Most of these traits are intrinsic to woks, particularly those made of carbon steel, a material that, like cast iron, can be seasoned to a jet-black, nonstick coating and unlike cast iron, can be cast or hammered thin and light enough to make tossing food a possibility.
When I was a test cook at Cook’s Illustrated, I conducted a number of blind taste tests, stir-frying noodles, beef and vegetables in Western-style skillets (the magazine’s recommended method at the time) side by side with a nonstick wok, and my own well-seasoned carbon steel wok. The carbon steel wok unanimously won those taste tests, producing flavors that tasters described as “grilled” or “caramelized.”
As Lan Lam has written in a more recent article from Cook’s Illustrated, this has to do with the chemical interactions between the food and the layers of polymerized oils on the surface of a seasoned wok. It’s also tied to the unique action of stir-frying in a wok. “Modernist Cuisine” describes how when a morsel is tossed up through the heavy cloud of steam that forms above a hot wok, that steam condenses on the surface of the food, a process that “deposits formidable amounts of latent energy that rapidly heats the food.” It then drops back down onto the hot surface of the wok where that surface moisture is re-vaporized, and the cycle repeats. A wok allows you to constantly toss food through its own vapors, speeding up its cooking, which concentrates flavor and promotes the development of new flavor compounds through the Maillard reaction better than a flat skillet can.
The wide rim of a wok factors into another key element of wok hei. In her 2010 book, “Stir-Frying to Sky’s Edge,” Ms. Young emphasizes the importance of adding soy sauce and other liquids around the perimeter of the wok, so as not to decrease the temperature of the searing zone in the center, which can cause meat and vegetables to steam rather than sizzle. This advice is common among Chinese chefs and cookbook authors.
But as I watched Sichuan chef Wang Gang splash soy sauce around the perimeter of a wok full of home-style egg fried rice on his YouTube channel, I noticed something: how rapidly it sizzles and sputters. I was reminded of a Mexican cooking technique I learned from the late chef David Sterling at his home in the Yucatán: As he tipped fresh salsa into a ripping-hot saucepan, it superheated in an instantaneous steamy sputter, giving it a richer color and smoky undertones. Could this concept of a seared sauce also be a factor in wok hei flavor?
To test this, I made two identical batches of lo mein, changing only the manner in which I finished them. For the first, I finished by splashing two tablespoons of soy sauce around the perimeter of the wok, while simultaneously splashing two tablespoons of water into the center of the wok. For the second, I swapped the water and soy sauce. (Adding water to the test ensured that both batches would experience the same cool-down effect of liquid added directly to the center, while only one would develop seared soy sauce flavors.)
The difference was stark. Adding soy sauce to the center of the wok left the noodles with a raw soy sauce flavor, while drizzling it around the hot edges of the wok created smoky flavors reminiscent of grilled meat. I’ve since found that adding a small splash of oil to the perimeter of the wok before adding the soy sauce will prevent the soy sauce from ending up caked onto the side of the wok.