Soak the grain mixture in a bowl of cold water so that the grains are covered by about 2 fingers. Cover and leave to swell overnight. Drain well the next day
Stir yeast with sugar until liquid. Mix both flours with salt and bread spice in a bowl. Add sourdough and mixed yeast, add 500 ml lukewarm water. Immediately knead with the dough hooks of the hand mixer for at least 5 minutes to a smooth, soft dough. Add the grains and knead briefly. Cover and leave to rise in a warm place for about 45 minutes
Grease a box mould (30 cm long; 2.3 litres capacity) and dust with flour. Add the dough and smooth it down. Cover and leave to rise for another 30 minutes in a warm place until the dough has increased considerably. Cut several times with a damp knife on the surface and dust with flour
Place in the preheated oven (electric cooker: 225 °C/ circulating air: 200 °C/ gas: see manufacturer), reduce heat immediately (electric cooker: 200 °C/ circulating air: 175 °C/ gas: see manufacturer). If necessary, place a small, ovenproof bowl of water on the floor. Bake for about 50 minutes. Let the bread cool down for about 5 minutes on a cake rack. Then carefully turn over and let it cool down
waiting time approx. 1 1/2 hours