Chris Young / January 2012
Sounds old-fashioned, but modern chefs are learning what grandma knew best: for speed and flavor extraction, a pressure cooker can’t be beat.
Food is basically water with a bunch of impurities. Cuts of meat, for example, are around 70 percent water by weight. Or consider the carrot: while it may seem solid under the heel of a knife, it’s really as liquid as a glass of milk. And lettuce? It’s nothing more than a crunchy water bottle, comprised of over 98 percent water!
Water, it turns out, affects nearly everything that happens in the kitchen, including imposing a speed limit on cooking. Turn up the burner underneath a simmering pot of water and suddenly steam bubbles erupt with enough force to churn the water to a rolling boil; the temperature, however, won’t have increased. The flames beneath the pot are thousands of degrees, the bottom of the pot several hundred degrees, but the temperature of the water in the pot is stuck at its boiling point.
Food is no different than the pot of water. The surface may be searing hot, but beneath the surface—where the food is still wet—it’s never more than boiling hot.
The upshot is that for many foods we enjoy, cooking can only happen so fast. Turning tough pork shoulder succulent and tender, for example, is all about dissolving enough collagen. Likewise, durable plant foods like rice or beans need time to swell with water and soften to the perfect toothsome consistency. For foods such as these, the total cooking time is limited by how fast these chemical transformations occur, not by how long it takes to heat the food.
The only way, then, to speed cooking is to accelerate these chemical reactions. Luckily, chemical change tends to happen faster at higher temperatures. As a general rule of thumb, for every 10°C/18°F increase in the temperature, chemical reactions go about twice as fast, which cuts the cooking time in half. This rule is far from perfect, but it will get you in the ballpark. Consider that succulent and tender pork shoulder again: at 60°C/140°F, it will take around 30 hours of cooking to reach the braised texture I enjoy. Increase the temperature to 80°C/176°F—a traditional oven-braised temperature—and the cooking time drops to around seven or eight hours. If you’re pressed for time, you could boil the pork shoulder and get a similar result in about two hours. Need to get it done faster? You’re stuck, because you can’t raise the temperature within the meat above the boiling point of water. Unless, that is, you raise the boiling point of water.
A good pressure cooker, then, can be a very handy tool in the kitchen. These heavy pots with clamp-on lids and safety valves accelerate cooking because the boiling point of water rises with increasing pressure. When the gauge or indicator on the pressure cooker shows 1 bar/15 psi of pressure above whatever is sealed in the pot, the boiling point of water inside can be as high as 120°C/250°F. Cooked at this temperature, chunks of pork shoulder will be meltingly tender after only 20 to 30 minutes. Remove these morsels from the pressure cooker and plunge them into a deep fryer for a few moments and you’ll have perfect crispy and tender pork carnitas.
OK, what else can I do with a pressure cooker?
For all their intricacies, cooking plant foods is simple. Compared to the cooking of meats and seafood, the way heat tenderizes plant tissues is straightforward and substantially more forgiving of a less-than-exact cooking temperature. Cooked above 90°C/195°F or so, the cell walls will spring leaks while the adhesive that binds cells to their neighbors begins to dissolve. This weakens the cells—making it easier for our teeth to break them open—and loosens their bonds to neighboring cells—making it easier for our teeth to separate them. The higher the cooking temperature, the faster and more completely all of this occurs.
Again, under pressure, the cooking temperature can exceed 100°C/212°F, which accelerates the softening process. For root vegetables, beans, or grains, a pressure cooker can dramatically shorten the cooking time. The speed-up is so dramatic, in fact, that it’s easy to overcook delicate plant foods and, thus, timing everything just right can be important if you don’t want to end up with a vegetable puree. Pressure-cooking is especially useful for making notoriously tough plant foods tender. Really hard grains, seeds, spices, and even nuts can be turned pleasantly tender with a pressure cooker, whereas conventional cooking methods would barely soften these ingredients past a crunch.
The elevated temperatures possible inside a pressure cooker not only speed softening but also cause characteristic flavors to develop far more quickly than they do with conventional cooking. This makes pressure-cooking particularly suited for transforming a long labor-intensive process into one hardly more time-consuming than a casual sauté. A prime example is the classic French onion soup, in which a combination of caramelization and Maillard reactions produce a rich flavor. The traditional approach takes very slow careful work to thoroughly brown the onions without burning them. By using a pressure cooker you can attain the same result in 30 to 40 minutes with no more effort than tossing the sliced onions into the cooker with a small amount of water or stock. It helps to add a bit of baking soda, which further speeds the reactions by making things slightly alkaline. After cooking, adjust the seasoning of the soup with a bit of vinegar or another acid to neutralize the alkaline taste and brighten the overall flavor.
A similar technique can be used to yield purees and soups with incredibly deep roasted aromas. Carrots or squash, for example, cooked with a knob of butter and a pinch of baking soda dissolved in water come out with a deep caramelized flavor and are easily pureed and thinned to the ideal consistency with whatever flavorful liquid you choose. Again, be sure to use some acidity to neutralize the alkaline taste and balance the seasoning.
Extracting flavor under pressure
Perhaps the very best use for a pressure cooker is preparing stocks, broths, and sauces. The relentless cycles of evaporation and condensation that come with a long slow simmer drive a wide range of reactions that build the complex flavors of a well made stock. By increasing the temperature 20°C/36°F above a normal simmer, diffusion and flavor-creating reactions run faster to yield a superb stock in a fraction of the time normally required.
Moreover, properly used, the liquid inside the pressure cooker will not come to a boil, despite the elevated temperature, unless you let the heat get out of hand. So long as steam is not escaping the pressure cooker, the pressure inside will always stay high enough to keep the liquid from boiling. Never reaching a boil is important because it keeps the stock clear. Turbulence from boiling emulsifies oils and small food particles from ingredients into a stock, thereby making it murky.
When the pressure-cooking step is done, you should let the cooker cool before removing the lid. That is a good safety tip, of course, but that’s not the only reason to do this. Cooling first means that volatile aromas in the vapor above the stock will condense back into the liquid rather than escape into the kitchen. When you remove the lid, you should be able to see all the way through a thin top layer of oil to the bed of spent meat and vegetable pieces at the bottom. The stock in between should be crystal clear—and you will be amazed by its flavor.
Some tips when selecting and using a pressure cooker
1) Get the largest size pressure cooker you can afford; you’ll regret it later if you buy a small one. Sadly, there are no pressure cookers on the market larger than 12 liters. Many of you will claim I’m wrong after spending some time on the Internet. A little Google-fu will turn up numerous large pressure canners that are reasonably priced. These are made from raw aluminum and are unsuitable for cooking food that isn’t packaged in glass jars. If you’re a DIY kind of person, you can have these large pots hard anodized, as many commercial cookware manufacturers do for their products. It’s not a terribly expensive process. Otherwise, you’ll have to settle for the 12-liter stainless-steel pressure cooker made by several manufacturers.
2) Electric pressure cookers are convenient, but it seems that all of these run at about half the pressure possible with stove top cookers; thus, the maximum possible cooking temperature is about 8°C/14°F lower. This means that cooking times will be about twice as long.
3) A few commercial appliance manufacturers, such as Electrolux, make very large tilt pressure braising pans in sizes ranging from 16 to 40 gallons for large volume production. Admittedly, these appliances are expensive, but they often result in savings. For example, at The Fat Duck in Bray, England, nearly all stocks have been made using such an appliance since 2005 and the upfront costs were quickly recouped from increased yields and labor savings. At the same time, the resulting stocks were considered superior to those prepared conventionally.
Using these tilting pressure pans is easy: The meats and bones are fried in the kettle at 250ºC/480ºF until brown; vegetables are then included and allowed to sweat before adding water to cover (be sure to install a long reach faucet near the appliance); finally, the pressure-tight lid is lowered and locked in place. The pressure will quickly come up to just under 0.5 bar/7 psi, an effective cooking temp of nearly 112ºC/234ºF, which yields gallons of deeply flavored stock in as little as one to two hours. Once the pressure is released, the entire braising pan tilts 90 degrees, making it easy to drain the stock and clean out the kettle.
4) All pressure cookers include safety mechanisms that vent excess pressure, but frequently overheating a pressure cooker can bend the flanges that hold on the lid.
5) Never force a pressure cooker open while it’s pressurized. The sudden release of pressured steam can inflict horrific burns. To rapidly cool and open a pressure cooker, set it into a sink of cold water and run cold water over the rim of the lid. Be careful, however; this cold-water quick release technique should never be done in a way that floods the valves with water. The water will prevent air from being drawn into the cooker as steam cools and condenses, which can cause a forceful internal suction that can bend the flanges and lid inward—irreparably damaging this pressure cooker.
By adjusting the pressure inside a pressure cooker, you can control the boiling point of water. At elevations above sea level, the total pressure will be lower, and thus the maximum possible temperature inside the pressure cooker will decrease.