Q. When we mention that chometz is prohibited ‘Bemashehu,’ even to the smallest amount during Pesach, does that mean that the prohibition applies to molecules of chometz too?
A. As far as when dealing with the prohibition created directly by consuming a small piece of the prohibited food, even a ‘mashehu’ or the smallest of parts, would have to be something that can be noticed by the common eyesight, as Poskim rule regarding insects.
However, when dealing with ‘nosen ta’am’ or the flavor imparted by the prohibited substance, since we are dealing with flavors (or smells). we would include even unseen particles at the molecular level.
The unique case of the chomets prohibition during Pesach, is that it applies even when mixed with “min bemino” (the same tasting substance), and therefore can not be sensed or perceived, and it is still prohibited even ‘bemashehu.’ That is a unique property of the prohibition of chometz during Pesach.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a