Legumes produce dense foam as they boil, and some of their skins rise to the surface. In a closed pressure cooker, the foam and skin fragments are carried upward with the steam and enter the steam channel and valve. A clogged valve cannot release pressure, creating a safety hazard. The precautions are simple: do not fill the pot more than one-third full, add one spoonful of oil, and let the pressure drop naturally.
A pressure cooker works with the pressure of steam that builds up inside, and the only thing that regulates this pressure is the valve in the lid. Steam exits there, and excess pressure is released there. Legumes produce dense foam as they boil, and some of their skins rise to the surface. In a closed cooker, this mixture is pushed upward and enters the steam channel. Because the channel is narrow, even a few pieces of skin are enough to restrict it. The same risk applies to rice and pasta; these 2 produce starchy foam as they boil.
A clogged valve causes two problems. First, cooking is disrupted: the pressure may not reach the expected level, or the indicators may give a wrong reading. Second, and more importantly, the route for releasing excess pressure is blocked. Modern cookers have backup safety mechanisms, but no mechanism can replace a well-maintained valve. For this reason, follow several rules from the start whenever cooking legumes in a pressure cooker. Cooking legumes in a pressure cooker requires more attention than cooking them in a regular pot.
The first rule is the fill level. For ordinary foods, the pot may be filled up to two-thirds; for ingredients that foam, this limit drops to one-third. Legumes, rice, and pasta fall into this group. The second rule concerns oil: adding 1 tablespoon of cooking oil to the pot disrupts the foam bubbles and noticeably reduces how much they rise. The third rule is to follow the instructions in the cooker’s own manual. If the fill line is not marked inside the pot, fill the empty pot with water and measure it once.
How you release the pressure also matters. For foods that foam, do not use quick release—that is, opening the valve to release the steam all at once; the sudden drop in pressure throws the foam upward and clogs the channel at that moment. The right approach is to take the cooker off the stove and wait for it to cool on its own. If you are in a hurry, hold the bottom of the cooker under cold water, but never force the lid open. Do not leave the cooker on the stove while it cools; the hot burner delays the pressure from dropping.
Maintenance is also part of the process. After every use, remove the lid, clean the valve and steam channel under running water, and hold them up to the light to check that the inside is visible. Check for cracks in the gasket. Repeat this inspection before using the cooker. Some manufacturers do not recommend cooking lentils and dried peas in a pressure cooker; follow any such warning in the manual. The gasket hardens and begins to leak over the years; it is a part that must be replaced.