Q: What is cake flour and is it available in Australia?
A: 'Cake' flour is a low protein flour which usually contains about 7-8% protein (which you will find expressed as grams of protein per 100g flour in the nutritional panel on the packet). In comparison, regular plain flour contains around 10-11% protein while ‘strong’ or bread flour contains around 12-15% protein.
Why the protein content is important is that it's an indicator of how much gluten can be developed in the batter or dough and how strong the baked 'structure' will be.
High protein = strong structure
Low protein = weaker, softer structure
High protein flour is used in bread baking where a strong structure is needed to effectively hold the carbon dioxide bubbles, while low protein flours are used when you want to achieve a more tender, softer crumb like in a sponge cake (and why it is called cake flour).
It is available at selected supermarkets and specialty food stores, usually as a self-raising flour, but if you are unable to get hold of some you can make your own by reducing the protein content of regular plain flour by substituting some of it with cornflour (which is gluten protein-free).
To make a homemade ‘cake’ flour substitute: Measure 150g (1 cup) regular plain flour and then remove 2 tablespoons of it. Add 2 tablespoons cornflour and sift together before using.