Flour : Ash Content

Didier Rosada talked at length about flour during last week’s Artisan III workshop, detailing the flour testing process by which the miller determines the level of enzymes, the protein content and the ash content.
What’s the ash content? The ash content represents the quantity of bran (outer layer of the kernel of wheat) remaining in the flour after the milling process. It is impossible for the miller to separate all of it from the endosperm (starchy part of the grain where most of the carbohydrates and proteins are stored). But the baker needs to know the amount of bran left in the flour as it will have an impact on water absorption, nutrition (mineral content), fermentation activity, breakdown of gluten during mixing, color of the dough, etc. The information will be present on the flour specs sheet, sometimes on the label as well although usually not in this country.
In order to determine the ash content, the miller incinerates some flour at 900 degrees C. All organic components burn and only the minerals are left (most of them are contained in the bran).
In France, the ash content is used to classify the flours. Type 55 flour (T55) : 0.55% of the bran is left in the flour Type 150 (T150) is whole wheat flour.
In Italy, 00 flour contains the least amount of bran and is used for cakes, whereas 0 flour is used for pasta, 1 is all-purpose and 2 is whole wheat.
In the old days, patent flour (the whitest flour) was very much sought after. Not anymore. Today in France, most bakers are moving to T65 because consumers are interested in complementing their diet with more fiber. Some bakers even put T80 in their baguettes as whole wheat is a good way to boost flavor and nutrition.
Yeast feeds on the minerals, so the greater the ash content, the more fermentation activity. For that reason, a smaller amount of yeast should be used in whole wheat dough. Otherwise the fermentation will be too fast. If you go from 100% white to 100% whole wheat, you should use 30 to 40% less yeast.
The bran will affect the strength of the dough as it interferes with gluten formation and punctures the dough, lowering gas retention. Therefore the baker needs to be very careful when fermenting whole wheat dough.
The larger the amount of bran, the darker the color of the crumb and the stronger the flavor ((except if using white whole wheat which is paler and sweeter), the denser the bread and the soggier the crust. For that reason, when baking whole wheat bread, it is best to open the door of the oven towards the end of the baking to make sure the crust stays nice and crispy.
According to Didier, a flour with 0.48 to .54 % ash content is good for artisan baking. You can got to 0.58% with no problem (except that the bread will be a bit darker). The equivalent in France would be T55 to T65.