In the same way that Americans speak of flours in terms of their protein (gluten) content, Europeans speak 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 the starch burns up almost entirely. What’s left are mostly minerals.
So what does this minerally “ash” tell you? More than you’d think. If you consider a wheat berry in the same way you would an onion: a thing made up of many layers. The layers on the outside are the tougher ones that contain less of the starch than the much purer inner layers.
The outermost layer is the shell of the wheat berry: the bran. It doesn’t have much digestible starch in it, but rather hard cellulose and other tough stuff. When all that burns up you get quite a lot of leftover “minerals”. The outer oily germ also contains very little starch, and so leaves more minerals behind when it’s burned. And indeed the outer layers of the starchy endosperm contain more non-starch matter than the innermost layers.
So you can see a pattern developing here, I think. Flours with higher “ash contents” are those that have more of their non-starch bits left in them when they hit the fire: bran, germ and such. So a high ash content (say, 1.4%) is going to be a whole grain flour, one that includes the bran, germ and outer endosperm layers, and a low ash content (0.3 percent or less) is going to be a cake flour, that includes only the inner endosperm.
So what else do you know if you know the ash content percentage? Well you know how coarse or fine the flour is since the outer regions of the wheat berry are tougher adn the inner ones softer. You also know the protein level more or less. Why? Because most of the protein in a wheat berry is in the outer layers of endosperm. So a higher ash content flour, in addition to having more bran and germ pieces in it, will also tend to have more protein (gluten). Here you start to see where ash contents begin to overlap to some degree with North American “extraction rates” and gluten percentages. They’re all ways of judging a flour’s relative texture and strength.