Folding: keeping the air in · The World Table

From the Lexicon, Creaming, Foaming & Cake Structure

Cakes rise on air you built before the oven. CREAMING: beating butter and sugar until pale: sugar crystals literally cut air pockets into the fat, which chemical leaveners then inflate (butter cakes, cookies where the long cream = lift). FOAMING: whipping eggs (whole: genoise; whites: meringue, angel food, chiffon) into a protein foam, the sponge family's entire structure. RIBBON STAGE: whole eggs and sugar whipped until the batter falls in a slowly-dissolving ribbon, the doneness test for sponges and molten cakes alike. The corollary rule: once air is in, FOLD, never stir: every rough stroke is height surrendered.

Read it in the Lexicon →

How to tell it is going right

The air was actually built first and it shows: whatever was creamed or whipped has changed colour and grown in volume, pale and light rather than dense.
The two mixtures were brought closer before being combined: a little of the light one worked into the heavy one first, so the rest is not beaten flat trying to move it.
Folded and not stirred: a broad implement down through the middle, across the base, up and over, with the bowl turned between strokes.
Stopped the moment the streaks disappear. It is still visibly aerated and holds its shape briefly when the spoon lifts away.
Into the tin and into the oven straight away: a batter left standing on the bench gives back the air it was built for.

Where it goes wrong It was stirred until it looked perfectly smooth. Every stroke past the point where the streaks vanish knocks out air that cannot be put back, so the bake comes out dense and short of the tin, and often wet through the middle where there was no structure to hold it.

A chosen film How to Fold in Ingredients | Baking Mad Silver Spoon Watch for: The spatula cutting down through the centre, sweeping the bowl's floor and turning the mass over while the other hand rotates the bowl: it is a flip, never a stir, and the difference is the air you keep.