Unleavened bread symbolizes both pilgrimage and judgment, as well as corruption. As a symbol of pilgrimage, it is associated, of course, with the Exodus. Now the Israelites, rushing to leave Egypt, took their dough before it was leavened, having their kneading bowls bound up in their clothes and on their shoulders (you find that in Exodus 12:34), leaving slavery behind as a pilgrim people.
