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Cake flour is nearly identical to pastry flour save one very small difference, and even then the difference is questionable. Surprise, it’s gluten. Cake flours have a reduced gluten content of 7-10%. Much like pastry flour, it is finely milled and available bleached or unbleached. The difference between cake and pastry flour is trivial, and as you probably noticed, the gluten content range is nearly identical save for one measly percent (maybe).
The ranges in gluten percentage account for the differences between producers, the wheats they use, and slight variations in their processing. It’s possible for a cake flour, like King Arthur, and an all-purpose flour, like Bob’s Red Mill, to have the same gluten percentage (in this case 10%).
Note that all-purpose isn’t as finely milled, so the difference between those products can be noticeable, and they are not interchangeable. However, regarding cake flour and pastry flour—two finely milled, low-gluten flours—even a trained pastry chef would be hard-pressed to notice the difference in the final product. If you need cake flour but your store only has the pastry variety, go ahead and buy it. For all intents and purposes, they are interchangeable.
