Vol. XVI · Moist Heat Plate 8 of 11pressure cooker, pressure cooking, 115 °C, 120 °C, beans, stock, collagen, time saving, natural release, Instant Pot, 高压锅HT

Pressure Cooking in Practice.

Beans that need ninety minutes at a simmer are done in about thirty under pressure. Ten or fifteen degrees make the difference.

A pressure cooker seals in the steam so the pressure inside rises, about 70–100 kPa above the atmosphere, and water boils at about 115–120 °C instead of 100 °C (III.4). The slow chemistry of moist cooking speeds up with temperature by the Arrhenius law (V.10): pectin breakdown in beans and vegetables, collagen to gelatin in tough meat, starch gelatinisation in grains. Fifteen to twenty degrees make these two to three times faster.

It suits exactly the foods that take longest: dried beans and chickpeas, stocks and bone broths, braises and curries of tough cuts, whole grains like brown rice. It is less good for food that must be watched (you cannot see inside) and for delicate things that overcook in seconds. Practical points: never fill more than two-thirds (half for beans, which foam); let pressure fall naturally for meats and beans (a quick release can make them tough or burst); and adjust for altitude, where the boiling point inside is lower too. The workshop compares times.

Below: how many times faster bean softening (course pectin rate) and collagen conversion (course gelatin rate) run at the cooker's temperature than at 100 °C, for the gauge pressure and altitude you choose, and what a 90-minute simmer becomes.

What you will learn
Workshop
Boiling point inside from steam tables at the gauge pressure plus local atmospheric pressure (barometric formula). Speed-up = rate at that temperature ÷ rate at 100 °C, for the course pectin breakdown (activation energy 90 kJ/mol) and collagen conversion (65 kJ/mol) rates. Real times also include coming up to pressure and releasing it.

Fifteen degrees, three times faster

At about 70 kPa a cooker at sea level boils at 115 °C. In this model, beans that take 90 minutes at a simmer need about 29 minutes, and a stew that needs three hours about 79 — the beans gain more because pectin breakdown is more sensitive to temperature than collagen conversion. At 2 000 m the open pot boils at 93 °C and the cooker at 111 °C: the cooker saves even more time there, which is why it is so popular in the mountains.

In practice: soak beans when you can (they cook more evenly and foam less); fill no more than half with beans, two-thirds otherwise; bring to pressure on high, then lower the heat to hold it; time from when pressure is reached; let it release naturally for meats, beans and stocks (ten to twenty minutes), quick-release only for vegetables and delicate foods; check that the valve and gasket are clean before each use. Electric multi-cookers do all this automatically but are not tested for canning (XIII.8).

At the stove

Cook the same dried chickpeas two ways, simmered in an open pot and in a pressure cooker, and note the time each takes to become creamy.

Denis Papin demonstrated his "steam digester" to the Royal Society in London in 1679, softening bones into jelly; his safety valve, added after an explosion, is the ancestor of every pressure-cooker valve.

Terms

Pressure cooker
A sealed pot in which steam pressure raises the boiling point.
Gauge pressure
Pressure above the surrounding atmosphere.
Natural release
Letting pressure fall on its own after cooking.
高压锅
Pressure cooker.
Pressure raises the boil to about 115–120 °C, making slow moist reactions two to three times faster: ideal for beans, stocks and tough cuts. Fill sensibly and release naturally for meat and beans.

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. ISO 2533:1975, Standard Atmosphere (the troposphere barometric formula).
  3. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.