For the sourdough, mix 250 g water with 250 g wheat flour type 1050 and 25 g topping and leave to rest for 12 hours at 28 ° C. Cut the potatoes into small cubes and boil in 250 g of water. Then puree the potatoes with the water and allow to cool to approx. 28 ° C. Mix the sourdough, mashed potatoes, flour and salt and knead well. Put the dough in a loaf pan and let rise in a warm place for about 3 hours. Preheat the oven to 250 ° C top and bottom heat. Bake the bread descending to 180 degrees for 100-120 minutes.