Egg white foam is doing all the lifting in a chiffon or angel cake

The theory of meringue-leavened cakes is well covered elsewhere. This is about the version you meet in practice.

What the foam is

Whipping egg white unfolds its proteins so they link into a film around air bubbles, with water held in that film. Sugar added gradually dissolves into the water phase and thickens it, which slows drainage and stabilises the foam.

The foam contains no fat, which is precisely why it can hold so much air compared with a creamed butter mixture. In the oven the trapped air expands and the proteins set, giving a very light structure with a large volume of gas. Chiffon cakes add oil and yolks to a separate base, then fold in the meringue as the only source of lift beyond a little baking powder.

Why fat ruins it

Fat molecules disrupt the protein film, so even a trace of yolk in the whites reduces the achievable volume noticeably. Separate eggs one at a time into a small bowl before adding to the main one, so a broken yolk costs you one white rather than six. Plastic bowls hold a residue of fat that washing does not always remove, which is why metal or glass is preferred.

Wiping the bowl and whisk with a little lemon juice or vinegar and drying them removes stray grease. A foam that refuses to gain volume is almost always a contamination problem rather than a technique one.

Whipping stages

Soft peaks flop over when the whisk is lifted, and this is usually the right stage for folding into a batter. Stiff peaks stand upright and hold their shape, which suits piped meringue but folds badly and tends to leave lumps.

Overwhipped whites look grainy, dull and slightly curdled, and they have started to lose the water they were holding. Once past that point the foam cannot be recovered, though adding one fresh white and whipping briefly sometimes helps. Sugar added too early slows the whipping considerably; added too late it does not dissolve and weeps in the oven.

Acid and stabilisers

A small amount of cream of tartar, lemon juice or vinegar lowers the pH and helps the proteins link more readily. The practical effect is a foam that reaches volume faster and is more tolerant of a little overwhipping.

About an eighth of a teaspoon of cream of tartar per white is a common proportion. Copper bowls achieve a similar stabilisation chemically, which is where the traditional preference came from.

None of this substitutes for a clean bowl and fat-free whites.

Folding and filling

Fold a third of the meringue into the base first to slacken it, then fold the rest in two additions. Use a wide spatula and large strokes, stopping while a few faint streaks of white remain rather than mixing to uniformity. The tin is never greased for angel food or chiffon, because the batter needs to grip the sides to climb.

Once the crumb sets, a non-stick tin defeats the same mechanism, which is why these cakes are baked in plain aluminium tube tins. Run a skewer through the batter once to break large trapped bubbles before baking.

Cooling upside down

The set structure is very weak while hot, and left the right way up the cake collapses under its own weight. Inverting the tin, usually onto its feet or over a bottle neck, lets gravity stretch the structure while it firms. Cooling takes at least an hour and often longer, and the cake should be completely cold before release.

Free it by running a thin knife around the edge and the central tube rather than trying to shake it out. A cake that shrinks dramatically as it cools was either underbaked or cooled the right way up.

The takeaway

Clean bowl, soft peaks, gentle folding, and cool it upside down or lose the lot.

Patience is an ingredient, and it is the one people leave out.