Sam2 –
We cannot be M’vatel Chametz before it becomes Assur because we hold that since Chametz is an Issur Mashehu, it’s Chozer V’neor on Pesach and becomes Assur, even if it’s .00001% of the mixture when Pesach starts.
You’re entitled to hold this way, but the mechaber (OC 447:4) cites an opinion that says chametz is batel before Pesach, and the Rema says that the minhag is to rely on this when the dealing with lach. Not only that, but the Mishna Berura says on the spot that if a dry item fell into a mixture 60 times its size, you may cook it l’chatchila before Pesach until it dissolves and thereby becomes lach and batel, and then eat it on Pesach. Therefore, people buying their non-certified yogurt that was manufactured before Pesach are certainly resting on large shoulders.