Will a Hot Wok and Cold Oil Really Prevent Sticking?
There's a basic rule of thumb in wok cooking that goes like this: a hot wok and cold oil will prevent your food from sticking. The idea is that you preheat your wok over high heat until it is ready to cook, then add your oil immediately before adding other ingredients.
But does this technique really work, and is it really the best way?
Let's quickly take a look at exactly what makes food stick in the first place. You may have heard that food sticks to pans by getting stuck in microscopic pores in the metal and that oil prevents this by filling those pores and creating a smooth surface. This is not true. Raw proteins interact with metal on a submicroscopic level, forming actual molecular bonds. Even on a perfectly smooth, polished surface with no cracks or imperfections whatsoever, meat will stick to hot metal.
How does preheating prevent this? The thing is, only raw proteins will form this bond. Heat causes proteins to fold in on themselves, or even to break down and form all new compounds. Once in their folded or rearranged form, they no longer stick. So the goal is to get surface proteins to cook before they even come into contact with the metal by heating a film of oil hot enough that it can cook the meat in the time it takes for it to pass from the air, through the film of oil, and into the pan.
With Western-style cooking, I almost always add oil to the pan before I start to preheat it. It's a useful method of telling how hot your pan is. When the oil becomes very loose and starts shimmering (around 300° to 400°F), you're in sauté territory. When it starts to smoke lightly (400° to 500°F, depending on oil type), you're ready to sear, and when it starts to smoke heavily or you see flames licking across the surface, it's a sign that you should probably stop reading your social media feeds and focus on cooking.
So what about this hot pan, cold oil thing? It never really made sense to me; when you add a relatively small amount of oil to a really hot pan, it nearly instantly heats up to the same temperature as the pan-precisely the same temperature it would hit if you heated it up with the pan in the first place. Naturally, I thought that preheating the oil with the wok would work just as well for stir-frying. So I tried it side by side.
I found that whether I heated up the oil with the wok or added oil to an already preheated wok, my food was equally unlikely to stick. But there was one big difference: flavor. With stir-fries, you typically preheat the wok even hotter than you would heat a Western skillet for searing. If you start with oil in a cold wok, by the time it's hot enough to start cooking, the oil will have already broken down a great deal, producing free radicals and acrolein, which gives stir-fries a burnt, acrid flavor.
It turns out that as with much accepted wisdom, this one is right, but for the wrong reasons.
Nonstick coatings like Teflon are specially engineered to be smooth and virtually nonreactive, meaning that these types of molecular bonds will not form even if you start cooking in a cold pan. The problem with nonstick is that most nonstick coatings can't be heated hot enough to sear properly, nor are they robust enough for the vigorous scraping and tossing required for a good stir-fry.
Incidentally, if you've watched any episode of Sichuan chef Wang Gang or Guizhou-based couple Stephanie Li and Christopher Thomas's excellent instructional videos on YouTube, you'll be familiar with the concept of longyau. It's a technique restaurant chefs use to season their woks to prepare them to accept food. The idea is that rather than adding a measured amount of oil to the preheated wok before cooking, you add a large amount of oil, swirl it around to coat, then dump the excess oil out, leaving the wok slicked and ready to go.
To successfully practice longyau, you need to have a separate saucepan or other heatproof container on your stovetop filled with oil, which you can ladle into your wok, then dump back in after swirling. If you stir-fry frequently enough to have a permanent longyau oil pot on your stove, this is a useful technique to keep in your arsenal.
Smoke Signals: How to Tell When Your Wok Is Hot Enough
But wait a minute. Without the visual cues that preheating oil provides, how can you tell if your wok is hot enough? Easy. The wok does this all by itself. A well-maintained and seasoned cast iron or carbon steel surface should always have a very thin layer of oil coating its surface, which means that even without added oil, you'll still see light smoking when it hits stir-fry temperatures. Just to make sure, I'll often rub a small amount of oil into the surface of the wok with a paper towel before preheating. This small amount of oil is not enough to produce off flavors in your stir-fries, but it's enough to indicate the surface temperature of the wok. Once it starts smoking, I add the remainder of my cooking oil, immediately adding my first stir-fry ingredients, which lowers the temperature of the wok and oil enough that the oil will not burn.
