In the same way that Americans talk of flours in terms of their protein content, Europeans talk of ash content. But what is this mysterious “ash” and why would you want it in your flour? The answer is that the ash isn’t in the flour, it what’s left over after a set quantity of flour (100 grams, I think) is burned — burned in such a way that all the starch, bran and germ is eliminated, leaving only the charred remains of the non-flour matter behind. The ash is then measured to determine how much of this non-flour matter the original batch of flour contained.
But just what is this “non-flour matter”? “Mineral content” some sources will tell you, “material related to fiber” others say. Both of those terms are essentially true, if non-specific. “Ash” is really a catch-all term for all sorts of non-harmful, non-starch items and/or impurities in the flour, which range from the naturally-occurring minerals in the tissue of the wheat itself to pieces of wheat stalk, bits of dirt and flecks of stone, right up to things like insect parts and rodent hairs.
What…disgusted? Don’t be, because American flours have all those things too (if you don’t believe me, read this sometime…just do yourself a favor and do it on an empty stomach). We just don’t like to talk about it, which — at least I theorize — is the reason we talk about things like protein levels and extraction rates instead.
What can you learn from the ash content of flour? Quite a lot, really. If you consider a wheat berry as you might, oh say, an onion or an egg — it’s a thing that’s made up of layers. Since wheat is grown outdoors, it stands to reason that the outer layers of that berry will contain more impurities than the inner ones (also most of the naturally occurring minerals are found in the outer bran of the berry). Grinding and sifting is a process by which those outer layers are removed, so the more you refine a flour — the more you purify it down to only the very inner, starchy endosperm — the fewer non-flour items you’re going to have in the finished product.
And so it is with European flours, where a high ash content (say, around 1.5 percent) is going to be a whole grain flour and a low ash content (0.3 percent or less) is going to be a cake flour. If this sounds a lot like what we know in America as an “extraction rate”, it is. However because different wheats naturally accumulate more minerals and “non-flour matter” than others according to what type they are and where they’re grown, it’s not a perfect comparison. But it’s close enough for jazz.
Why can’t American flours simply be graded this way as well to make things easier on all of us internationally-minded bakers? It’s been tried. However it turns out that there are fundamental differences in the way Americans harvest and store wheat compared to Europeans. For example, we dry our wheat to a greater degree than Continentals do, which is one of the factors that throws the calculations off.