Question:
If the matzah has wheat and water, then why can’t we use flour at Passover?
Answer:
Your question is correct.
It is permitted to use flour on Passover, bake matzoh or anything else that will not be sour, but there is not much to do with flour, since every touch of flour in the water causes it to be sour quickly and the sour is forbidden at Passover.
Besides, we do not use Passover in flour found in stores, for the simple reason that we need to know for sure that the flour did not touch the water and is not sour, in most flour mills they wet the wheat slightly in order to grind easier, so any regular flour there is a concern that the flour is sour unless kept that did not touch it at all and ground in dry and is packaged and kept in a dry place.
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