The pressure cooker, invented by the physicist Denis Papin, works by expelling air from the vessel, and trapping the steam produced from the boiling liquid inside. This raises the internal pressures and permits high cooking temperatures. This, together with high thermal heat transfer from the steam, cooks food far more quickly, often cooking in between half and a quarter the time for conventional boiling. After cooking the steam is released so that the vessel can be opened safely.
In 1938, Alfred Vischer presented his invention, the Flex-Seal Speed Cooker, in New York City. Vischer's pressure cooker was the first designed for home use, and its success led to competition among American and European manufacturers. At the 1939 New York World's Fair, the National Pressure Cooker Company, later renamed National Presto Industries, introduced its own pressure cooker.
These operate with a spring-loaded valve that is often hidden from view in a proprietary mechanism. This generation is characterized by two or more pressure settings. Some of these pressure cookers do not release any steam during operation (non-venting) and instead use a rising indicator with markings to show the pressure level. These only release steam when the pan is opened, or as a safety precaution if the heat source is not reduced enough when the pan reaches the required cooking pressure. Others use a dial that the operator can advance by a few clicks (which alters the spring tension) to change the pressure setting or release pressure; these release steam during operation (venting).
These include an electric heat source that is automatically regulated to maintain the operating pressure. They also include a spring-loaded valve (as described above). This type of pressure cooker cannot be opened with a cold water quick-release method and should be operated with caution when releasing vapour through the valve, especially while cooking foamy foods and liquids (lentils, beans, grains, milk, gravy, etc.)
At higher altitudes, the boiling point of liquid is somewhat lower than it would be at sea level. When pressure cooking at high altitudes, cooking times need to be increased by approximately 5% for every 300 m (980 ft) above 610 m (2,000 ft) elevation. Since the regulators work off the pressure differential between interior and ambient pressure, the absolute pressure in the interior of a pressure cooker will always be lower at higher altitudes.
Pressure cookers are available in different capacities for cooking larger or smaller amounts, with 6 litres' capacity being common. The maximum capacity of food is less than the advertised capacity because pressure cookers can only be filled up to 2/3 full, depending on ingredients and liquid (see Safety features section).
Modern pressure cookers typically have two or three redundant safety valves and additional safety features, such as an interlock lid that prevents the user from opening the lid when the internal pressure exceeds atmospheric pressure, preventing accidents from a sudden release of hot liquid, steam and food. If safety mechanisms are not correctly in place, the cooker will not pressurize the contents. Pressure cookers should be operated only after reading the instruction manual, to ensure correct usage. Pressure cooker failure is dangerous: a large quantity of scalding steam and water will be forcefully ejected and if the lid separates it may be propelled with considerable force. Some cookers with an internally fitted lid may be particularly dangerous upon failure as the lid fits tighter with increasing pressure, preventing the lid from deforming and venting around the edges. Due to these dangers pressure cookers are generally over-engineered in a safety regard and some countries even have regulations to prevent the sale of non-compliant cookers.
A common mistake is for the user to start timing when a colored pop-up indicator rises, which happens when there is the slightest increase in pressure, instead of waiting for the cooker to reach its selected pressure level. The typical pop-up indicator only shows that the cooker has pressure inside, which does not reliably signal that the cooker has reached the selected pressure. This pop-up indicator often acts as an interlock, preventing the lid from being opened while there is internal pressure. Manufacturers may use their own terminology for it, such as calling it a "locking indicator."
The natural release method allows the pressure to drop slowly; this is achieved by removing the pressure cooker from the heat source and allowing the pressure to lower without action. It takes approximately 10 to 15 minutes (possibly longer) for the pressure to disappear before the lid can be opened. On many pressure cookers, a coloured indicator pin will drop when the pressure has gone. This natural release method is recommended for foods that foam and froth during cooking, such as rice, legumes, or recipes with raising agents such as steamed puddings. The texture and tenderness of meat cooked in a pressure cooker can be improved by using the natural release method. The natural release method finishes cooking foods or recipes that have longer cooking times because the inside of the pressure cooker stays hot. This method is not recommended for foods that require very short cooking times, otherwise the food overcooks.