Pressure Cooking

Pressure cooking is the process of cooking food, using water or other cooking liquid, in a sealed vessel (known as a pressure cooker) that does not permit air or liquids to escape below a pre-set pressure. Pressure cooking allows food to be cooked with greater humidity and higher temperatures than possible with conventional boiling or steaming methods. In an ordinary non-pressurised cooking vessel, the boiling point of water is 100 °C (212 °F) at standard pressure; the temperature of food is limited by the boiling point of water because excess heat causes boiling water to vaporize into steam. In a sealed pressure cooker, the boiling point of water increases as the pressure rises, resulting in superheated water. At a pressure of 15 psi (103 kPa) above atmospheric pressure, water in a pressure cooker can reach a temperature of up to 121 °C (250 °F).

Pressure is created initially by boiling a liquid such as water or broth inside the closed pressure cooker. The trapped steam increases the internal pressure and temperature. After use, the pressure is slowly released so that the vessel can be safely opened. Pressure cookers heat food quickly because the internal steam pressure from the boiling liquid causes saturated steam (or "wet steam") to bombard and permeate the food. Thus, higher temperature water vapour (i.e., increased energy), which transfers heat more rapidly compared to dry air, results in faster cooking times compared to conventional cooking methods.

Several variants of the pressure cooker exist. An early pressure cooker, called a steam digester, was invented by Denis Papin, a French physicist, in 1679. Large pressure cookers are often called pressure canners in the United States because of their capacity to hold jars used in canning. Pressure canners are specifically designed for home canning, whereas ordinary pressure cookers are not recommended for canning due to the risk of botulism poisoning, because pressure canners hold heat and pressure for much longer than ordinary pressure cookers and these factors are a critical part of the total processing time required to destroy harmful microorganisms. An autoclave is a type of pressure cooker used by laboratories and hospitals to sterilize equipment. Pressure fryers are used for deep fat frying under pressure — note that ordinary pressure cookers are not suitable for pressure frying. In the food industry, pressure cookers are often referred to as retorts or canning retorts.

Pressure cooking can be used to quickly simulate the effects of long braising or simmering.

Almost any food which can be cooked in steam or water-based liquids can be cooked in a pressure cooker.