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under pressure to cook
June 2016

 

In a world of modernist cuisine, sous vide baths and microwave ovens, pressure cookers feel like a throw-back to the past. Yet they deserve a special place in the kitchen, particularly when energy efficiency, speed or tenderness matters.

But why does a pressure cooker faster than quickly steaming food in a covered pot? For a technology that goes back almost three centuries-- surprisingly few explanations for how pressure cooking works are correct.

At some level, a pot of steaming water and a pressure cooker are pretty similar. If primed with a small amount of water, flavors concentrate in a pool of jus. Both devices are saunas delivering 100% humidity, which helps break down tough meat collagen into slippery gelatin. Both take advantage of the enormous latent heat energy steam deposits on cool food. It's the same reason you can place your hand in a 250F oven and feel comfortably warm, while your hand above a steaming pot of water quickly scalds. But the latent heat per molecule of steam in a stove pot or in a pressure cooker is nearly constant. So latent heat is not the source a pressure cooker's 50-100% faster speeds.

Which begs the question- how does a pressure cooker work?

A pressure cooker is nothing more than a sealed chamber, fitted with a safety valve. The pressure rises as water heats on the stove, turns into steam, and the trapped steam molecules move faster and faster. Raising the internal pressure. Depending on the setting of the cooker's safety valve, the pressure vents and plateaus at around twice atmospheric pressure1.

Water normally boils at 212F at sea level (e.g. one atm.). But, inside a sealed container, pressurized steam molecules constantly impinge on the liquid's surface, forcing-back liquid water trying to evaporate. All of which suppresses boiling, until rising temperatures propel water molecules past the barrage of incoming steam. At two atmospheres of pressure, the evaporation and venting rates come into equilibrium, and the boiling temperature increases by 38F to 250F.

So-- are higher pressures, or higher temperatures, or both responsible for the faster cooking speed?

 

Many people claim high pressures "force" braising liquids into the food, so they cook faster. But this assertion is quite muddleheaded, as we discuss more fully in an article on vacuum marination.

Almost all foods are dense with liquid, protein and carbs. No space for steam to enter.

Nor can the steam pry open food. Steam pressure (known as hydrostatic pressure) surrounds and squeezes each piece uniformly. Any attempt to push steam inward at say a crack in a piece of carrot, is exact countered by hydrostatic pressure forcing the carrot's crack closed. A standoff.

Food compresses VERY slightly until the pressure inside matches the hydrostatic pressure outside. Any liquids inside the food won't squirt out, because they are pushed on equally from all directions. It takes a pressure DIFFERENCE to make things move, and there is no pressure difference across the food. There is no "out" direction.

If food contained pockets of air, or was highly compressible like a sponge, it might shrink under pressure, and when the cooker is vented, expand and suck in a bit of liquid. But few foods are that compressible. People (intelligent hunks of meat) aren't that compressible either- when you swim down fifteen feet (about half atmospheric pressure, typical of an electric pressure cookers), your capillaries don't burst, your eyeballs don't pop out, and even the air in your lungs compresses, matching the external water pressure. Once the pressures equalize, which is almost instantaneous, not much happens.

Pressure is NOT the explanation.

As to temperature, it is true 250F is 18% higher than 212F. But the Fahrenheit scale is based on setting 32F as the freezing point of water - an arbitrary choice. Its like claiming you're six foot tall because you are standing on a stool. The scientific scale for temperatures sets zero to the absence of all physical motion, not to the freezing point of water. Known as the Kelvin scale, water boils at 373K, and 2 atm steam reaches 394K, or an increase of 6%. A pretty small thermal advantage2.

No, the real reason pressure cookers cook faster is DENSITY. At two atmospheres (remember your ideal gas law from high school), steam contains twice as many water molecules as the air over a pot of steaming water. Depositing twice as much latent heat every second to the food. And, roughly speaking, cooking up to twice as fast.

Not temperature. Not pressure. Just density.

 

 

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Additional articles on kitchen science can be found HERE.
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In the practice of all-things barbecue, we appreciate the support and conversations with Meathead at AmazingRibs.com, Sterling at BigPoppaSmokers, along with numerous competition pitmasters and backyard chefs.

1 As fans of Mythbusters appreciate, high pressure steam in a sealed container is a recipe for a ballistic missile. Not beef stew. So home pressure cookers are limited by a safety valve (and good sense) to under two atmospheres of pressure. Here, as throughout this article, I reference pressure to a vacuum. In other words, the pressure inside the cooker is one atmosphere greater than outside. And since outside is at one atmosphere above vacuum by definition, the ABSOLUTE pressure inside is two atmospheres.

2 There are reasons other than speed to prefer 240F over 212F. For example, some microbes, like botulism, shrug off boiling water. To kill botulism bacteria, 250F for over 30 minutes is required. This is why people pressure-cook canning jars to preserve foods that are consumed without reheating, or are not acidic. The high temperatures also accelerate browning reactions, destroys enzymes that might lead to rotting or mushiness, and so on.

 


Contact Greg Blonder by email here - Modified Genuine Ideas, LLC.