Cake flour
A very soft, low-protein, finely milled flour — and in Britain largely a concept rather than a product on the shelf.
Also called soft flour (UK); low protein flour (INTERNATIONAL); cake flour substitute (US); what is cake flour (INTERNATIONAL)
Cake flour is milled from soft wheat to a lower protein content than plain flour, and milled finer. Both matter.
The LOW PROTEIN means less gluten can form, which is the whole point: a cake wants a tender crumb held together by set egg protein and gelatinised starch, and a developed gluten network makes it chewy. The FINE MILL means it hydrates quickly and disperses evenly through a batter, which helps a delicate mixture stay smooth without being beaten.
American cake flour is also usually chlorinated, a treatment that alters the starch and lets the flour carry more sugar and liquid than it otherwise could. That is a real functional difference and it is one reason American cake recipes can specify sugar levels that behave oddly with untreated flour.
In British shops cake flour is uncommon. The standard domestic substitution is to replace a portion of plain flour with cornflour — commonly around two tablespoons per 125 g — which lowers the protein of the mixture without changing the bulk. It reproduces the protein effect and not the fine mill or the chlorination.
What changes if it is left out
Plain flour with part of it replaced by cornflour approximates the protein content. The result is slightly less tender than true cake flour and far closer than plain flour alone.
A widely shared domestic convention. The proportions vary between sources and BakeHQ reports the common figure as a convention rather than a specification.
RecordedEstablished practice, not a measured result
Related
What it does
Structure building
Low protein and finely milled: it builds the least gluten of any wheat flour, which is why a cake made with it is tender rather than chewy.
Covered in
The flours, and what actually separates them
The low-protein end.
The science behind it
The low-protein end, and where regional treatment differences matter.
Used in
A small quantity of weak flour, for as little structure as will do.
Compared in
One side of the comparison.