for the spongecake:

Cake flour is a highly refined, low-protein wheat flour specifically formulated for baking fine-crumbed cakes and delicate pastries. Milled from soft wheat varieties, it contains approximately 7-9% protein content—significantly lower than all-purpose flour—and is chlorinated to alter the pH and starch structure, promoting better moisture absorption and tender crumb structure. The resulting flour is exceptionally fine, with a silky texture that distinguishes it from standard wheat flours. Its composition yields lighter, more tender cakes with superior volume and a more refined texture than alternative flours.

Cake flour may also be enriched with added starches (typically corn or potato starch) in some formulations to further reduce protein content and enhance tenderness. Regional variations exist, with some producers offering bleached or unbleached versions, though bleached cake flour remains most common in professional and home baking.

Cake flour is the foundational ingredient in fine cakes, particularly sponge cakes, angel food cakes, and layer cakes where a tender, delicate crumb is desired. It is essential in recipes specifically developed for sponge cake, where minimal gluten development is crucial to achieve the light, airy texture characteristic of the genre. The flour's low protein content and fine particle size allow eggs and sugar to incorporate air more effectively, resulting in superior volume and a velvety mouthfeel.

Beyond cakes, cake flour is employed in pastries, muffins, and quick breads where tenderness is prioritized over structure. It should not be substituted 1:1 for all-purpose flour in bread or yeast-based recipes, as it lacks the gluten strength required for proper rise and crumb development. Sifting before measuring or using the spoon-and-level method ensures consistent results.