Pour cold water over the grain mixture (it should be covered by about 2 fingers). Leave to soak overnight in the refrigerator.
The next day, drain the grains. Stir yeast and sugar until liquid. Mix both flours, 2 teaspoons salt and bread spice. Add sourdough and yeast. 1⁄2 l Pour on lukewarm water. Mix everything with the dough hooks of the hand mixer at least
Knead for 5 minutes. Knead the grains briefly at the end. Cover and leave to rise in a warm place for about 1 hour.
Grease a box form (30 cm long; approx. 2.3 l capacity) and dust with flour. Pour in the dough and smooth it down. Cover and leave to rise for another 30 minutes. Cut the surface more often with a damp knife and dust with flour.
Place in the preheated oven (electric cooker: 225 °C/circulating air: 200 °C/gas: see manufacturer) and switch down immediately (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). If necessary, place a small fireproof bowl with water on the bottom of the oven.
Bake bread for about 50 minutes. Cool for about 5 minutes, turn out and let it cool down.