Ash is the mineral material in flour. The ash content of any given flour is affected primarily by the ash content of the wheat from which it was milled and its milling extraction. The test for determining the ash content involves incinerating a known weight of flour under controlled conditions, weighing the residue, and calculating the percentage of ash based upon the original sample weight.
The ash content of wheat varies from about 1.50 to about 2.00%. The pure endosperm contains about 0.35% ash. Considering that the wheat kernel contains about 80% endosperm, it becomes clear that the non-endosperm parts of the kernel (pericarp, aleurone, and germ) are very high in ash when compared to the endosperm. Thus, the ash content is a sensitive measure of the amount of non-endosperm material that is in the flour.
The goal of milling is to separate the endosperm from the non-endosperm parts of the wheat kernel. This separating is difficult and never clean. Thus, there is always contamination of endosperm with non-endosperm and visa versa. As flour yield is increased, the amount of contamination with non-endosperm increases and the ash content increases. Thus, the ash content is a good and sensitive measure of the contamination of the endosperm.
Millers will often comment that the ash does not affect the baking performance of flour. This is probably true. However, the non-endosperm parts of the wheat kernel are known to decrease baking quality and as the ash content increases so does the level of non-endosperm material.
The ash content of white pan bread flour has increased over the years from 0.45% in the 1950s to the current level of 0.50-0.55%. This has undoubtedly resulted from negotiations where the miller has agreed to the flour buyer’s price but only if he can raise the ash content of the flour a couple of points (0.02%).