All-purpose flour or plain flour is designed to be used for most baking purposes. It's a wheat based flour that consists of a blend of hard (high protein) and soft (low protein) flour. All-purpose flour usually has an medium protein content in the range of 9% to 12%.
Due to this medium protein content, it's great for things where you don't need lots of dough elasticity like cookies, cakes and pastries. It usually won't yield great results in things like pizza crusts and other applications where you're interested in getting a chewy dense bread.
All-purpose flour is available in bleached and unbleached. Bleached all-purpose flour is usually treated with benzoyl peroxide or chlorine dioxide to after milling to make it whiter.
If you only have two types of flour in your pantry, it's recommended you have whole wheat flour and all-purpose flour.
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