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.
Read more about this topic: Pressure Cooking
Famous quotes containing the words science of, science, pressure and/or cooking:
“The so-called science of poll-taking is not a science at all but mere necromancy. People are unpredictable by nature, and although you can take a nation’s pulse, you can’t be sure that the nation hasn’t just run up a flight of stairs.”
—E.B. (Elwyn Brooks)
“... my one aim and concentrated purpose shall be and is to show that women can learn, can reason, can compete with men in the grand fields of literature and science ... that a woman can be a woman and a true one without having all her time engrossed by dress and society.”
—M. Carey Thomas (1857–1935)
“The mother whose self-image is dependent on her children places on those children the responsibility for her own identity, and her involvement in the details of their lives can put great pressure on the children. A child suffers when everything he or she does is extremely important to a parent; this kind of over-involvement can turn even a small problem into a crisis.”
—Grace Baruch (20th century)
“For the writer, there is nothing quite like having someone say that he or she understands, that you have reached them and affected them with what you have written. It is the feeling early humans must have experienced when the firelight first overcame the darkness of the cave. It is the communal cooking pot, the Street, all over again. It is our need to know we are not alone.”
—Virginia Hamilton (b. 1936)