Excerpt: The Science of Woks

MATERIAL	DENSITY g/cm3	HEAT CAPACITY in J/(g K)	VOLUMETRIC HEAT CAPACITY in J/(cm3 K)	CONDUCTIVITY in W/m K, at room temperature
Carbon steel	7.85	0.49	3.85	54
Stainless steel	7.5	0.5	3.75	45
Cast iron	7	0.46	3.22	80
Aluminum	2.7	0.92	2.48	204
Copper	8.94	0.38	3.40	386

If we were to look purely at heat capacity and conductivity, aluminum is the clear winner here. It stores plenty of heat (nothing has it beat!) and transfers energy quickly (only copper is more conductive). However, aluminum hits a snag in the density department. Because it is so light, you’d need a ludicrously thick aluminum pan to achieve the volumetric heat capacity of carbon or stainless steel. 

In other words, all else being equal, a 2-millimeter-thick carbon steel pan will hold about 60 percent more energy at a given temperature than a 2-millimeter-thick aluminum pan, though an aluminum pan will be about four times more conductive.

So What About This Reactivity Thing?

Reactivity is a pan’s ability to react quickly to changes in heat input. If I switch off the heat, do the contents of the pan continue to sizzle and sweat, or do they cool rapidly? If I need a quick blast of heat, does the wok react fast enough when I turn the dial up?

With a Western-style pan, we typically value consistency over reactivity. We want the pan to maintain a slow, steady sizzle as we add finely diced vegetables to sauté, and we want it to maintain a vigorous sizzle as we sear. Rarely in Western cuisine do you find yourself needing to go from a simmer to a sear to a gentle bubble in the course of a few moments. With wok cooking, this is a frequent occurrence. 

This quality is related to the conductivity of a wok’s material (the more conductive the more reactive) and its thickness (the thinner the more reactive), but more importantly, the geometry of your wok and how it fits with your burner can have a big impact. Ideally, your wok should fit over your hottest burner with a wide enough flat-bottomed area that the ring of flames heats mostly the bottom with a little bit of flame riding up the flared sides. This maximizes reactivity in the center of the pan, where you need it the most.

Stored Heat vs. Heat Input

When talking about stir-fries, there’s one important distinction to learn: the difference between stored heat energy and heat flux.

Stored heat energy is precisely what it sounds like: the energy that is stored in the pan itself. We measure this energy in degrees. When people ask you what the temperature of a given object is, they are really asking how much energy that object has stored up inside its molecules. 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, functionally, its weight).

Now, with Western cooking vessels, the stored energy in a pan is of vital importance. Most of the highest-quality Western cookware is thick and made of dense metals like cast iron or steel. 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 drop much in temperature, allowing you to cook your food in an even, predictable manner. If we go back to the bucket metaphor from above, a heavy Western skillet is like a big, fat bucket of energy. It takes a long time to fill that bucket (which translates to lengthy preheating), but once full, it’s got energy to spare—adding food to it will displace relatively little of its overall stored energy.

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 additional 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. It’s a small bucket that will quickly lose its energy stores as soon as you add food to it.

Thus, for good wok cooking, high heat energy input is required. Not only must the pan be ripping hot to start, but with most recipes you need to keep it above a high flame the entire time you cook in order to replenish the energy being pumped into the food.

Stir-Frying in Woks vs. Skillets

So why this distinction between stored heat and heat input/output? How are the two styles functionally different? 

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, maintaining a relatively high cooking temperature from start to finish.

When you add food to the pan, both a wok and a skillet show a rapid drop in temperature as heat gets transferred to the food. Assuming a starting temperature of 650°F, a Western 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 the energy that was 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.

Aha!, 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 data set, 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 reactive heat.

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 heat 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 column of hot, steamy air. When food is pushed up the sides, the bottom very rapidly comes back up to hard-searing temperatures, priming it for the next ingredients. 

To demonstrate this difference, I used a thermal imaging camera to show the heat patterns in a Western skillet vs. in a wok. 

A preheated wok has a high-temperature zone in the very bottom and gets progressively cooler toward the edges. 

So the key to good stir-frying on a home burner is to cook in small enough batches to allow this bottom section of the wok to remain hot throughout cooking. In a traditional Chinese-style meal with multiple small courses, this occurs naturally, as you are only cooking small batches of each dish. If you want to cook fewer dishes but larger portions, this means cooking in batches.

Whether I’m using a skillet or a wok for my stir fry, I divide my meat and vegetables into one-pound portions (or even half-pound portions if I’m working on an especially slow burner) and add them to the wok one at a time, allowing them to sear and begin cooking before transferring them to a bowl on the side. Once the wok has regained some energy and starts smoking again, I cook the next batch, and so on. I can then mix everything back together at the very end just before adding my aromatics and sauce.