Flour labels protein as a percentage of weight, and for white flour it roughly predicts gluten strength. Pastry flour around 8 to 9 percent is tender, all-purpose around 10 to 12 percent is versatile, and bread flour above 12 percent holds structure for tall, open loaves.
The measurement is crude: two flours at the same percentage can behave differently because of wheat variety and milling. Ash content and extraction rate matter for flavor, and wholemeal flours report protein that includes proteins that never form gluten.
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For a rustic open crumb, choose a bread flour around 12 to 13 percent and add a small portion of whole wheat for flavor; for soft rolls, choose lower protein so the crumb is fine, not chewy.