Tevul Yom, Chapter One, Mishnah Three
1) The nail shaped knob on the back of the loaf, or the small globule of salt, or the burnt crust less than a finger’s breadth: Rabbi Yose says: whatever is eaten with the loaf becomes unclean [when touched by the tevul yom].
2) And one doesn’t need to say, this is so [when touched] by other unclean things.
Section one: If there is a lump of bread on the back of a loaf of terumah bread, or a small globule of salt or a small burnt piece on the crust, they are all considered as being part of the loaf, because when one goes to eat the loaf, she will eat these parts too. If the tevul yom touches one of these parts, the entire loaf of terumah is disqualified.
Section two: If the law is stringent with regard to the tevul yom, who is really pure, all the more so the law will be stringent with regard to other defiling things. If they touch such parts of the loaf, the entire loaf is impure.