Flour Ages, And The Change Is Oxidation Rather Than Drying

Flour is not inert. It changes measurably in the weeks and months after milling, and those changes affect dough handling long before the flour is anywhere near spoiled.

Freshly milled flour is weak

Straight from the mill, flour produces slack, sticky doughs with poor gas retention and loaves that spread rather than rising.

The proteins are present but the bonds that give gluten its strength form less readily, so the network built during mixing is weaker than expected.

Millers historically stored flour for weeks before sale precisely because of this, and the improvement was known long before its chemistry was.

Oxygen does the strengthening

Exposure to air slowly oxidises sulphur-containing groups in the flour proteins, allowing more of the cross-links that give gluten its elasticity.

The dough becomes stronger, less sticky and better at holding gas, which is the same direction that chemical improvers push it in far more quickly.

This is why the treated and untreated versions of the same flour behave differently, and why a bag opened months ago handles differently from a new one.

The fats change too, and not favourably

Flour contains a small quantity of oil, mostly in the germ, and that oil oxidises over time into compounds with a stale, faintly bitter character.

White flour has had most of the germ removed, so it keeps for a long time. Wholemeal retains it and goes rancid within months.

This is the practical limit on storing wholemeal, and it is why bakers who mill their own store the grain rather than the flour.

Enzyme activity continues quietly

Enzymes in flour remain active at low levels, slowly breaking starch into sugars and modifying proteins even in a sealed bag.

Warmth and moisture accelerate this, which is why flour stored in a hot kitchen changes faster than flour kept cool and dry.

The effects are gradual and usually helpful in moderation, but they contribute to why an old bag of flour ferments and browns differently.

What this means in practice

Flour bought and used within its normal shelf life is unlikely to cause problems, and the variation between brands exceeds the variation from age.

Bakers milling their own grain do see the effect clearly and often rest the flour for a period before using it for bread.

Storage in a cool, dry, sealed container slows every one of these processes, and it is the single measure that keeps flour behaving predictably.