Vol. III · Water Plate 6 of 11steam, condensation, latent heat, steaming, burns, dumplings, bunsHS

Steam.

A gram of steam condensing on a cold surface gives up as much heat as five grams of boiling water cooling to warm.

Every gram of steam carries about 2 257 joules more than a gram of boiling water — the latent heat it took to boil it. When steam meets anything cooler than 100 °C, it condenses back to water and hands that heat over, right at the surface. That is why steaming is one of the fastest ways to cook (II.3): the steam keeps condensing on the food until the food's surface reaches 100 °C, and it never dries the surface the way an oven does.

It is also why steam burns are worse than splashes of boiling water. A puff from a lifted lid deposits its latent heat on the skin in an instant. The workshop compares the heat delivered by steam and by hot water, and steams a bun.

Below: heat given up per gram by steam condensing and cooling, against water at the same temperature cooling; and the centre of a 50 mm bun in a steamer and in boiling water.

What you will learn

Explain latent heat of condensation.
Explain why steaming is fast.
Explain why steam burns are severe and how to avoid them.
Compare steaming with boiling.

Workshop

Left: joules per gram released by steam at 100 °C condensing and cooling to the chosen temperature, against water at 100 °C cooling to it. Right: centre of a 50 mm bun (the course's bread-dough composition) in a steamer and in boiling water (thermal bench).

Heat hidden in the vapour

A gram of steam that condenses on skin at 40 °C releases about 2,508 J — 10.0 times as much as a gram of boiling water cooling to the same temperature, and nearly all of it in the first instant. Lift a lid away from you, so the first puff goes the other way, and keep hands and faces out of the column above a steamer or a kettle.

In the steamer the same heat cooks food quickly and gently: a 50 mm bun reaches 90 °C at its centre in 20 min, about as fast as in boiling water (21 min), but without soaking, dissolving or washing its surface. That is why Chinese cooking steams fish, buns, dumplings and custards (XVI.3), and why bakers inject steam into ovens for the first minutes of baking bread (XXIV.9): condensing steam heats the dough surface fast while keeping it moist and extensible. Steam's temperature is capped at the boiling point, so steaming never browns.

At the stove

When you next steam vegetables or dumplings, open the lid by tilting it away from you, and notice how the cloud goes where the lid sends it. Put a folded cloth under the lid of a Chinese bamboo steamer, or wrap a metal lid in a towel, to stop condensed drops falling back onto buns.

The latent heat of steam was first measured by Joseph Black and his student James Watt; Watt's separate condenser, which saved that heat instead of wasting it, made the steam engine efficient.

Terms

Condensation: Vapour turning back to liquid on a cooler surface, releasing its latent heat.
Latent heat of condensation: The heat released when steam condenses: about 2 257 J per gram at 100 °C.
Steaming: Cooking by steam condensing on the food, at the boiling point.
Steam injection: Adding steam to an oven at the start of baking to keep a dough's surface moist.

Steam carries latent heat and releases it where it condenses: fast cooking for food, severe burns for skin.

Sources

IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (IAPWS-IF97), region 4 (saturation).