bleached flour [bleech ed flouuhr, flou-er] noun
Flour that has been lightened with a bleaching agent during processing. Most home-baking recipes call for all-purpose flour, cake flour or bread flour (there are many more kinds, but these are the most common types of flour) but within these types there are variables including protein amount, type of wheat and whether the flour is bleached. Technically, all all-purpose flours are bleached. A naturally occurring carotenoid pigment in wheat gives freshly milled flour a faint yellowish tinge. Over the course of about three months, the pigments oxidize and the flour becomes a lighter, whiter color. This is a preferred look by consumers, so manufacturers figured out how to hasten the process using chemicals – typically with either benzoyl peroxide or chlorine gas. Both are considered safe, and while both whiten the flour, the gas also lessens the protein content, which is a significant modification.
Are unbleached and bleached flour interchangeable in recipes? It depends how sensitive your palate is, how picky you are and what recipe you are using. If you are using a couple of tablespoons to thicken a pastry cream, they probably won’t act very differently, nor will the taste be different to most people. In cakes, however, the protein content could greatly affect the textural outcome. In a delicate white cake, the flours might taste different as well; bleached flour can taste metallic or even flat to some with discerning palates.