How does a pressure cooker work, and is it safe?

By raising the boiling point of water above 100°C, so food cooks at a higher temperature and therefore much faster — and modern ones are considerably safer than their reputation, which derives from equipment of a different era.

The physics. Water boils when its vapour pressure equals the surrounding pressure. In a sealed vessel, steam cannot escape, so pressure rises — and with it, the temperature at which water boils.

At typical domestic pressure-cooker settings, water boils at around 120°C rather than 100°C. Since reaction rates roughly increase with temperature, the additional twenty degrees reduces cooking times substantially — commonly by around two-thirds for braises, stocks and pulses.

This is why it is transformative for specific foods: tough cuts, dried beans without soaking, stocks, and anything where collagen conversion or softening takes hours conventionally.

It is also why cooking takes longer at altitude in an open pan — lower atmospheric pressure means water boils below 100°C, and a pressure cooker directly corrects for it.

What it does not do. It does not cook "under pressure" in the sense of pressure itself tenderising anything — the pressure exists to raise the temperature, and that is its entire function.

The safety features on modern cookers, which are the reason the old reputation is outdated:

A primary regulating valve releasing steam above the set pressure.

A secondary safety valve as backup.

A lid interlock preventing the lid opening while pressurised — mechanically impossible on modern designs, which removes the failure mode behind most historical accidents.

A gasket release as a final path, designed to fail safely.

Electric models add thermal cut-outs and cannot be left on a forgotten burner.

What actually causes problems now:

Overfilling. Never more than two-thirds full, and half for foods that foam or expand — pulses, pasta, grains — because foam can block the vent. This is the main genuine risk.

A blocked vent from food debris, which must be checked before each use.

A perished gasket, which should be replaced periodically.

Forcing the lid, which modern designs prevent.

Releasing pressure incorrectly — natural release for foamy or liquid contents, quick release for vegetables.