Sponge
biscuit
A sponge is a baked base whose lightness comes from whipped eggs rather than chemical raising agents. Air bubbles trapped during whipping expand in the oven, and the setting protein and gelatinised starch lock the structure in place.
There are two main methods. In the whole-egg method, eggs and sugar are whipped to the ribbon stage and flour is folded in. In the separated method, whites and yolks are whipped apart and combined at the end, giving a lighter, drier sponge.
Flour goes in with folding strokes and without delay: prolonged mixing develops gluten, which turns the sponge rubbery and stops it rising. For the same reason sponges often use low-protein flour, or replace part of it with starch.
A baked sponge is cooled completely and usually rested for several hours: fresh from the oven it crumbles when cut, while a rested one slices cleanly and takes syrup better.