cchemistry :pressure cooker

Pressure cookers are especially designed cookware of aluminum or stainless steel. All models have a lock-on lid and a vent or pressure relief valve. Most pressure cookers are designed to be used on top of your stove, but there are some electrical models and also a one that is supposed to work in the microwave.

Food is cooked by the high temperatures inside the cooker. how the internal pressure compares to temperatures. This high temperature is made possible by raising the pressure to a point greater than atmospheric pressure. Cooking times are based on pounds of pressure.
Steam cooks many foods quicker than dry cooking methods. Steam is efficient in transferring cooking heat rapidly to foods upon contact without burning or damaging the final product, and for less energy than either electricity or gas. As a result, steam can be used to cook anything from delicate, tender-crisp veggies to large chunks of meat, either fresh or hard frozen.

Consider the difference in cooking between an oven and steamer for example. You can put your hand in a 400oF oven and not burn yourself, but put your hand over a boiling tea kettle and the 212oF steam will scald immediately. This is due to the different methods of heat transfer: Air is a poor conductor of heat; but water is a good conductor. Think of being outside when it's 70oF (quite pleasant) versus being in a pool of water at 70oF (feels very cold).

The increased pressure inside the pot delays the water and/or other liquids inside the pot from boiling under it reaches a much higher temperature. As a result, the cooking process is sped up considerably. A pressure cooker works by building up pressure from steam in the pot, which cooks food at a very high temperature. This method can cut cooking time by up to two-thirds, and will also retain the nutritional value and moisture of foods.

Pressure cookers create a closed environment that literally forces steam through foods to effect heat transfer. When the tightly sealed cooker is set over high heat, steam pressure builds and the internal temperature rises. This environment ranges from 5 to 10psi in low and medium pressure units and 15psi in standard high-pressure units which enables cooking at temperatures of 257o F. Under high pressure (15 pounds per square inch), the fiber in food is tenderized and flavors mingle in record time. What's more, fewer nutrients are lost because cooking is so speedy and nutrient-rich steam condenses in the pot instead of being lost in the air.

To know how a pressure cooker works you must know the physics behind it. The boiling point of water is 100°C. When boiling with in a pot with no cap, no matter how much you heat, it will never go past the temperature of 100°C because of evaporation. Also the vaporized steam is the same temperature of the boiling water. So when you cook with a pot of water this time with a sealed cap, as you try to add more heat by increasing the temp., all that will happen is that the vapor will try to escape but because it is a sealed environment it will not be able to escape resulting in the build up of pressure (or force/area). The temperature and pressure will have a direct proportional relationship so as one increase so does the other. Also steam has 6 times the heat potential when it condenses (changes from gas to liquid state of matter) on a cold food product. The build up of pressure and heat potential are two factors which enable pressure cookers to cook foods faster and more efficiently.

In pressure cooker the pressure develops inside the vessel as time goes on. As pressure increases the boiling point of water also increases. Food inside the pressure cooker cooked very fast because of high boiling temperature, means the food is not cooked at 100 C but much higher temperature than 100 C.
Rapid S

Pressure cookers generally operate at 15psi (~760mmHg) above atmospheric pressure. Since atmospheric pressure is 14.7 psi, the pressure within the cooker is about double the atmospheric pressure. Once the operating pressure is attained, the temperature in the pot stabilizes at the boiling point for water at that pressure, which is about 120 C (248 F) at 2 atmospheres of pressure. Further temperature increase is prevented since the pressure is stabilized by the constant venting of steam from the cooking vessel.

If the temperature is raised by only 20 C above open pot boiling, why is the cooking time so much faster? The answer is that cooking results from chemical reactions in the food, and the rate at which all chemical reactions occur depends on the temperature. The temperature dependence of reactions is variable, but a rough rule of thumb is that the rate will double for every 10 C increase in temperature. Therefore the reactions that occur during cooking will occur roughly 4 times faster in a pressure cooker at 120 C, and the food will cook in one quarter of the time.