What the Numbers on a Flour Bag Actually Tell You

Flour is the ingredient a bakery lives and dies by, and yet it is the one most home bakers reach for without a second thought. At Boulangerie de la Baie, choosing a flour is closer to choosing a variety of grape than picking a generic pantry staple: two bags that both say “wheat flour” can behave in completely different ways once water and time get involved. Learning to read the information printed on the bag, and to understand what the milling process did to the grain, is one of the fastest ways to stop guessing and start baking with intention.

The French type system and what ash content means

French flour is graded with a “T” number: T45, T55, T65, T80, T110, T150. Many people assume this refers to how finely the flour is ground, but it actually measures ash content, the amount of mineral matter left behind when a sample of flour is incinerated in a laboratory oven. Those minerals are concentrated in the bran and the outer layers of the wheat kernel, so a higher ash number tells you the flour contains more of the whole grain, not just the starchy white centre.

In practice this means the numbers describe how complete the flour is. T45 is a very white, refined flour with most of the bran removed, prized for delicate pastry and viennoiserie. T55 is the everyday white bread and pastry flour of most French kitchens. T65 carries a little more of the grain and is the classic choice for a rustic baguette or a country loaf, giving more flavour and a faintly cream-coloured crumb. By the time you reach T110 and T150 you are into wholemeal territory, with pronounced nutty, earthy flavours and far more of the germ and bran intact.

North American flours use a different vocabulary altogether, labelled by use rather than by ash: cake flour, all-purpose, bread flour, whole wheat. A rough bridge is that French T45 to T55 sits near a soft all-purpose, T65 leans toward a bread flour, and T110 upward corresponds to whole wheat blends. The translation is never exact, because the wheat itself differs, but knowing the logic lets you substitute sensibly rather than blindly.

Protein and gluten, the engine of structure

If ash content tells you about flavour and colour, protein content tells you about structure. When flour is hydrated and worked, two proteins, glutenin and gliadin, link together into gluten, the elastic network that traps fermentation gases and lets a dough rise into an airy loaf rather than collapsing into a dense patty. A flour with 12 to 14 percent protein has the strength to hold a long fermentation and a tall, open crumb. A flour at 8 to 9 percent protein is deliberately weak, which is exactly what you want for a tender cake or a short pastry that should crumble rather than stretch.

This is why using the wrong flour sabotages a recipe before you have made a single mistake of technique. Try to build a chewy sourdough on cake flour and it will never develop the elasticity to hold its shape. Use a strong bread flour for a sponge cake and you risk a tough, rubbery result because you have handed the batter far more structural protein than it needs. The gluten potential of the flour sets the ceiling on what the dough can do.

A practical note that surprises many home bakers: protein content is not fixed by variety alone. It varies with the harvest, the weather, and even the field. Professional bakeries adjust hydration and mixing time from one sack to the next precisely because the same labelled flour can drink a different amount of water in a wet year than in a dry one. If a dough that normally behaves suddenly feels slack or stiff, the flour may simply have changed under you.

How milling shapes behaviour

Two flours can share an ash number and a protein figure and still perform differently because of how they were milled. Stone-ground flour, crushed slowly between rotating stones, keeps more of the germ oils and tends to have a coarser, more irregular particle size. That gives fuller flavour but also a shorter shelf life, because the oils from the germ eventually go rancid. Roller-milled flour, produced on steel rollers that separate the grain into fractions before recombining them, is more uniform, more stable, and easier to control at scale.

Particle size affects how quickly a flour absorbs water. Finer flours hydrate fast and can feel deceptively wet at first, then tighten as they fully absorb. Coarser wholemeal flours keep drinking water for much longer, which is why a wholemeal dough often benefits from an autolyse, a rest of twenty minutes to an hour after mixing flour and water, before salt and leavening join. That pause lets the bran soften and the flour hydrate evenly, and it makes the dough noticeably easier to shape.

Matching the flour to what you are baking

Once you understand ash, protein, and milling, choosing becomes straightforward. A few reliable pairings:

• Croissants and laminated pastry: a strong but pale flour around T45 to T55, with enough protein to survive lamination yet fine enough to stay tender.
• Baguettes and country loaves: T65, sometimes with a small percentage of T80 or wholemeal blended in for depth of flavour.
• Cakes and delicate sponges: a low-protein soft flour, so the crumb stays tender and fine.
• Rustic sourdough: a bread flour with 12 percent protein or more, often cut with 10 to 20 percent wholemeal for character.
• Shortcrust and sablé: a weaker flour that discourages gluten development, giving that clean, sandy snap.

Blending is where things get interesting. A loaf built entirely from wholemeal can be heavy and tight, but replacing 15 or 20 percent of a white bread flour with wholemeal adds aroma and colour while keeping the crumb open. Small adjustments, tested and repeated, teach you more than any single recipe ever will.

Storing flour so it stays at its best

Flour is not immortal. White refined flours are relatively stable and keep for many months in a cool, dry, sealed container away from light. Wholemeal and stone-ground flours are far more perishable because of their germ oils, and are best used within a couple of months or kept in the refrigerator or freezer for longer storage. A faintly bitter, crayon-like smell is the warning sign of rancidity, and it will carry straight through into the finished bread.

Keep flour sealed not only against moisture but against pantry pests, which find an open bag irresistible. If you buy in bulk, decant into airtight containers and label them with the type and the date. It sounds fussy, but there is nothing worse than reaching for what you think is bread flour mid-recipe and discovering it is a soft pastry flour, or a bag that turned three months ago.

None of this requires a laboratory or a professional mill. It simply asks you to treat flour as an ingredient with character rather than a neutral white powder. Read the number, note the protein, learn how your favourite bag behaves in your own kitchen, and the bread will start to tell you that you finally understand it.