I recently learned in Sefer HaChinuch the Mitzvah of not seeing chametz in
one's r'shus (Parshas Bo). The Chinuch quite clearly says the issur
includes grain that has turned to chametz as well as leavening ("s'or").
I assume that yeast is prominent among leavening agents and at least,
according to the Chinuch, should be treated like Chametz on Pesach.
I'm not offering Psak Halacha here; only think the question needs a little
more looking into.
Moshe Polon