Cake Flour: US cups to ounces

Cake flour is the softest wheat flour you will ever weigh, and that softness is exactly what makes it slippery to convert. Milled from soft winter wheat with a protein content around 7 to 9 percent, cake flour is also chlorinated, a treatment that weakens the gluten further and, more relevant for measurement, alters the surface of each starch granule so the particles slide past each other almost like talc. The grind is much finer than all-purpose, which means cake flour packs differently depending on whether it has just been opened, has been sitting in a humid pantry, or has been jostled in transit. A cup scooped straight from a freshly opened bag can weigh substantially more than a cup spooned from the same bag two weeks later, because the fine particles compress under their own weight over time. King Arthur Baking's ingredient weight chart pins unbleached cake flour at 120 grams per US cup at a density of 0.5072 g/ml, but that figure assumes a spooned and leveled cup, not a packed one.

The single most common conversion mistake is treating cake flour as if it weighs the same as all-purpose flour. Recipes ported from volume to weight, or scaled up from a single layer to a sheet pan, often get plugged into a calculator at 125 grams per cup (the AP standard) when the cake flour reference is 120. That five gram gap per cup compounds fast: a three cup cake recipe ends up with fifteen extra grams of flour, and because cake flour's job is to produce a tender, low-gluten crumb, that extra flour is exactly what you do not want. The cake comes out drier and slightly tougher, and the baker blames the recipe.

The right way to measure cake flour is to fluff the bag first. Cake flour settles more than coarser flours, so stir or whisk the flour in its container before you do anything else. Then spoon it lightly into your measuring cup, never dipping the cup into the bag, and level the top with the flat edge of a knife or a bench scraper. Better still, set a digital scale to grams, tare a bowl, and weigh out 120 grams per cup the recipe calls for. If your recipe specifies sifted cake flour, sift first and then spoon into the cup, because sifting cuts the weight per cup by roughly ten percent. The order matters: "1 cup sifted" and "1 cup, sifted" are two different amounts in baking shorthand, the first sifted before measuring, the second after.

Density precision matters most in the recipes cake flour was bred for. Genoise and chiffon cakes rely on whipped eggs to do almost all the lifting, and an extra ten or fifteen grams of flour will deflate the foam and produce a dense, gummy crumb. Angel food cake is even less forgiving, since there is no fat to mask a heavy hand with the flour. Hot milk cakes, sponge rolls meant to be rolled without cracking, and delicate butter cakes like a classic 1234 cake are all engineered around cake flour's specific weight, and they will reward you for weighing rather than scooping. Cookies and quick breads tolerate the slop. Layer cakes do not.

For substitutions, the workable rule is to take 1 cup of all-purpose flour, remove 2 tablespoons, and replace those 2 tablespoons with cornstarch, then sift the mixture three times. By weight, that is roughly 109 grams AP plus 16 grams cornstarch to produce 125 grams of homemade cake flour analog, which gives you something close to the protein dilution and tenderness, though not the chlorination. Pastry flour at 8 to 10 percent protein is a closer one-to-one swap by volume, and you can use it gram for gram in place of cake flour with only a slight increase in chew. Bleached cake flour from a different brand should still be measured at the King Arthur 120 gram reference; the chlorination process is industry standard and the densities cluster tightly. Avoid swapping in self-rising cake flour without recalibrating the leavening, because you will end up with both too much flour by weight and too much baking powder for the recipe.