Vol. XVI · Moist Heat Plate 3 of 11steaming, steam, condensation, fish, buns, dumplings, 清蒸鱼, bamboo steamer, latent heat, timing by thicknessHT

Steaming.

Cantonese cooks steam a whole fish by the rule "about eight minutes for a fish two fingers thick". The physics agrees.

Steam carries a great deal of energy: each gram releases 2.26 kJ as it condenses back to water on the cooler food (III.6), so condensing steam heats a surface faster even than boiling water, while holding it at 100 °C. That makes the steamer an oven set exactly to 100 °C with very fast heat transfer: food cooks quickly and evenly, stays moist, and does not lose flavour and nutrients into a pot of water. It cannot brown, which is why steamed dishes rely on sauces, garnishes and hot oil poured over at the end (清蒸鱼 finished with sizzling oil over scallions).

China's cooking has steamed for thousands of years — buns (包子, 馒头), dumplings, whole fish, egg custard (蒸蛋, XVII.5), rice cakes, pork with rice powder — in stacked bamboo baskets over a wok. The timing follows the square law (II.2): double the thickness, about four times as long. Water must be at a full boil before the food goes in, and the lid kept on. The workshop steams fish fillets.

Below: minutes for the centre of fish of the thickness you choose to reach 55 °C in steam, in simmering water and in a 180 °C oven (thermal bench), and the steaming time across thicknesses.

What you will learn
Workshop
Thermal bench: cod (as a slab of the thickness chosen) from 5 °C; steam at 100 °C (condensing, heat-transfer coefficient 1 500 W/m² K); simmering water at 95 °C (500); a 180 °C oven with wall radiation (12). Fish is cooked at about 55 °C at the centre, when it just flakes.

An oven at exactly 100 °C

In this model a fillet 20 mm thick steams to 55 °C at its centre in about 4.8 minutes and one 40 mm thick in 18.5 — 3.9 times as long for twice the thickness, as the square law predicts. A 30 mm piece steams in about 10.5 minutes against 28.5 in a 180 °C oven: steam is far quicker than hot air at almost twice its temperature, because condensation delivers heat so efficiently.

Practical steaming. Bring plenty of water to a full boil first; put the food on a plate or lined basket so juices are kept; space it so steam can flow; cover tightly and keep the water boiling; time from when steam is pouring out. For 清蒸鱼, score the thick part, steam just until the flesh flakes at the bone, pour off the steaming liquid, then top with shredded scallion and ginger and pour over smoking-hot oil and seasoned soy. Lift the lid away from you: steam burns are severe.

At the stove

Steam two pieces of the same fish, one twice as thick as the other, checking with a thermometer. Note how much longer the thick one takes: close to four times, not twice.

The oldest Chinese steamers found are pottery 甗 (yan) from Neolithic sites more than five thousand years old: a pot below for water, a perforated vessel above for food — the same design as a modern bamboo steamer over a wok.

Terms

Steaming
Cooking in condensing steam at 100 °C.
Condensation
Steam turning back to water on a cooler surface, releasing its latent heat.
清蒸鱼
Cantonese steamed fish finished with hot oil, scallion and soy.
Bamboo steamer
Stacked woven baskets set over boiling water.
Condensing steam heats food fast but never above 100 °C: gentle, moist, no browning. Time it by thickness squared; start with water at a full boil.

Sources

  1. IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (IAPWS-IF97), region 4 (saturation).
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.