Its other names are “family”, “occident” or “plain” flour. It is by far the most common type of flour sold in stores, but at the same time, the hardest to define. Why is that? Because all-purpose flour is a blend of hard and soft wheats that almost every miller combines in different proportions. Part of the reason for that has to do with local availability. Harder wheats grow better in the plain states and softer ones grow better in southern states and the Pacific Northwest.
However different mills also have different ideas about what a good “all-purpose” flour is. “AP” flour is by definition a utility player in the kitchen, so it must be of at least passable use for everything from bread to cakes to brownies, as a thickening agent or a coating for fried foods. That’s a tall task for a single product, so no wonder regional mills have historically tried to tailor their flours to meet the needs of their local markets. For example, in the North where people have historically eaten more yeast breads, higher protein (gluten) flours are favored, since more gluten gives bread a taller rise and a lighter crumb. In the South, where the biscuit is king, home bakers prefer a softer low-protein flour for a finer, more tender quick bread to eat with their country ham. Switch the two, and you’ve got trouble, buster.
Southern all-purpose flours can have a protein content as low as 7.5 percent. That’s almost as low as the lowest protein cake flours. Try making a rustic bread with that! Big national brand AP flours (mostly made in northern locales) are quite high in protein, about 11.5 percent on average, which is very good for bread, since even commercial bread flours top out at around 13 percent protein. But think of that for a second: the jump from, say, King Arthur all-purpose flour to King Arthur bread flour is 1 percent protein. The variation between different brands of American all-purpose flours, however, can be as much as 5.5 percent. Amazing.
Be aware that if you buy all-purpose flour from a local mill that’s unbleached, it will have a shelf life of roughly eight months. After that the fat in the flour (and yes, even white flour has fat) will start to go rancid. Conventional bleached flour will keep for an almost unlimited period without turning, though it will eventually dry out (because yes, flour has water in it, too).