Flour grades aren’t things you hear about too much in home baking circles, and then usually only among very dedicated bread bakers. The reason is because “grades” are terms that are mostly used in the milling industry, and not so much in the baking industry, except when specialty types of flours are needed for specific applications.
So grading is all about what happens in the mill. To get a sense for that, imagine a conveyor belt of wheat berries, fresh off the truck. The first thing that happens is they’re separated from impurities of all kinds, bits of stalk, any little bits of dirt or small rocks, bugs, whatever. Next they’re cleaned and steeped in water to help stiffen the bran and make it easier to remove. When ready the wheat berries are sent through a set of rollers for what is called the “first break”. That is, the berry is crushed. All the various layers of the endosperm (and there are many of them, because the endosperm is onion-like) break into pieces.
The bran and the toughest, least pure outer layers of the endosperm —which contain bits of residual bran and lots of protein and nutrients — break into the biggest pieces. The middle, softer, layers with less protein and nutrients break into smaller pieces, and the inner, softest-of-all, least protein-heavy layers break into the smallest pieces of all.
At that point the whole mess is sifted through a set of giant, stacked sieves. In the very the top sieve you have the bran and the biggest, least pure endosperm pieces. Next down are smaller, slightly purer pieces that have fallen through, below that the slightly purer pieces and so on, until the very bottom where you have the smallest, softest pieces of all. These sieves contain the “streams” from which all the flours in the mill are made.
The contents of the top sieve (bran and toughest parts of the berry) are separated into pure bran (for cereal), shorts (very tough bits of endosperm with a little bran and germ used for animal feed), and a very coarse, brownish flour know as “clear” flour, which is mostly used for rye breads and other very rustic-style loaves. Clear flour is for hard core country bread lovers only. It comes in three grades: fancy (the lightest), first and second.
But I digress. If you were to combine the contents of all the remaining sieves (streams) together and ground them you’d have what’s known as “straight” flour, a slightly coarse, 72% extraction, light brown-colored flour that doesn’t turn up in stores, and that American bakers don’t often bake with. But the French do, which is another reason why it’s so difficult to replicate French breads in the US.
But I digress again. It is all these lower streams that are ground and processed in various ways to produce all the flours we buy at the store. These are called “patent” flours, which are by far the largest family of flours, and include everything from all-purpose (AP) flour to bread flour, high gluten flour, pastry, and cake flours. Patent flours can have highly variable extraction rates. “Extra short” patent flours have the lowest extraction rates (i.e. are cake and pastry flours) and “medium” or “long” patent flours have the highest and are used for bread.
The very “low grade” flour, the leavings of straight flour after it is sifted, is sometimes called “red dog” flour and is only sold as livestock feed or to make pet food.