I am going to make a loaf of sourdough potato bread - and have a good, active starter on hand that is several years old. I don't know how much starter would be the equivalent to the biga in this recipe. The recipe calls for 7 ounces of a biga. My starter is 100% hydration. How would I figure this out? Thank you!
(The recipe is from Reinhart's BBA)
His biga is at 66.7% hydration, so that means there is 4.2 oz of flour, and 2.8 oz of water in 7 oz of biga. There are 2 ways to deal with this (most people will probably do the 2nd way).
The first way ... To substitute different pre-ferments at different hydrations, you need to match the flour and then adjust the water from the total formula. So if your starter is 100% hydration then you need to use 8.4 oz of starter (4.2 oz of flour and 4.2 oz of water). Then when you put the final bread dough together, you need to subtract 1.4 oz of water from the water called for in the recipe (since you added 4.2 oz of water instead of 2.8 oz... 4.2-2.8=1.4).
The other option is to do a build of your starter so that it ends up at 66.7% hydration (2/3 water). To do that, you feed it at a 2:3:5 (starter:water:flour) ratio and let it ferment until peaked like normal (usually over night). If you use 2oz of stater, you will end up with 10oz, but it is easier to weigh out this way. The extra 3 oz can either be tossed, fed, or used another way.
You might find you need to adjust the fermentation / proofing times and temperatures for a 'wild yeast' firm starter versus a commercial yeast biga as well. Keep an eye on the dough and make notes about it's performance so you can adjust next time as required.
It would also be interesting to do a side-by-side taste test - one loaf with the biga, one loaf with the firm levain starter.
Thank you both. I will try it in the morning!