How Do You Make Water Hotter Than Boiling? Hold the Lid Down

Water boils when its vapour pressure matches the pressure pushing down on it, which at sea level happens at one hundred degrees. Raising the surrounding pressure means the water must be hotter before its vapour can push back hard enough, so the boiling point rises. A sealed pot achieves that automatically, since steam produced inside cannot escape and accumulates, raising the pressure until a valve opens. At roughly twice atmospheric pressure the contents reach about one hundred and twenty degrees, which is twenty degrees hotter than anything an open pot can achieve regardless of how hard it is heated, since an open pot simply boils faster without getting hotter.

Twenty degrees sounds modest and its effect is large:

• •Chemical reaction rates roughly double for each ten degree rise
• •So the processes that soften food run around four times faster
• •Tough connective tissue converts to gelatine in a fraction of the time
• •Dried beans cook without soaking, in a third of the usual time
• •Stocks extract more in less time
• •Less energy is used overall despite the higher temperature

A reputation for exploding kept these pots out of many kitchens for decades, and it was not entirely unearned. Older designs relied on a weighted valve and a rubber gasket, and a blocked valve, a perished gasket or an overfilled pot could produce a failure that sprayed scalding contents across a room, with injuries well documented. Modern designs address this with several independent safety features, including a primary regulating valve, a secondary pressure release, a gasket designed to extrude through a slot if pressure rises too far, and a lock preventing the lid from being opened while pressure remains. Those changes have made failures rare, and the reputation has outlasted the hazard by a long way.

The device is far older than most people assume and its inventor had a difficult time with it. Denis Papin, a French physicist working in London, demonstrated a sealed vessel with a safety valve to the Royal Society in 1679, cooked a meal in it for the fellows, and published a description. The safety valve was his own invention and was necessary, since without it the vessel was a bomb, and the same principle protects every pressure system since. The device did not become domestic equipment for two and a half centuries, being too dangerous and too expensive, and it was used industrially for extracting gelatine from bones. Domestic versions arrived in quantity only in the 1930s and 1940s.

The device is most valuable where an open pot performs worst. At altitude, water boils at a reduced temperature, which lengthens cooking times severely and makes some foods effectively impossible, and a sealed pot restores normal conditions, which is why they are standard equipment in high-altitude regions. Sterilising equipment requires temperatures above boiling, which is the principle of the autoclave used in laboratories and hospitals and which is the same device at a larger scale. Home preserving of low-acid foods requires that temperature to destroy the organism responsible for botulism, and a boiling water bath cannot reach it, which is why the pressure method is the only safe route for those foods.

Water boils when its vapour pressure matches what is pushing down on it, so raising the pressure raises the boiling point, and a sealed pot reaches about one hundred and twenty degrees. Reaction rates roughly double per ten degrees, so softening runs around four times faster. Modern designs carry several independent pressure releases, and the technique is essential at altitude and for safely preserving low-acid foods.