A steamed pudding is cooked at a temperature that cannot rise

The ceiling is the whole method

Water at ordinary pressure will not go above its boiling point however hard you heat it, because any further energy goes into converting liquid to vapour rather than raising the temperature. A pudding sitting in or over that water is therefore held at approximately a hundred degrees for as long as you like. An oven has no such limit, and a great deal of what makes an oven-baked cake what it is comes from surfaces reaching two or three times that.

Steam is nonetheless an efficient way to deliver heat, because condensation gives up a large quantity of latent energy at the moment vapour becomes liquid again on a cooler surface. That is why a steamed pudding cooks thoroughly rather than slowly, despite the modest temperature. It is gentle in temperature and quite aggressive in transfer, and the two facts together shape everything that follows.

No browning, and therefore a different kind of flavour

Maillard browning needs surface temperatures considerably above boiling, so a steamed pudding cannot brown, and its surface stays pale, soft and slightly damp. Everything a baked cake gets from its crust — the roasted, caramel notes that a cook rarely credits separately — is absent. The flavour has to come from the ingredients themselves, which is a large part of why the traditional repertoire leans so heavily on dried fruit, spice, treacle, citrus peel and alcohol.

A boiled pudding, cooked in a floured cloth rather than a basin, does develop something like a skin. Flour on the cloth gelatinises against the surface and forms a thin, tough layer that holds the shape and keeps water out. It isn’t a crust in the baked sense and it doesn’t taste like one, but it is a structural element and the reason a cloth pudding can be turned out as a sphere.

Moist heat works on starch and fat in different directions

Starch gelatinises fully and comfortably below a hundred degrees, so a long steaming leaves it thoroughly swollen and set with none of the drying that a hot oven would cause. Protein coagulates well within the range too. Nothing in a pudding batter needs the ceiling raised — the cooking is complete, it is simply unhurried, and the extended time also softens dried fruit and lets it exchange moisture with the batter around it.

Suet is the traditional fat for a reason connected to that same ceiling. Beef fat melts higher than butter does, so it holds its shape well into the cooking and only releases late, leaving small voids behind and giving the characteristically open, light texture. Butter in the same recipe melts early and produces something denser. Vegetarian suets are formulated to melt in roughly the same range, which is the property being copied rather than the flavour.

The clock is long and the reason is transfer, not temperature

A large pudding steamed for several hours is not being cooked slowly out of tradition. Heat has to travel from the outside of a dense, wet mass to its centre by conduction, and a low temperature gradient makes that unhurried. The advantage is that the outside cannot overcook while it waits, since it is already at the maximum temperature the method allows. Overcooking a steamed pudding takes real effort, which is exactly why the method suited a kitchen where nobody could stand over it.

Pressure changes the arithmetic directly. Raise the pressure and water boils higher, so a pressure cooker lifts the ceiling by twenty degrees or so and cuts the time substantially. It also changes the texture, and traditional recipes that specify hours aren’t always improved by the shortcut. That is a matter of judgement rather than physics.

Water gets in unless you stop it

The one real hazard is condensate. Steam condenses on the lid and on the inside of the covering and runs down, and a pudding that takes on water goes soggy at the top and can be genuinely ruined. The pleated paper or foil cover that recipes insist on serves two purposes at once: the pleat allows the pudding to expand without splitting its cover, and the tie keeps the run-off out. Neither is decoration.

The other constant is the water level. A pan that boils dry stops being a steamer and becomes a very hot base, at which point the ceiling disappears along with the method, and the pudding scorches. Topping up with boiling rather than cold water keeps the temperature steady. Both of these are the sort of instruction that reads as fussiness until the mechanism is clear.

Why the method survived and where it sits now

Steaming asks for a pot and a heat source, not an oven, which mattered enormously for most of the history of domestic cooking, and it produces something that keeps well and reheats without complaint. A rich fruit pudding made months ahead and steamed again on the day is not a compromise; the second cooking is part of the plan. Sugar, fat and alcohol between them do most of the preserving.

The form is now largely confined to a handful of traditional puddings in Britain and its former colonies, and to sponge puddings that are steamed for texture rather than necessity. That is a shame in a minor way, because the technique has one genuine advantage that has nothing to do with nostalgia: it is almost impossible to dry out something cooked in saturated steam.