This will replace the bakers yeast in the dough. You will make your own yeast and the quality of the yeast will define the quality of the bread you make with it.
It is important that the room temperature is quite high (optimally = 20-25°C or 68-77°F) and avoid drafts or your starter might not "take".
1) In a jam jar with a large opening put ½ the volume of whole meal flour and 1/2 the volume of bread flour (type 55) with a pinch of sugar and enough water to make a thick batter.
Put the lid on without screwing it on so that air can circulate inside and outside the pot.
morning and evening for 3 - 5 days.
2) When the mixture is quite bubbly and gives off a fermented smell which you will recognize as your experimentations continue (it must be done at just the right moment), you throw half of it away and reconstitute the equivalent volume with bread flour and a bit of sugar and water. This time the mixture should be a bit thicker, half way between a thick batter and a bread dough.
6-12 h, the mixture should have gone bubbly and doubled up (and might
3) Through half away again and reconstitute the mixture this time with no sugar and let rise. Start over again.
There you go, the starter is ready to use......
It will keep maximum one week in the part of the refrigerator which is not so cold (minimum 8°C= 47°F). After 5-6 days, feed your starter by replacing half the volume with flour, water and a bit of sugar.
When you recuperate some of the bread dough to make up some new starter you put it straight in the refrigerator. You should not wait too long, even by feeding it once in a while, it might go off.
In theory, you only make a starter once (thank goodness as it is the hardest part in making sourdough bread) and then you can recuperate it from the bread. But that is the theory, because in practice at one time or another, who knows why, but you loose it.
starter is not easy to make and it might take you several attempts before
you succeed. But it is quick to make so you can indulge yourself.