Science of Pressure Cooking
The boiling point of water (and water-based liquids) is determined by the ambient pressure. Pressure cookers always require liquid in order to cook food under pressure. At atmospheric pressure and at sea level, the boiling point of water is 100 °C (212 °F) and excess heat only increases the rate at which water evaporates into steam vapour; more heat does not increase the temperature of the water. At higher altitudes above sea level, the atmospheric pressure is lower and thus the boiling point of water is lower, because the lower pressure pushing on the water makes it easier for the water molecules to escape to the surface compared to higher pressure. Inside a pressure cooker, once the water (liquid) is boiling and the steam is trapped, the pressure from the trapped steam increases and this pushes on the liquid, which increases its boiling point, because it becomes harder for the water molecules to escape from the surface as the pressure increases on it. The heat applied to the liquid by the heat source continues to create more steam pressure and the extra heat also raises the temperature of the liquid under this increased pressure. Both the liquid and steam are at the same temperature. Once the selected pressure level is reached, the pressure regulator on the lid indicates this and now the heat source can be lowered to maintain that pressure level and save energy, since extra heat will not increase the temperature of the liquid if the pressure is not allowed to rise — excess pressure will only escape as fast-flowing steam from the lid.
Steam and liquids transfer heat more rapidly than dry air. As an example, the hot air inside an oven at, say 200 °C (392 °F), won't immediately burn your skin, but the wet steam from a boiling kettle at 100 °C (212 °F) will scald your skin almost instantly and 'feel' hotter, despite the steam (and water) in the kettle being at a lower temperature than the air inside a hot oven. Since steam and liquids heat substances faster, the pressure cooker can cook food quicker — under pressure — compared to ordinary cooking methods.
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