There's really no difference. Durum wheat is the plant. Semolina
is a coarse grind of the durum wheat berries. You can also grind
them finer to make durum flour.
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Durum wheat is used in pasta because it is easy to digest.
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The origin of Durum wheat is believed to have been in either
Abyssinia or the southern Mediterranean. Wikipedia has a full
article on Durum wheat - see related link.
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Pasta flour you can buy it from all main supermarkests.