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The Food Lab: For the Best Stir-Fry, Fire Up the Grill
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Wanna know why your Chinese food never tastes as good as it does at the restaurant? It's not better ingredients, it's not ancient Chinese secrets, it's not even MSG (though all of those things can help). It's this: ridiculously high heat. And we're not talking Atlanta on a hot day high heat, we're talking campfire-set-by-a-Red-Dragon-who-came-straight-from-the-depths-of-Mount-Doom-if-Mount-Doom-were-on-the-sun hot.
Take a look in a Chinese restaurant kitchen and you'll see the wok chef tossing the contents of his wok with one hand while scooping up bits of sauce and seasonings with the wide flat ladle held in his other, all the while adjusting two valves set by below the surface with a flick of his knees. To achieve the massive amount of heat output needed for the best stir-fries, these knee-valves control not only gas output, but oxygen as well, in order to burn faster and hotter.
They're connected to specially made burners with giant rings that blast out an inferno of heat that resembles a jet engine afterburner more than anything. It's a beautiful four-limbed dance.
An no, we're not going to building one of these at home today.
What we are going to do is work our way through a few different common home methods of stir-frying to see if we can come up with the ideal way to approximate restaurant-quality dishes.
The Science of Stir-Fries
Perfect stir-fries should have a complex smoky, singed flavor known as wok hei—the breath of the wok. It comes from a combination of polymers and oil breaking down within the skillet, and from microscopic droplets of fat vaporizing as you toss food up and over the edge of a wok into the hot column of air created by the intense burner below. The food must be cooked fast enough that it can develop these flavors while simultaneously retaining a crisp, fresh crunch. There's nothing worse than wok-cooked vegetables that have been sitting around in a steam table until they are dull and drab.
There's the common thought that in order to get this type of food, you require extremely high heat cooking, and while true, there's a bit more complexity to that statement.
When talking about stir-fries, there's one important distinction to learn: the difference between stored heat energy (that is, temperature measured in degrees) and heat energy input/output.
Stored heat energy is precisely what it sounds like: the energy that is stored in the pan itself. Most cooking vessels we use are made of metal, and the amount of heat energy it can store is based on its specific heat capacity (the amount of energy a specific weight of the material contains per degree of temperature), and its mass (that is, its weight). There's a little bit of math here, so for those of you who were into 9th grade algebra or physics, you can go ahead and jump to the bottom for a slightly more in-depth explanation.
Now, with Western cooking vessels, the stored energy in a pan is of vital importance. Most of the highest quality Western cookware is made of heavy gauge metal—cast iron or stainless steel—often sandwiched around a core of aluminum. (The aluminum is there in order to take advantage of its good heat conduction and distribution properties). The idea is that with proper preheating, a Western pan will have enough energy stored in it that even when you add food to it—a big fat steak, for example—it won't lose much temperature, allowing you to cook your food in an even, predictable manner.
With a thick enough skillet and enough preheating—say to around 650°F or so—you could completely remove a pan from the heat, throw a steak in it, and still get a good sear from the stored energy. Once you've got your pan ripping hot, relatively little heat energy input is required to get good results.
A wok, on the other hand, relies on a different principle. Woks are thin, relatively lightweight vessels. The average wok is about a third the thickness of a standard Western pan. This means that at a given temperature, it's got relatively little stored heat. Take a 650°F wok off of the burner, throw some food in it, and its temperature will very rapidly drop, barely singing the food inside. For good wok cooking, high heat energy input is required. Not only must the pan be ripping hot to start, but you need to keep it above a high flame the entire time you cook with it if you want to effectively generate wok hei.
We'll get back to why this distinction is important in a moment. For now, just know that a standard home range has a maximum heat output about 20 to 25 times lower than a restaurant wok range.
Let's take a look at a few of the options you have working in your own kitchen at home.
Option #0 Any kind of Non-Stick Pan
Non-stick is not a viable option for good wok cooking.
Even the best non-stick coatings will begin to break down at around 475°F, and by 650°F, they are actively decomposing into toxic vapours. Most should not be heated above 450°F or so. This is far, far too low a temperature for effective wok-cooking. You will achieve no wok hei, no intense smoky, seared-and-crunchy flavors. The best you'll get is food-court level Chinese food. Any source that advises using a non-stick anything (be it a skillet or a wok) should not be trusted for a wok-based recipe.
Option #1 and #2: Home Burner, Skillet or Wok
It's possible to get good-but-not-great results with a regular skillet on a home burner. The key is to use a heavy pan (stainless steel or cast iron), and to work in batches, making sure to allow the pan to reheat after each batch. A flat-bottomed wok on a home burner is even better.
In a restaurant, wok cooking is a non-stop, start-to-finish, once-it-hits-the-pan-it-stays event. Here's a basic breakdown of the process:
- Step 1: Oil Flavorings. These ingredients include things like hot chiles and spices like Sichuan peppercorns. They are briefly cooked in the hot oil, then removed and discarded or saved for garnish. These are the only elements that will be removed from the wok once they are added.
- Step 2: Proteins*. Marinated or brined (or both) proteins generally cut into bite-sized pieces go in next. They are stir-fried briefly to achieve searing and smokiness. As soon as they've just started to sear, they get pushed up to the cooler regions on the side of the wok to make room in the hot base for...
- Step 3: Vegetables. Like proteins, vegetables require extreme heat to sear properly without losing their crunch. After allowing them to sear briefly in the bottom of the wok, the proteins are pushed back down, everything is tossed together, and the center is cleared again to make room for...
- Step 5: Aromatics. These are finely chopped ingredients like garlic, ginger, scallions, pickled vegetables, or herbs. They need only the briefest of cook time to release their aroma before everything gets tossed together again and you add your...
- Step 6: Sauce. Most traditional Chinese sauces are added sparingly—just enough to barely coat the morsels in the pan. Westernized Chinese dishes will often contain a lot more sauce, as we have a habit of stirring it into our rice. In either case, they are added right at the end and should start boiling as soon as they hit the wok
- Step 7: Garnishes. Final touches, like fresh herbs, sliced scallions, toasted/fried nuts, or seeds.
*In Chinese cooking, very rarely do you see dishes in which vegetables and proteins make up equal parts of a dish. It's usually either a protein with vegetable accents, or a vegetable with protein accents.
So now, back to the difference between stored heat and heat input/output. Why, exactly, is this distinction important? It's because with Western cooking vessels, even heating is of utmost importance. You want the pan to be of a consistent temperature from the edge to the center. Thick gauge metals with an aluminum core help achieve this. You pan loses less heat when food is added, maintaining a relatively high cooking temperature from start to finish. Indeed, if you graph the average temperature in a Western skillet vs. a wok over a home range, you'll find that this is the case:
The leftmost edge represents the maximum pre-heat temperature of 650°F. When you add food to the pan, it rapidly loses heat. A Wester skillet will drop to around 480°F, while a wok, because of its lower amount of stored heat energy, will drop all the way down to nearly 400°F. As the cooking continues, a Western skillet will slowly regain some of that energy lost when the food was added. Remember: a Western skillet is designed to heat and cool very slowly and evenly.
The wok, on the other hand, will regain that lost heat at a faster rate, but it's beginning with a lower initial temperature—it takes nearly a full two minutes before it makes up for lost time and overtakes the Western skillet.
A-ha!, you may be thinking. So a Western skillet is superior to a wok after all for cooking on a low-output burner!
Well let's take a look at a different graph, this time focusing only on the temperature of the bottom of the wok—the high-heat searing zone. For these readings, I added food, stir-fried it until it developed a reasonable sear, then pushed it to the sides of the skillet or wok.
Now we see that in fact, in the area where it matters most—the very bottom of the wok where searing is taking place—a wok actually regains its heat significantly faster than a Western skillet does. Again, this is because a Western skillet is designed for even heat, while a wok is designed for fastheat.
With wok cooking, you want different temperature zones inside the pan. You want a screaming hot part at the very bottom. You want slightly cooler regions around the edges. You want flames rising up the sides of the wok and curling over the edge so that when you fling food up in the traditional stir-fry method, it hits a rush of hot air that helps eliminate the steam and moisture that are the bane of good stir-fried dishes. When food is pushed up the sides, the bottom very rapidly comes back up to hard-searing temperatures, priming it for the next ingredients.
See all those lovely bubbling juices in between those slices of beef? That's not good. It's an indication that your pan is not regaining heat fast enough and that the meat is boiling and steaming rather than searing and browning. It's a surefire road to overcooked meat and under-flavored stir-fries.
When working with a low-output home burner, even with a wok, however, you can't cook more than a half pound of ingredients or so at a time—this is barely enough to feed one person. The key to good stir-frying on a home burner? Cook in batches.
Whether you use a skillet or a wok, divide your meat and vegetables into half pound portions. Add them to the wok one at a time, allowing them to sear and begin cooking before transferring them into a bowl on the side. Once the wok has regained some energy and starts smoking again, cook the next batch, and so on. You can then mix everything back together at the very end just before adding your aromatics and sauce.
One more reason to use a wok instead of a stainless steel skillet: wok hei is not developed in stainless steel, at least not as readily, as it largely comes from the burning of the patina of fats and polymers that have embedded themselves in a well-used carbon steel or cast iron wok (some flavor also develops from droplets of oil that vaporize as you toss your food). For this reason, if you have a cast iron skillet, it's preferable to use it over stainless steel.
A carbon steel is how I cook stir-fries indoors.
That said, there just has to be a better way—a way to pump more heat into that wok—doesn't there?
Indeed there is: take it outside.
Option #3: Wok Set On Chimney Starter
This has been my go-to method for a while now, though I can't remember where I first saw it being utilized. I was pretty sure it was an Alton Brown technique, but now I can't see where he ever mentioned it. (If anyone knows, please inform me!)
Its a pretty self-explanatory method: just fill up your chimney starter with coals, light the sucker up, and once the coals are completely covered in gray ash and hot, put the wok on top, and let'er rip.
A couple notes:
- Make sure that your coals are 100% lit before you start cooking. Placing the wok on top of the starter cuts off the air-flow to the coals at the top, meaning they'll completely stop combusting. If they aren't already glowing hot, they'll end up blocking the heat from the hot coals below. A more enterprising backyard cook than I might try cutting out a few notches for ventilation around the top of the chimney using tin snips. Let me know how that works out if you do it.
- Cook one dish at a time, and let the coals pre-heat for a few minutes in between dishes. Again, this has to do with airflow and the top coals cooling when the wok is in place.
You have the advantage of not smoking out your apartment, but the only problem with this method? No flames licking up the side of the wok. The bottom heats up super fast, so you've got half the battle done right there, but the sides done heat properly, meaning you've still got to work in relatively small batches.
This leads us to...
Option #4: Wok Set In Outdoor Coal-Fired Grill/Turkey Fryer
A full-fledged turkey fryer burner hooked up to a propane tank will get pretty dam close to approximating the heat output of a Chinese restaurant range, though you still need to use a flat-bottomed wok. Even better is to use real live coals. In the past, I've done this succesfully on camping trips or by carefully balancing a wok on top of a pile of burning coals inside a kettle grill (do not try this at home), but I recently snagged myself a Weber 8835 Gourmet BBQ System Hinged Cooking Grate ($30 or so) which is pretty much custom-designed for the task.
It's got a removable central grate that opens up to reveal a circle in which you can nestle a real-deal, round-bottomed wok (I got a 20-incher made out of carbon steel in Chinatown for $14).
With this setup, you've got essentially what amounts to a perfect restaurant arrangement. Light up a full chimney of coals, pile them in the center, and you end up with intense heat below, moderately high heat on the sides, plenty of airflow up and around the edges, the stability of a round-bottomed wok, it really can't be beat. I've produced some of the finest home stir-fries to date on my Weber since picking this puppy up.
Oh, ok—wanna see one more graph? Here ya go. This is average temperature (taken as an average of the center, the middle, and the edge of each cooking vessel) for all of the cooking methods from pre-heating up until 2 minutes after food is added.
See that lovely green line soaring above the rest? Yep, that'd be the wok-on-the-grill setup.
Words of Warning/Advice
A few notes before you jump in and start grill-wokking: This is not the world's safest cooking method. Things are hot, fast, and furious. Before you begin, I suggest the following (lest you learn the hard way):
- Use an all-metal wok. Things get hot. I've scorched the wooden helper-handle on my indoor wok and nearly set the handle on fire trying to use it on a coal setup. Get yourself an all-metal wok, preferably carbon steel—aluminum can melt at the temperatures this can achieve.
- Make sure you have plenty of clearance. Things can and will get very hot and the last thing you want to do is find out that the flowers two feet away from the grill are wilting from the heat or that the stairwell is about to catch fire.
- Have a stack of clean, dry dishtowels or heat-proof gloves. To move the wok around, you're going to need hand protection. I use dry dish towels (wet towels will steam and burn your hands) for both the hand that shakes the wok around, and the hand that wields the spatula. If you are using a heavy wok, you'll probably need both hands to lift it to get the food out at the end. It's a good idea to have another helper nearby to scrape the food out with the spatula while you tilt the wok over a serving platter.
- Have a heat-proof area nearby to place the wok. In the midst of cooking, there may be times when you find that your wok is getting too hot. during these time, you'll want a safe place to rest it while it cools a bit. I use a small galvanized steel trash can and rest the wok right on top (it's the same can I use to store my spent ashes before discarding). This is also a good place to rest the wok while you clean it between dishes (I use water and a soft scrubby held with a long pair of tongs)
- Bring friends. Nothing sadder than letting a good stir-fry go to waste. That is, nothing sadder except for re-heated stir fries.
Get The Recipes!
Stay tuned this week for a new stir-fry recipe every day. Most are my own renditions of Chinese-American staples (of which I am a closet fan).
And in case you missed it, how about some...
Appendix: Specific Heat, Density, and How It Relates To Stored Energy
Specific heat capacity is the amount of energy it takes to raise a specific amount of material by a specific number of degrees. In metric, this is measured in kilojoules per kilogram degrees Kelvin. For instance, aluminum has a specific heat of .91. That means that it takes .91 kilojoules of energy to raise 1 kilogram of aluminum by 1° Kelvin (about 2° degrees Fahrenheit for those who are imperially inclined). Conversely, this means that for every degree Kelvin a kilogram of aluminum is at, it has 1 kilojoule of energy to give up either to the surrounding environment or the food in the pan. Cast iron, with a heat capacity of .46 holds about half as much energy as aluminum per unit weight. This means that given identical weights and starting temperatures, an aluminum pan will contain about twice as much heat energy as a cast iron pan. This gets more complicated when you take into account...
Density. The density of a material is the proportion of how much it weighs versus how big it is. Aluminum has a density of 2.7 grams per cubic centimeter, whereas cast iron has a density of 7.2. This means that given two pans of identical shape and size, a cast iron pan will weigh about 2.5 times more than an aluminum pan. Thus ince cast iron holds about half as much heat energy per unit weight, given identical shapes and sizes, a cast pan will hold about 1.25 the amount of heat energy as an aluminum pan.
About the author: J. Kenji Lopez-Alt is the Chief Creative Officer of Serious Eats where he likes to explore the science of home cooking in his weekly column The Food Lab. You can follow him at @thefoodlab on Twitter, or at The Food Lab on Facebook.