Ingredient

Cake flour

Low-protein flour, around 8%, milled fine and usually chlorinated. Less protein means less gluten means a crumb that tears softly instead of pulling — the difference between a sponge and a bread roll.

Sourcing
Baking aisle. In the UK 'soft' or 'sponge' flour is the near equivalent; plain flour cut with cornflour approximates it well enough for most recipes.
Varieties
Bleached, Unbleached, Self-raising
Quality cues
Noticeably finer and whiter than plain flour, and it clumps in the hand rather than running through it.
Substitutions
Plain flour with 2 tbsp per cup replaced by cornflour, Italian 00 flour
Storage
Airtight and cool. The fine grind makes it stale faster than plain flour.
Shelf life
6-8 months
Nutrition

Per 100 g

Calories
362 kcal
Protein
8.2 g
Fat
0.9 g
Carbs
78 g
Fibre
1.7 g
Sugars
0.3 g
Sodium
2 mg

From USDA FoodData Central (#169723). Raw, edible portion.