When I was a member of a Messianic Jewish congregation, the rabbi considered yeast, baking powder, and baking soda to all be leaven. During the feast of unleavened bread, we were not to bring anything leavened for the after-service snack table. Leaven is anything that makes dough light and fluffy as opposed to thin and compact.

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Mary Vanderkooi
Kale Heywott Church (KHC)
Soddo, Ethiopia