Peel the yams and mash them with a fork. Let cool. Combine water, starter, wheat germ, yam, and flour and mix or knead for 4 minutes. The dough will be wet and quite sticky, if you’re doing this by hand, use a spatula. Add salt and knead for another 10 minutes, or until the dough is elastic and stretchy and can pass the windowpane test.

Put the dough into a well oiled bowl and cover with plastic wrap. Let rise until double. Gently shape into a boule and let rest for 1 hour. Refrigerate for 8-10 hours to develop flavor. Remove from fridge and let warm up until it increases in size by one half, then bake at 450°F for 30 minutes.