Back in the Biscuit post comments (here, if you're interested), there was a mini discussion about flour, the different types and what this MEANS for your finished product. Flour is probably the single most important staple in my pantry. Flour is the basis for just about everything I make as a pastry chef (not actually everything......but almost).
There are many different types of flours. You may have noticed this happening on the grocery store shelves. There's bread flour, all purpose that's "better for bread", all purpose, and pastry flour, just to name a few. And YES, they really do serve very different purposes.
Every flour has a protein content. Protein in flour is what becomes the gluten strands when you add the liquid, and the yeast and salt, and you get it all going. The higher the protein content, the more gluten you're activating. The more gluten, the denser and chewier the product is going to be.
Cake flour has the lowest percent of protein (6-8%), followed very closely by Pastry flour (7.5-9.5%). You don't want a lot of active gluten happening in your cake or pastries, you want them to be light and tender. Bread, on the other hand, is a denser product; it needs a good network of gluten (from a higher protein content flour, like bread flour) to support all of the rise that will happen between the yeast fermentation, and the heat and steam of the oven.
Now: here's where stuff gets interesting.
Enter, butter. Butter acts as a shortening agent in bread dough- it will actually coat and shorten the individual gluten strands, so if you're making an enriched bread (such as challah), you need to compensate for that gluten shortening effect with a higher protein flour. Think of it this way: your dough is going to need ALL THE HELP IT CAN GET to bounce back (literally) from all of the butter (and, frankly, sugar) that an enriched bread needs in order to be.....enriched. And here's a tip: if you're making brioche or challah at home, before you add the warmed butter, let that dough work for a good 10 minutes to get the gluten as active as possible. Then when you've finished adding all of your butter, give the bread another 10 minutes in the mixer to get that gluten network going again. The more you work a dough, the more active the gluten. And THAT RULE right there is why, on the opposite end of the spectrum, you should try not to over work your pie crust. No one wants a chewy pie crust, right?
(if you overwork your pie crust: DO NOT FREAK OUT. Wrap it up, stick it in the fridge and walk away. let it rest. come back to it later, it will be fine. There is NO CRYING IN PATE BRISEE)
All of these rules about protein content in flour, and now I'm going to tell you not to worry. Don't give your all purpose the heave ho- all purpose flour will serve you well. The protein contents aren't so far off that an all purpose is going to contain too much protein for a good pate brisee (pastry flour, for instance has a protein content of 7.5-9.5%, while all purpose generally has a protein content of anywhere between 10-13%. See? Not so far off). BUT, I would recommend taking your recipe into consideration, and knowing what you now know about butter, if you're working on an enriched bread recipe, opt for the bread flour (which has a protein content of 12-15% and will give you juuuuust that much more of an edge).
Conversely, and some of you already know this, I like to use a higher protein content flour for things that you might not expect, like chocolate chip cookies. The higher gluten content in bread flour gives the cookie a chewy quality that I can't get with all purpose, and the gluten helps to hold the moisture better than an all purpose or lower protein flour. I find that it really DOES make a noticeable difference. Try it next time you make chocolate chip cookies, and let me know if you see a difference in the end result!
Something else that's interesting to think about is whole wheat flour. Have you ever noticed that you never really can find a whole wheat bread without another flour mixed in? I'm talking a pure, whole wheat only loaf? There is a really real reason for this: the fragments of cracked wheat in whole wheat flour will cut the gluten strands, so another flour needs to be incorporated into the dough to help compensate for the effects of the whole wheat. Whole wheat flour has a pretty high % of protein (13-14%), but it needs the boost of another flour in order to work well.
So there you have it: cake flour and pastry flour have the lowest percent of protein, and they'll give you a light, tender product. All purpose is somewhere in the middle of the pack and will yield fair results for most baking, but if you're working with something that has specific needs (ie. enriched bread), then you'll do well to swap out your all purpose for a flour that is job specific.