I can't find bread flour!
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It's the amount of protein (gluten). Bread flour has a higher percentage. You could use whole wheat flour and add wheat gluten.
Bread flour as you buy in the store is usually white flour made either with wheat which has a naturally higher gluten (protein) content than all-purpose flour, or is flour with added gluten. Whether or not a flour is specifically bread flour has to do only with the amount of gluten and not whether it's white or whole wheat. If you can't find bread flour, adding gluten to all-purpose flour or a whole wheat flour will work. It's also important that hard red wheats (either spring or winter) will have more gluten than soft red or white wheats and are better for making bread.
From a baking perspective, gluten content is the biggest difference which affects the bread's structure (rise and texture). There are several other factors some nutritional, some textural, some in flavor.
Where, if you don't mind me asking, do you live that there is no bread flour at your local grocery?
I disagree with the above answer. The biggest difference between the two is that bread flour is a white flour made only of the endosperm and whole wheat includes the germ and bran. The closest flour to bread flour would be regular unbleached white flour. Bread flour does have a slightly higher protein content than regular white flour but the folks at King Arthur Flour say it can be substituted without harm.
Cynthia is a trusted source on Bread/Baking.
Hope Preston is correct about the contents of whole wheat flour. It actually has a significantly higher protein content than bread flour. Bread flour's protein, or gluten, content is 12-12.5%, while that of whole wheat flour is 14-14.5%, equal to that of high gluten flour which is used to make bagels. That's a difference of 17%, which I think is significant. The reason for whole wheat flour containing more protein is to compensate for the scissoring action that can result from the sharp edges of the bran. During the kneading process, it tends to literally cut, or shorten protein strands. And since protein is what literally holds bread up, well, I imagine you're getting my drift. Bread flour can mean many things to many producers. Look for any flour labeled "best for bread," or even "good for bread machines." If the work bread is mentioned anywhere on the label, you can trust that it is bread flour.
Hmmmâ€¦ Reading my answer after the one above it suggests something other than what I was trying to say. That was careless, sorry. Protein / gluten is a matter of relativity.
First off, the protein content of different brands (and types within each brand) varies significantly and thus their interchangeability. What may (or may not) be true for King Arthur is definitely not true for other brands and recipes. Regardless, even with a high protein whole wheat, there still isn't enough to compensate for the bran's effects. To get comparable results, you need to add either white flour or gluten.
Additionally, since gluten and bran are both absorbent, whole wheat flours require considerable additional water in the mix. And since bran takes a while to absorb that water, it's difficult to judge just how much to add. In other words, for these reasons and others, use a whole wheat recipe if you want to use whole wheat flour, preferably one which specifies the exact brand used.
Perhaps a more useful answer to the original poster would be this: To substitute AP flour for bread flour, add an additional 1 1/2 Tbs. flour for each cup called for in the recipe. The additional flour will boost the protein (gluten) content of the dough.
Giovanni50, I just sent you an email with a bit more information about true bread flour, and a link to a site which rounds it all out for you. Happy baking.