The "Old-fashioned 100 Percent Whole-Wheat Bread" recipe in Beth Hensperger's The Bread Bible calls for 3 cups of warm water and 1 cup dried buttermilk to create the sponge. (Other ingredients in the sponge include 3 cups whole wheat flour.) The sponge rests for about an hour before the remaining ingredients are added in.
Does anyone have any advice for substituting the dried buttermilk for real buttermilk? I understand from Googling that dried buttermilk is reconstituted 4 parts water to 1 part dried buttermilk, but I'm not sure how reverse engineering the volumes for this recipe might work given the wet and dry ratios for the sponge.
I also have nonfat dried milk powder on hand. You can view the full recipe through Google Books.