90 Percent Whole Wheat Bread with Walnuts

Prepare the mise en place for the levain and the whole wheat sponge.

To make the levain, combine the bread flour and water with the culture in a large mixing bowl, stirring with a wooden spoon to blend. When blended, scrape down the edge of the bowl, cover with plastic film, and set aside to ferment at 75°F (25°C) for 12 hours.

To make the whole wheat sponge