- Crumble the bread and put it into a large mixing bowl.
- Pour in some lukewarm water and mix. Stir in the rye flour.
- Cover the bowl with a tea towel and leave the dough root to stand at room temperature for 24 hours, until bubbly and pungent. Add the salt and yeast. Add the rye flour and knead until smooth and firm.
- Let the dough rise until doubled. This takes longer than ordinary yeast bread. Divide the dough into 3 pieces and leave a small part for the next dough (Use that as the sourdough root. Freeze it and soak it in warm water, then proceed as normal according to the recipe).
- Shape into loaves and let it rise under a tea towel.
- Bake at 200°C (400F) for about 1 hour.
Source: Visit Finland