Much of Italian cooking is rooted in simplicity. Each ingredient and flavor serves a specific purpose, and for this reason, quality is of the utmost importance. Especially when it comes to something like fresh pasta - it’s all in the ingredients and technique. There’s nothing to hide behind, so choosing the correct type of flour (and eggs) can make or break the texture and taste of your finished product.
It’s important to understand the Italian flour categorization system, especially when outside of Italy, so you can best choose a substitute.
In Italy, flour is categorized based on the ash content - basically how refined the flour is. The higher the ash content, the more germ and bran left in a flour, and the less refined it is.
Flour is also categorized as grano duro (also listed as semola flour) and grano tenero (a softer type of wheat). Grano duro is slightly yellow, more granular, and more commonly used for pasta and some breads in the south of Italy. It's also knows as durum wheat flour. Grano tenero is generally what we think of as white flour, and is more broadly used in bread, pizza, and pastry, and northern pasta doughs.
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Flours made from grano tenero are classified in the following way. Note that the lower the number, the softer and finer the flour. The higher the number, the more coarse the grain - going up all the way to whole wheat:
