Authentic Ruisleipä is prepared only from rye flour, water, and salt using a starter culture (ruisjuuri). It is often baked into thin, flat, round loaves with a hole in the middle, called 'reikäläipä', but this recipe is for a classic loaf.
The dark color and sour taste are the result of long fermentation and the high content of whole grain rye flour.
If you don't have an active rye starter, prepare it in advance by mixing rye flour and water and feeding it over 2-3 days.
For this recipe, mix 100g of active starter (ruisjuuri) with 100g rye flour and 100ml warm water. Leave in a warm place for 4-8 hours until it doubles in volume and begins to bubble.
In a large bowl, mix the active sourdough starter with 300ml of warm water and salt.
Gradually add 500g of rye flour. Rye dough is not kneaded like wheat dough; it is very sticky. Just stir it with a wooden spoon or silicone spatula until the flour is combined. The dough will be dense and heavy.
Cover the bowl with plastic wrap or a towel and let it proof in a warm room for 3-5 hours. The dough should slightly increase in volume, and small bubbles should appear on the surface.
Preheat the oven to 200 °C (392 °F).
Transfer the dough to a bread pan (or shape it into a round loaf on a baking sheet lined with parchment paper). Smooth the surface with a moistened hand.
Let it proof in a warm place for another 30 minutes while the oven reaches temperature.
Place the loaf in the oven and bake at 200 °C (392 °F) for approximately 90 minutes. For a crispier crust, you can brush the loaf with a little water every 20 minutes.
The bread is done when it has a dark, firm crust and the internal temperature reaches 95 °C (203 °F).
After baking, immediately remove the bread from the pan and let it cool completely on a wire rack. Rye bread should only be sliced after it has completely cooled (preferably after 12 hours).