70-75˚F.
Bulk Fermentation: 12 hours at 75˚F, until dough is domed, lightly bubbly, and just doubled in volume. A peek at the underside of the dough along the edges of the bowl should also show a web of fine bubbles.
Dust top of dough lightly with flour, loosen from bottom of bowl, and invert onto floured countertop. Using flour on hands and top of dough as necessary to prevent sticking, gently press dough into 8-inch circle, then fold edges toward middle to form round.
Cover with inverted bowl and let rest for 30 minutes.
Flour top of dough and surrounding countertop. Using bench scraper, loosen dough from counter. Shape as desired.
Transfer to floured banneton or lined basket. Cover and let sit for 30 minutes, then transfer to refrigerator for 8 to 24 hours (loaf will not rise appreciably during this time).
One hour before baking bread, adjust oven rack to middle position, set covered heavy-bottomed Dutch oven on rack, and heat oven to 475 degrees.
Lay 12- by 12-inch sheet of parchment paper on counter. Remove loaf from fridge and invert onto center of parchment paper.
Score loaf as desired. Pick up loaf by lifting parchment edges and lower into Dutch oven. Carefully cover pot and transfer to oven. Bake until fully sprung, about 20 minutes.
Carefully remove pot from oven. Reduce oven temperature to 425. Remove loaf, transfer to bare oven rack, and continue to bake until deep golden brown, 10 to 20 minutes longer. Transfer loaf to cooling rack and allow to cool for at least two hours before serving.