PHYSICS IN ENGLISH J;HCE:?D7C?97 How does a pressure cooker work? To know how a pressure cooker works you must Pressure cookers operate above atmospheric know the physics behind it. The boiling point of pressu re. Once the operating pressure is attained, water is 100 °C. When boiling water in a pot with the temperature in the pot stabilises at the boiling no lid, no matter how much you heat it, the temper- point for water at that pressure, which is about a ture will never go over 100 °C because of evapo- 120 °C at 2 atmospheres of pressure. Further tem- 3 I passaggi di stato I passaggi ration. Also the vaporised steam is at the same tem- perature increase is prevented since the pressure is perature as the boiling water. So when you cook stabilized by the venting of steam from the cooking with a pot of water but this time with a sealed cap, vessel. as you increase the temperature, all that will happen If the temperature is raised by only 20 °C above is that the vapour will try to escape but, because it open pot boiling, why is the cooking time so is inside a sealed environment, it will not be able much faster? The answer is that cooking results to escape resulting in the build up of pressure. The from chem ical reactions in the food, and the rate temperature and pressure have a directly propor- at which all chemical reactions occur depends on tional relationship, so as one increases so does the the temperature. The temperature dependence of other. Steam also has six times the heat potential reactions is variable, but a rough rule of thumb is when it condenses on a cold food product. that the rate will double for every 10 °C increase In the pressure cooker the pressure develops in- in temperature. Therefore the reactions that occur side the vessel as time goes on. As pressure increas- during cook ing will occur roughly 4 times faster in es the boiling point of the water also increases. The a pressure cooker at 120 °C, and the food will cook food inside is cooked very fast because of the high in one quarter of the time. boiling temperature, which means that the food is not (Taken from http://wiki.answers.com/Q/How_does_a_pressure_ cooked at 100 °C but at a much higher temperature. cooker_work) EXCERCISES 1 True or false? pressure food pot pressure faster increase a. Temperature and pressure are change higher direct chemical useful boiling indirectly proportion. T F 3 Match questions and answers. b. Steam has less heat potential when it condenses. T F QUESTIONS ANSWERS c. Pressure cannot increase the boiling A How does 1 The venting of steam point of water. T F temperature from the cooking d. Water boiling in a pot with no lid stabilise inside a vessel prevents pressure cooker? the temperature will never exceed a temperature from continuing to T F of 100 °C. increase. 2 Complete. B What happens 2 Because the rate of A .............................. cooker is a very ...................... inside a pressure chemical reactions cooking instrument, because it makes cooker when that cause the the pressure cooking of food will ......................... cook faster. This is possible increases? double with every because of a ............................ in the boiling point 10 °C increase in of water due to higher ............................... When temperature. you close a .............................. and increase the C Why is the cooking 3 The boiling point temperature the pressure will .............................., time 4 times faster of water also because pressure and temperature are ................... if the cooking increases, so that proportional. .............................. pressure temperature is just food is cooked at a makes the .............................. point of water 20°C higher than temperature higher increase, and the higher temperature makes the the normal boiling than 100 °C. .............................. reactions in food work faster. point of water? For this reason food cooks .............................. when you use a pressure cooker. A ......... B ......... C ..........