BBA Potato Rosemary coverting to sourdough question

Here are the "rules of thumb:"

1. Keep the total amount of ingredients constant (flour, water, salt) except eliminate the commercial yeast.

2. Decide on the percentage of the flour you want to pre-ferment. If the formula includes a biga, you could use that as a guideline. In general, 20% pre-fermented flour would be an average amount, but the range is huge.

3. Decide on your levain hydration (liquid or firm).

4. Make your levain with the percentage of flour you want to pre-ferment and the amount of water you need to achieve the target levain hydration. (This flour and this water is subtracted from the amounts you will mix in the final dough.)

5. Now, you have to determine your procedures - autolyse? machine mix vs. hand mix? retardation? etc.

6. Do it!

Hope that gives you the information you need. If you need a more concrete/specific example, just ask.

Happy baking!

David

The fermentation and proofing times will likely be different too (probably longer). Best advice here is to give yourself lots of time and give the dough lots of attention, and write down the timings and temperature, along with the results, so you can do it again (or make needed changes if the result isn't what you wanted).

Speaking from recent experience, I did this not long ago with a Hamelman potato bread.  It was an IDY formula which I converted to levain, no additional IDY.  Although your BBA is biga based, the conversion to levain may present an autolyse hydration issue.

 Depending on the recalculated hydration, excluding the levain, you may find the autolyse to be extremely dry, clumped and difficult to deal with.  The further incorporation of the levain may also yield a difficult task.  If this will be the case, then mix the levain into the water and flour at the outset and "autolyse" with the levain.  This will mark the start of fermentation so you should be aware of that when trying to determine total bulk rise timings.