Pressure Cooking with PV=nRT - Mark Y.

Author: Mark Yaney

Pressure-cooking is the process of cooking food, using water or other cooking liquid, in a sealed vessel, known as a pressure cooker. Pressure cookers are used for cooking food faster than conventional cooking methods which also saves energy.

Pressure is created initially by boiling a liquid such as water or broth inside the closed pressure cooker. The trapped steam increases the internal pressure and temperature. After use, the pressure is slowly released so that the vessel can be safely opened.

Pressure-cooking can be used for quick simulation of the effects of long braising. Almost any food which can be cooked in steam or water-based liquids can be cooked in a pressure cooker.

Standard(s)

MS-PS1-4 Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.

How do Pressure Cookers Work?

Normally water will boil at 212°F (100°C) at sea level Atmospheric pressure at 1,01325 bar, i.e. normal atmospheric pressure on the sea level at 0°C --'in an open system'. At this temperature liquid water turns to gas. Applying more heat to an open pot of water will only increase the rate that liquid water turns to vapor but will not increase the temperature of the liquid.

Leading Question

Ask the students which will cook faster, the rice in the small container at full heat, or the larger container (the pressure cooker) with half the heat?

Sample Investigation and Materials needed

At the beginning of class, start 2 different batches of brown rice, on 2 identical hot plates. Put one batch of the rice in smaller pot with the lid on it, preferably a noisy one. Place the other batch in a larger pressure cooker and make sure it remains relatively silent by turning the heat down when the gauge reads full pressure. After 10 minutes, check both pots. Feed the students from the pots they guessed about.

Principles Illustrated

The Ideal Gas Law, or combined gas law, basically states that Pressure times volume is equal to the number of moles of a gas times the gas constant times temperature. If one was to increase the temperature, then pressure would naturally increase. Since Volume is held at a constant in a semi-sealed pressure cooker, then pressure must rise along with the rising temperature. What is discrepant are the dramatically reduced cooking times. Dry, hard black beans typically need to be cooked for hours to be mushy. With a pressure cooker, 180 minutes can be reduced to less than 30.

This is mostly because the steam, which normally no more than 212 degrees F is increased to nearly 300, where it denatures the proteins much more rapidly, inducing edible squishiness.