Flour ash content tells a buyer how refined the flour is. Minerals sit mainly in the bran, so a low ash means a white, refined flour and a high ash means more bran and a darker, stronger-tasting flour. Ash on dry matter is the basis of German and French flour type numbers.
What is measured
A sample is burned in a furnace until only mineral residue remains. The residue, as a share of the sample, is the ash. ISO 2171 is the reference method. The result can be on dry matter or as is; the two differ by the moisture content, so the basis must be stated.
What ash predicts
- Flour type: German Type 550 is roughly 550 mg of ash per 100 g of dry flour; French T55 is read the same way. These are national conventions, not EU law.
- Colour and taste: more ash, more bran specks and a fuller flavour.
- Mill performance: at the same extraction rate, a lower ash shows cleaner separation of bran.
What ash does not tell you
Ash says nothing about baking strength. Two flours with the same ash can behave very differently if their protein, gluten index or falling number differ. Ash also depends on the wheat: wheat grown on mineral-rich soil gives a higher ash at the same refinement.
In a contract
Specify an ash band on dry matter with ISO 2171 named, rather than a grade name. The flour specification guide explains the full set of numbers, and the glossary entry on flour ash gives the definition.