Go to any supermarket, head to the baking aisle, and you'll be faced with a huge array of different types of flour. Most of them will be wheat flour. And most of that wheat flour will be refined wheat flour—the powdery white substance from which the bran and germ found in whole wheat flour has been removed.
In an American supermarket, those refined wheat flours are further categorized by the amount of protein in the flour, which in turn relates to their specific baking properties.
Cake flour has the least protein at 6 to 8% (depending on brand), and produces a light, fine-textured crumb.
Bread flour has the most, generally around 12 to 14%, which makes for a chewier crumb with more structure, allowing you to form the large bubbles present in good bread.
All-purpose flour comes up in the middle, at around 10 to 12%. It's what most people have on hand as it does a decent job at both cakes and breads.
For most home bakers, that's about all the information you need, but for more advanced bakers and pizzamakers, flour gets a lot more complicated than that. Disclaimer: this one is for all the hardcore bakers out there.
Types of Wheat
The three major distinctions that matter to most bakers are the color, season, and hardness of the wheat.
Red vs. White: This refers to the color of the bran, which is the outer protective coating of the wheat kernel. Bran color is less important in refined flours than in whole wheat flours.
Winter vs. Spring: While it probably doesn't matter to you when your wheat was planted or harvested, it's useful to know that flour from winter wheat has an average protein content of 10-12 percent and medium gluten* strength, while flour from spring wheat has an average protein content of 12-14 percent, and thus high gluten strength.
*Gluten is the protein matrix formed by water and flour that gives bread structure. Thus high protein flours produce more gluten and in general, a more robust structure for heartier, chewier bread
Hard vs. Soft: This is the most important category. Flour from hard wheat has a higher protein content and stronger gluten-forming proteins, making it better for yeasted products and breads with an open, chewy crumb. Flour from soft wheat has less protein, low gluten strength, and is are better for chemically-leavened products like cakes, muffins, biscuits, and cookies, all of which have a very fine crumb.
Any flour can be any combination of those three categories, so you could have a hard red spring wheat or a soft white winter wheat, for example.
One thing to keep in mind is that protein and gluten are related, but not exactly the same. The proteins in the flour combine with water to create gluten, but a high-protein flour may not always create the best quality gluten. So the strength of the gluten is as important as how much protein there is to start with. However, the more protein there is in a flour, the more potential gluten there is as well. You with me?
At the Flour Mill
Flour quality isn't just about the grains themselves. Milling also plays an important role.
When grain is milled, the first product is straight flour, which contains some bran and germ. Bakers in France might use straight flour, but it's uncommon in the US, where straight flour is usually sifted into other categories to make the blends that you'll commonly see on store shelves.
Patent Flour is the name for the white flour that's sifted out of the straight flour. Patent flour can be further categorized into five different types, but most of them are not available retail, so it's really beyond the scope of even this article.
Clear Flour is what's left over after the patent flours have been removed from straight flour. It is darker than the patent flours and has a higher ash content. First clear flour is often blended with rye or whole wheat flours to lighten the texture of those breads without lightening the color as well. Because of the high mineral content (referred to as ash content*) in first clear flour, many people like to use it to feed sourdough starters.
*It's called ash content because when the flour is burned for analysis in a lab, the amount of ash remaining is proportional to the mineral content of the flour.
American vs. European Flour Classifications
European flours are more likely to be categorized by ash content, while American flours are more concerned with protein content. In theory, as ash goes down, protein goes up, but it also depends on the protein level that was present to begin with.
Italian flours are classified by the fineness of the grind and the amount of bran and germ removed. Type 00 flour is the finest grind, is very powdery, and has the least bran and germ remaining. Following Type 00 are Types 0, 1 and 2, with Type 2 being the coarsest flour. Since this is all about milling and not about protein content, it's possible for 00 flour to have varying amounts of protein depending on the original amount in the grain—indeed, you can purchase Type 00 flours intended for various baking purposes. Lower-protein flours are often labeled as "grano tenero", while the higher protein flours (like those for bread or pizza) are "grano duo."
Many bakers choose to substitute lower-gluten American flours for relatively high-gluten Type 00 flour used in Italy for pizza, but you can't substitute any type of American flour and expect the same results. For one thing, the gluten quality in those Italian flours is not equivalent to the gluten in American hard red wheat that's typically found in bread flour. The gluten in American bread flours is hard and springy and results in a very elastic dough, whereas the Italian gluten is firm but not as elastic, making it better for stretching or rolling pizza and pasta.
The "Italian-Style" flour made with American wheat sold by King Arthur Flour has a protein content of 8.5 percent, putting it in the same general range as lower protein Italian flours.
Unfortunately for American bakers, flour labels aren't always very forthcoming about what sort of wheat is in the bag or exactly how much protein or ash it contains. However, much of that information can be found at product websites or by emailing the companies.
While bread and pizza-makers concern themselves with the amount and quality of gluten in flour because of the structure it provides, that protein also has another effect on dough—higher-protein flours absorb more water. What that means to the baker is that a 67-percent hydration dough might feel wetter or drier depending on the brand and type of flour being used.
For commercial bakeries or die-hard bakers who use the same suppliers consistently, the inconsistencies among manufacturers aren't an issue. But for casual home bakers, it's just one more reason why the same recipe might have different results in different hands or on different days. Moral of the story: For consistent results, make sure you use the same brand and variety of flour each time you bake.