Why do some recipes call for BOTH starter/levain & instant yeast? Why NOT just levain?
I'm working on my 5th iteration of Ken Forkish's "Field Blend #2" loaf, from his 'Flour Water Salt Yeast' tome.
To date, it's been follow-the-recipe, and work for consistency.
That's, now, pretty good. Now, I'd like to better understand, & potentially tweak, some of the bits ...
Atm, it's a question about the 'Final Dough' ... which calls for both
360g mature levain
2g instant yeast
IIUC, from what I've been reading, one cup of sourdough starter is roughly equivalent to the rising potential of one packet of commercial yeast ... depending on activity etc.
So my *general* question is -- why are BOTH being used here? Why not just increase the levain?
I'm assuming there's a _reason_ ... I just haven't found a clear statement of it.