Vol. III · Water Plate 4 of 11pressure cooker, boiling point, gauge pressure, collagen, beans, kinetics, safety valveHMT

The Pressure Cooker.

A pressure cooker does not cook with pressure. It uses pressure to let water get hotter, and heat does the rest.

A pressure cooker is a pot with a lid that seals and a valve that lets steam out only above a set pressure. As it heats, steam builds up inside until the pressure is about one atmosphere above the air outside, and at that pressure water boils at about 120 °C instead of 100. The food is cooked by water and steam twenty degrees hotter than an open pot can manage.

Twenty degrees matter because the slow reactions of cooking — collagen dissolving into gelatin in tough meat, pectin breaking down in beans and root vegetables — speed up steeply with temperature (V.10). The workshop shows the boiling point at each gauge pressure and how much faster those two softening reactions run than in a simmering pot.

Below: the boiling point against the pressure above the outside air, and the speed of collagen-to-gelatin conversion and of pectin breakdown at that temperature relative to a simmer at 95 °C.

What you will learn
Workshop
Boiling point inside the cooker against gauge pressure (outside air plus gauge, IAPWS saturation curve). Rates of collagen solubilising and pectin breakdown relative to 95 °C, from the course's Arrhenius parameters (activation energies of 95 and 90 kJ/mol).

Twenty degrees, several times faster

At a typical high setting, about 100 kPa above the outside air, water inside boils at 120.4 °C. At that temperature collagen dissolves into gelatin about 3.9 times faster than in a pot simmering at 95 °C, and pectin breaks down 6.7 times faster, so a braise that needs three hours on the hob is tender in about 46 min, and dried beans soften in a fraction of their usual time. At a low setting of 50 kPa the cooker reaches 111.6 °C. At 3 650 m the same high setting reaches only 114.1 °C, because the outside pressure it adds to is lower — still far better than the open pot there.

The safety rules come straight from the physics. Never force a cooker open under pressure: the water inside is above its boiling point at room pressure and will flash to steam. Release pressure as the manufacturer says, keep the valve clear, and do not fill it more than about two-thirds (half for beans and grains, which foam). Home pressure cookers are not for canning low-acid foods unless the method is a tested one written for that cooker (XIII.8).

At the stove

Cook a cupful of dried chickpeas, soaked overnight, in a pressure cooker for 15 minutes at high pressure and let the pressure fall naturally; cook the same amount in an open pot and time how long it takes them to become as tender. Expect the open pot to take about an hour or more.

Denis Papin demonstrated his "steam digester" to the Royal Society in London in 1679, cooking bones until they were soft enough to eat; its safety valve became a model for those of the first steam engines.

Terms

Gauge pressure
Pressure above the surrounding air, as a cooker's gauge or valve reads it.
Safety valve
A valve that releases steam if the pressure rises too far.
Natural release
Letting a cooker cool until its pressure falls by itself.
Collagen to gelatin
The slow dissolving of connective tissue in hot water that makes tough meat tender (VI.6).
A pressure cooker cooks at about 120 °C instead of 100, and the slow softening reactions run several times faster.

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. USDA National Institute of Food and Agriculture, Complete Guide to Home Canning, 2015 revision; National Center for Home Food Preservation.
  3. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.