Simple sourdough
(assuming you have the starter)
The starter will keep for at least two weeks in the fridge; so you only have to bake twice a month to keep it healthy. It will grow in the fridge to a small degree but not very much, mostly the yeasty-beasties just sleep.
If you leave the starter for longer than two weeks without using it you just have to give it twice as long to get started in the first phase (sometimes called the 'sponge'). The longest I have ever left it was over two months, and it was fine after it got going!
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Stage 1
Put the starter in a bowl and add warm water and flour to make a thick batter - thicker than pancake batter - some people call this 'the sponge'. Water should be warm but not hot.
Try to use rye flour for the sponge, even if you are not planning to make rye bread. You can make the sponge with other flour once or twice but I have found that the long-term health of the starter is maintained if you use rye regularly.
Cover the sponge with damp tea towel and leave in warmish place.
The fermentation time depends on temperature, health of your starter, and the degree of 'sourness' you enjoy in the bread. 6 hours is too short, 60 hours is too long. If the starter is strong, then 24 hours is just fine but 48 hours gives a stronger taste.
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Stage 2
Add enough flour to the sponge and stir to make a very loose dough. I use 30:30:40 rye/strong/wholemeal but it is not critical.
Take 1-2 cups of the resulting loose dough and put it it a plastic tub in the fridge for next time - this is your starter.
(All the rest is just standard bread baking)
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Stage 3
Add more flour, oil and salt according to your custom, to make a proper kneading dough. Knead for 5 mins at least. Grease the container(s) it is going to be baked in. Shape and score loaves and put in containers. Cover and wait until it doubles in size.
Bake!