Durum wheat flour has the highest protein of all flour. However, the gluten that forms when water is added is not elastic, so durum wheat needs to be used in combination with other flours. Bread can be made with up to 26% durum wheat flour.

Wheat and other related grains (including barley, and rye) contain a mixture of two proteins glutenin and gliadin. When flour made from grinding these grains is mixed with water the two proteins combine and form gluten. Without water, gluten is not formed. The more the dough is mixed, the more gluten is developed.

Large holes develop inside the product during baking, and the structure must be strong enough to hold without collapsing. Thus, bread flour is used, and the batter is mixed well to develop the gluten. The high percentage of egg in popovers also helps build structure.

Although bread flour contains more protein than all-purpose, you can generally swap one for the other, if needed. If you use all-purpose flour in a bread recipe that calls for bread flour it will still turn out fine—it just won’t have the same chew or structure as it would if you use bread flour.

The main difference between bread flour and all-purpose flour is a matter of protein. Bread flour, which comes in white and whole wheat varieties, has a higher protein content than all-purpose, usually 11-13%. It’s called “bread flour” because most bread requires higher amounts of protein to produce lots of gluten.

Traditionally, all-purpose flour is made from a mix of soft and hard wheat (sans germ and endosperm). It has a medium protein content that can be used in a great number of recipes. However, if you make all-purpose flour at home, you can prepare it from any whole grain.

You can also use self-rising flour for recipes that have up to 1 1/2 teaspoons baking powder per cup of all-purpose flour. Be sure to omit any added salt from the recipe (self-rising flour has added salt). High in protein, bread flour provides the most structure in the baked goods world.