The Dough Handbook
Flour decides what a dough can become: its protein sets strength, its starch sets thirst, its milling sets flavour.
A wheat berry is mostly endosperm, and milled white flour is mostly that endosperm: about 70–75% starch, 8–14% protein, the rest moisture and trace minerals. The protein fraction is where flours diverge. Two of those proteins, glutenin and gliadin, are inert until wetted; once hydrated and worked they link into gluten, the stretchy web that lets a dough trap gas instead of leaking it.
Strength follows protein, but only roughly. Bread flours at 12–14% protein form tight, elastic networks suited to tall, chewy loaves. All-purpose flour at 10–12% forms a looser web that still holds gas. Cake flour at 7–9% barely networks at all, which is exactly what tender crumbs require.
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Milling leaves its own signature. Ash — the mineral content measured by burning a sample — rises as more of the bran and germ remain, and European type numbers such as T55 or T650 state that ash directly. Higher-ash flours carry more flavour and ferment faster, because the outer layers bring enzymes and nutrients, but their bran edges cut gluten strands and lower the rise.
Starch deserves equal billing. It absorbs the bulk of the dough's water, and the fraction damaged during milling absorbs several times its weight and is converted by amylase enzymes into fermentable sugars. Too little damaged starch and fermentation crawls; too much and the dough turns sticky and grey.
Choosing a flour is therefore choosing a behaviour: how much water it will hold, how fast it will ferment, how high it will rise, and how it will chew. The companion entry on hydration shows how to match water to the flour at hand.
Further reading