Challah 3:7-8

Challah 3:7

Let’s say that a person makes a dough from both wheat and rice. If it tastes like the wheat, the dough is obligated in challah and one can use it to fulfill the mitzvah of matzah on Pesach. [Obviously, this does not reflect later enactments regarding rice on Pesach.] If it doesn’t taste like wheat, it’s treated as regular rice dough: it is exempt from challah and it may not be used to fulfill the mitzvah of matzah on Pesach.

Challah 3:8

If one takes leaven from dough whose challah has not yet been separated and he puts it into dough whose challah has already been separated, if he can take challah from another dough for the liable portion, he should do so. If this is not an option, he separates challah for the whole thing from the dough in which the leaven was added.