Pressure cooking is a very old cooking method that’s experienced a recent resurgence thanks to a new generation of cookers that promise a speedy, set-it-and-forget-it path to dinner. These devices, whether electrical or stovetop, are all much safer than those of decades ago (which tended to explode), with mechanisms to prevent them from accumulating too much pressure. If you read the owner’s manual carefully, however, you’ll still find one important precaution to take when cooking under pressure: adding water. Water and pressure cooking go hand in hand. In fact, it’s water that helps generate the high-pressure environment that makes your food cook faster.
Most pressure cooker instructions state a minimum amount of water required for pressure cooking even a tiny amount of food. Inside the tightly sealed pressure cooker, the water is heated and eventually boils into steam. Since the steam cannot escape, it collects above the food. All those trapped water molecules increase the pressure inside the cooker.
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As temperature increases, gas molecules move faster, which increases the pressure inside the cooker.
So what do water and pressure have to do with cooking food faster? Cooking generally involves raising the temperature of food until chemical reactions take place, like those that break down the tough tissue in meat or soften the starch in vegetables. Those reactions usually happen faster at higher temperatures.
Imagine a conventional pot full of meat and water. As the pot is heated, the temperature inside increases until the water starts to boil. At that point, all the heat energy goes into boiling the water, and the temperature doesn’t increase unless all the water boils away—water in liquid form cannot be a higher temperature than its boiling point.
The same thing happens in a pressure cooker, but the temperature inside is much higher. The temperature at which a liquid boils is dependent on the surrounding pressure. When you cook in a regular pot at atmospheric pressure (14.7 pounds per square inch [psi]), water boils at 100°C (212°F). Inside a pressure cooker, the pressure can increase by an additional 15 psi, to almost 30 psi. At that pressure, water boils at 121°C (250°F). That means food can cook at a much higher temperature than it ever could at atmospheric pressure—and since cooking reactions speed up at higher temperatures, your food cooks faster. It also doesn’t dry out, since the water stays in liquid form.
The same phenomenon explains why cooking at high altitudes can be tricky. Air pressure decreases as you move higher above sea level. At lower pressures, water boils at a lower temperature. That means something simmering away is cooking below 100°C (212°F) and will take longer to cook. At high altitudes, by raising the pressure and boiling point above what happens at sea level, a pressure cooker can really boost the reactions cooking your food.
Want to feel what’s it’s like under pressure? Try out the Feeling Pressured Science Snack to experience just a 10% increase in pressure on your body. In a pressure cooker, your food experiences a 100% increase!
Trying to cook rice in a pressure cooker? You’ll need to add less water than you typically use. Since the pressure cooker is sealed, none will evaporate, and you only need to add enough water for the rice to absorb. Learn more about what happens to the water in your rice.
If you have a plastic syringe, you can get the pressure low enough so that water will boil in your own hands.
