1 hour ago, ElsieD said:
How do they differ?
Here's a rather detailed treatment (about cakes) from Rose's website: http://www.realbakingwithrose.com/2010/03/the_power_of_flour_part_one_of.html#.WLClpBiZP6A
She generally takes a very empirical approach to questions like this (her master's dissertation was about the effect of sifting on yellow cake), though the explanations could sometimes be clearer. Several statements from the conclusions:
"4. bleached flour results in the best flavor.
5. bleached flour results in the best volume.
6. bleached flour results in the most tender and velvety texture.
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7. unbleached flour results in less volume.
8. unbleached flour results in a coarser, chewier texture.
9. unbleached flour results in a cornbread-like flavor."
And from her book The Pie and Pastry Bible: "Although the bleached and unbleached all-purpose flour from the same national brand have essentially the same protein content, the flours will not behave in an identical manner. Bleaching destroys the extensibility, or stretching quality, of the flour, so using bleached flour would result in a strudel dough full of holes. Bleaching also diminishes the strength of the gluten formed, so using an unbleached flour for a pie crust would make a tougher crust."
All of that being said, I use King Arthur unbleached AP flour for all my routine baking without much thought. I do make pie crusts per a RLB recipe with a mix of bleached AP flour + cake flour (I make them in batches and freeze) and I will break out the (bleached) cake flour for cake recipes, when Rose tells me to, because I find that following her recipes faithfully yields predictable results..