How does a pressure cooker work? How does it cook the food faster and why the whistle?
In: Engineering
Put a pot of water on a flame. The heat energy from the flame will go into the water and start raising the temperature of the water. Higher and higher, until the water hits the boiling temperature of 100 Celsius 212 Fahrenheit.
At this point, the heat energy from the flame goes into “breaking apart” the molecules of water, from the tightly packed “liquid” phase to the very spaced out “steam / gas” phase. The temperature of the water remains constant at 100 / 212 until the last drop of water has boiled off.
So basically the reason why your food doesn’t char like a steak left too long on the barbecue, is because water limits the temperature (of the soup or meal or what have you) to 100 / 212. Without water, your food would char to a burned crisp because that flame can reach much higher temperatures than the water.
In any case, that temperature of 100 / 212 *depends on pressure*. It’s 100 / 212 only in 1 atmosphere of pressure like we have on Earth (outdoors). If you get a pressure cooker and allow steam pressure to build up inside, the pressure can build up to 2 atmospheres of pressure, so, water will start boiling at 121 Celsius / 250 Fahrenheit, and that “chars” (cooks) the food faster.
You’re cooking at 121 / 250 instead of at 100 / 212. It’s cooking “hotter”.
Again, in both cases, water will limit the temperature of the food so it can’t go past the boiling temperature, which prevents the food from charring (as long as it has water).
The whistle is a pressure indicator, to notify when the 2 atmospheres have been reached. Basically you have pressure in there, so if there’s a defect and the pressure gets too high, things could explode.
By cooking at high pressure you discourage water from boiling. It’s 100 C at standard sea-level atmosphere. By raising the pressure you allow temperatures above 100 C to be used in combination with water (steam).
The whistle is the pressure valve which prevents the utensil from exploding, there comes a point where the pressure is becoming too much and this valve vents. The reason it produces sound is much the same as why deflating balloons do.
Pressure cookers as a concept are about 350 years old, pretty neat
They trap the boiling liquid so this results in higher temperatures inside. And the food cooks faster.
Wistle is a valve that regulates pressure inside. Without this it would explode eventualy
Water boils at 100 degrees Celcius at sea level. Depending on where you live, it might boil at a few degrees less. Basically, the higher you go, the less air pressure on the water, thus lower the boiling point.
Water cannot go above 100 degrees unless you put in inside a place with high pressure (Hence the name Pressure Cooker). With the cooker sealed, the temperature can go higher (Around 120 degrees) and transfer more heat to the food, making it cook faster.
The “whistle” is some kind of a safety measure. When the temperature/pressure goes too high it opens and lets some steam out to prevent an explosion.
The pressure cooker creates an area of high pressure within, which raises the boiling point of the water inside. This means the water can be heated to a higher temperature without boiling off meaning it can cook the food faster because it can get hotter.
Normally water boils at 100 degrees. So you can not get the water in a regular pot hotter then this. It takes some time for the heat to get into the food and cook it. But if you could get the water hotter then it would cook the food faster. This is what a pressure cooker does. You close it off so that when you heat the water to 100 degrees it does not boil because there is nowhere for it to go. It will just build up pressure and heat. And because the water is hotter the heat will move faster through the food and cook it faster. But there is one problem and that is that the pressure of the water might cause the pressure cooker to explode. In order to prevent this the cooker itself is built very strong and if it fail it will do so in a safe way. But there is also a safety valve or two that will release the pressure inside well before the cooker is in danger of exploding. When the steam is released it will create a whistling sound telling you that you should turn down the heat a bit.
When you bake cookies in an oven, a recipe specifies a certain temperature. But when you boil pasta, recipes only specify lengths of time, and those change depending on elevation. This is because substances undergoing phase transitions (with boiling, water is changing from a liquid to a gas) are restricted to one temperature. No matter how high or low you turn your stove, boiling water will remain at the boiling point. The boiling temperature, however, changes with the pressure. At higher pressure, water molecules are pushed together more and they can’t “escape” their mutual attractions to become a gas as easily, so the boiling point is pushed to a higher temperature. Inside a pressure cooker, the water is able to reach a higher temperature. This cooks the food faster. The whistle is a pressure valve to keep the pressure inside the pressure cooker from reaching an unsafe level and turning it into a bomb.
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