How do they control the amount of protein in flour?

Since we know that the difference between bread, all-purpose, pastry, and cake flour is the amount of protein in that flour, how do they control the amount of protein that the flour has in it? Do they simply add protein to the flour until they get the correct amount?

No, that is not what happens at all. It turn out that there are a number of different types wheat that can be grown here in the United States and they all have different amount of proteins in them. The types are basically red or white wheat and the color pertains to the color of the outer shell (bran). The other difference is winter or spring and that has to do with when the wheat is planted. Winter wheat actually is planted in the fall but lives in the field through the winter as a seedling. The last classification is hard or soft with hard wheat having a higher protein content than softer wheats. So you put a combination of those types together such as Hard Red Winter Wheat or Soft White Spring Wheat etc. and you end up with a wheat that has a different protein content. So to get flours to have different protein content, the miller simply mills a different type of wheat. Different types of wheat require different climates to grow in, so one farmer may not be able to grow all of the different types of wheat successfully.

Just look at the ingredient list on a bag of bread flour and you will most see that its main ingredient is Hard Red Winter Wheat.