Pressure cookers speed up cooking by making water boil at a higher-than-normal temperature.
In the process, they may hiss, rattle, and sizzle like an infernal machine, threatening to redecorate your kitchen in shades of goulash. But your mother’s pressure cooker has been re-engineered to be more mannerly and nearly foolproof.
As with all cooking appliances, though, safety is a matter of understanding. Unfortunately, the instructions that come with the pressure cookers are full of scary dos and don’ts that make no sense unless you understand how the things work. That’s what I’m here for.
Pressure cookers burst, pardon me, appeared, upon the scene after World War II as the “modern” way to cook for homemakers whose time was over-programmed with cooking, cleaning and kids. Today, those baby-boom kids have grown up and are themselves over-programmed with jobs, gyms, and Jeeps. Any gadget that promises a gold medal for speed in the Kitchen Olympics is a sure sell.
No matter how many shortcuts you take, though, there are two unavoidable, time-consuming steps in all cooking. One is heat transmission, getting the heat into the interior of the food. That can be the bottleneck in many a “quick” recipe, because most foods are poor conductors of heat. The other slow step is the cooking reactions themselves. The chemical reactions that change our foods from raw to cooked can be quite slow.
Microwave ovens circumvent the slowness of heat conduction by generating the heat right inside the food itself.
But many dishes such as soups and stews benefit from the slow marrying of flavors that takes place in water-based cooking methods such as braising: the searing and simmering of meats and vegetables in a small amount of liquid in a covered vessel. You can’t do that in a microwave oven because the microwaves, not the simmering liquid, will do the cooking.
To speed up braising, we would like to use a higher temperature, because all chemical reactions, including those in cooking, go faster at higher temperatures. But there is a big obstacle: Water has a built-in temperature limit of 212°F, its boiling point at sea level. Turn up the heat to flame-thrower intensities and the water or sauce will certainly boil faster, but it won’t get one bit hotter.
Enter the pressure cooker. It boosts the boiling point of water up to 250ºF. How? I’m glad you asked, because the cookbooks rarely tell you, nor do the instructions that come with the cookers.
For water to boil, its molecules must gain enough energy to escape from the liquid and go flying off freely into the air as a vapor or gas. To do that, they have to push against the blanket of atmosphere that covers our entire planet. Air is light, but it goes up more than 100 miles and the blanket is therefore quite heavy; every square inch of it weighs about 15 pounds at sea level. Under ordinary conditions, water molecules must achieve energies equivalent to a 212ºF temperature before they are able to push through that 15-pound-per-square-inch (psi) blanket and boil away.
Now let’s heat a small amount of water in a pressure cooker, a tightly sealed container with a small, controllable vent for releasing air and steam. As the water begins to boil, it generates steam and, with the vent closed, the pressure inside the container builds up. Only after it has reached a total pressure of 30 psi, 15 from the atmosphere plus an extra 15 from the steam, does the vent controller allow the excess steam to discharge into the kitchen. Thereafter, it maintains the pressure at that 30 psi level.
To push through this higher “blanket” pressure and keep on boiling, the water molecules must now achieve a higher energy than before. To overcome 30 psi of pressure, they require an energy equivalent to 250ºF, and that becomes the new boiling temperature. The high-temperature, high pressure steam speeds cooking by permeating all parts of the food.
As you start to heat the sealed pressure cooker, the vent releases air until the water begins to boil and steam forms. The steam pressure is held at the desired 30-psi level by some kind of pressure-limiting device.
In many cases it’s a little weight on top of the vent tube. During cooking, the weight wobbles aside to release all higher-than-30-psi steam, which hisses as it escapes and scares people into thinking the thing is about to explode. It’s not. Newer pressure cooker designs use a spring valve instead of a weight to maintain the pressure at the desired level.
During cooking, you adjust the burner so that the contents boil fast enough to maintain the steam pressure, but not so fast that an excessive amount of steam is lost through the vent. In any event, the pressure regulator won’t let you turn it into a bomb. After the allotted cooking time, you cool the pot down, so that the steam inside condenses, returns to liquid, and relieves the pressure.
A safety device assures you that the pressure is gone (some models won’t even let you open them until then), whereupon you may open and serve.
