THE SCIENCE OF A LIGHT SPONGE

The lightest ingredient
is the air.

A cake batter is a liquid full of tiny bubbles, and the cake is only as good as those bubbles.

A ceramic mixing bowl of pale cake batter on a wooden worktop, a metal spoon folding in flour, with eggs and a sieve nearby in soft daylight — AI-generated image, not a photograph of a real person or place.

Cream for bubbles
Gas only grows them
Bake it soon
The number of bubbles is set in the bowl

Creaming butter and sugar traps the air, eggs coat it, baking powder and steam make it grow, and the oven sets it in place. Here is what each step does, in plain English.

Updated 16 October 2026

THE SHORT ANSWER

Bubbles first,
rise second.

Raising agents do not make new bubbles.
They inflate the ones you beat in.

CREAMING

Air goes in with the sugar

Beating fat with sugar traps air on the faces of the sugar crystals. A Cambridge researcher called air the most important ingredient in a cake.

Check a mixing step

RAISING

Gas grows bubbles, not new ones

Carbon dioxide from baking powder joins bubbles that already exist. The number of bubbles in the batter is the number in the finished cake.

See the numbers

THE OVEN

Heat sets the sponge

In the oven bubbles swell with heat, steam and gas. Egg proteins set and flour starch forms a gel, which locks the bubbles in place.

Mixing habits

A QUICK MIXING CHECK

What does this
step do?

Pick a step in a classic creamed sponge to see what it does to the bubbles. The bar shows how much that step adds to the cake's lift.

Based on food science teaching material and research. General kitchen information only.

Mixing step check

Choose a stepCreaming butter and sugarBeating in the eggsBaking powder gets wetLeaving batter to standThe first minutes in the oven

Creaming butter and sugarMakes the bubbles

Air is trapped in the fat on the faces of the sugar crystals. Finer caster sugar creams better than granulated.

Beating in the eggsCoats the bubbles

This step is about covering the fat-coated bubbles in egg rather than adding more. On heating the egg proteins set and hold the air.

Baking powder gets wetInflates bubbles

Bicarbonate and a dry acid react once wet, giving carbon dioxide. The gas enlarges existing bubbles; it cannot start new ones.

Leaving batter to standLoses bubbles

In standing batter small bubbles merge into bigger ones, which can rise and escape, giving lost volume and even tunnels.

The first minutes in the ovenBig expansion

Bubbles grow by thermal expansion, water vapour and carbon dioxide until egg protein and starch set around them.

Sources: Naked Scientists interview with Amy Chesterton, University of Cambridge (2011); Hong Kong Education Bureau food science notes (2016); University of Valladolid Innograin (2024); Royal Society of Chemistry, The chemistry of baking powder.

TWO NUMBERS WORTH KNOWING

Small bubbles,
big growth.

One figure on how much bubbles grow in the oven,
one on what creaming does to the mix.

3–5× more

Bubble volume in a finished sponge compared with the air held in the batter

In an ideal case, the Innograin team says the bubble volume in the final sponge is between 3 and 5 times the volume enclosed in the batter, though some gas is always lost.

GROWTH · UNIVERSITY OF VALLADOLID

1–7°C

Temperature rise of fat and sugar during creaming

In tests from 1932 to 1936, longer creaming warmed the mix more. Five minutes of creaming by hand gave volumes similar to 30 minutes on a slow electric mixer.

CREAMING · IOWA STATE, 1939

The same 1939 bulletin found that creamed volume rose to a maximum and then slowly fell with further beating, and that cakes mixed at 25°C scored higher than those mixed at 30°C.

A sliced round sponge cake on a board, the cut face showing a soft, even crumb full of small holes — AI-generated image, not a photograph of a real person or place.

FOUR HABITS TO TRY

Lighter sponges,
no special kit.

These follow from how bubbles behave in batter. They are habits, not rules: recipes vary.

Use soft fat and caster sugar
Butter and solid fats trap air when creamed; liquid oil cannot. Caster sugar's finer crystals give more surface for air than granulated.
Cream, then add egg gradually
Get the air in first, then beat the egg in to coat the bubbles. The 1939 Iowa tests found adding egg later in mixing generally gave the best cakes.
Fold the flour gently
Flour is there for structure, not more air: its starch gels in the oven. Mix just until it is smooth so the bubbles you made survive.
Get it in the oven
Wet baking powder starts giving off gas straight away, and in standing batter bubbles merge and escape. Bake soon after mixing.

WHY CAKES SINK

Held up by gas
until it sets.

Early in baking the cake is held up by what the Cambridge researcher called pneumatic supports: bubbles surrounded by liquid. Take it out too soon and the bubbles contract as they cool, and the rise falls back, just as a soufflé sinks.

Once the egg proteins and starch have set, the bubbles can no longer expand, so they pop into a connected network. That is why a cut sponge shows an open crumb rather than separate round holes.

Close-up of soft butter and caster sugar being creamed in a bowl with a wooden spoon, the mixture pale and fluffy — AI-generated image, not a photograph of a real person or place.

Questions

Questions people ask

Why cream butter and sugar for so long?

Creaming beats air into the fat, held on the faces of the sugar crystals. The bubbles made now set how many bubbles the cake can have, because no new ones form later.

What does baking powder actually do?

It is bicarbonate of soda mixed with a dry acid. When wet they react to give carbon dioxide. The Royal Society of Chemistry notes most modern powders are double acting, with a second acid that waits for the oven.

Can I make a creamed sponge with oil?

Not by creaming. Hong Kong Education Bureau food science notes say oil, which is 100% fat, cannot trap air and so does not help leavening. Oil cakes rely on other raising methods.

Why did my cake sink in the middle?

One common reason is that the structure had not set. Until egg and starch set, gas bubbles hold the cake up; if they cool early, they contract and the cake falls back.

Does it matter if batter sits before baking?

Yes. The Innograin team says bubbles in standing batter merge and rise out, giving cakes with less volume, an uneven crumb and sometimes tunnels.

The lightest ingredient
is the air.