[Regarding] a proselyte who converted and had a dough, [if] it was made before he converted, he is exempt [from having to separate challah]; but [if it was made] after he converted, he is obligated [to separate challah]. And if it is uncertain [when the dough was made], he is obligated [to separate challah]. But one is not liable on its account to [the penalty of] a “fifth.”  R' Akiva says: Everything follows the formation of a crust in the oven.