Some recipes call specifically for either unbleached or bleached all-purpose flour. The difference between the two types is negligible; bleaching is done with chemicals that give the flour a whiter hue, while unbleached is slightly creamier in color. Bottom line: All-purpose flour was created so that home bakers wouldn't have to buy a multitude of different flours for home baking needs, and interchanging bleached for unbleached in any recipe won't produce a significant difference.
The Gluten Factor
The chemical bleaching process destroys some of the proteins in flour, arguably changing the outcome of certain baked goods. Protein, or gluten, in flour contributes to the stretchiness factor in dough, and when making yeast products, gluten is important to lock in the gas bubbles produced by yeast. When baking with chemical leaveners such as baking powder, however, gluten's stretchy heaviness actually gets in the way. In these cases, less protein in flour is actually desirable to produce a lighter baked good. With this in mind, unbleached flour may produce a better yeast bread dough than bleached, and bleached might be more desirable in cookies, but the difference will be marginal.
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