Principles of Stir Fry, Part One
Over the years I've developed close relationships with my kitchen instruments. In my arsenal of tools, the cast iron skillets are my pride and joy. Slick from constant use with duck fat and lard, the patina of the skillets is nonstick and the browning abilities, virtually indisputable. Without exception, my most difficult working relationship has been with my collection of woks.
The majority of them are stacked in a dark corner in the kitchen. I take one out when I need an extra receptacle, but mostly, the woks are reminders of past hopes and plans. They are the unwanted ones--the vessels that have, for one reason or another, failed to produce the right results.
First Attempt: The Carbon Steel Wok
I started my search for the perfect wok by gathering information about the different materials available. Touted as the material used by traditional Chinese cooks, the 16-inch wide carbon steel wok was my first purchase. The wok was beautiful the day I brought it home: shiny with a blue tinge, the surface was supposed to develop a dark patina once seasoned.
"Patina" is a culinary term for the acquired change in the appearance of a surface. Ideally, you want a cooking vessel with a distinguished patina. With cast iron, a patina develops when fat comes into contact with the surface of the metal. The fat penetrates the material and in doing so, transforms it from a dull and dry surface into a slick and supple one.
Patina development is an intimate process for a cook and his or her vessel. Pouring acidic ingredients, like tomato sauce or wine, can wear away at the patina, whereas fat builds it up. A patina is an organic, dynamic surface that is sensitive to its surroundings. When a skillet is seasoned enough to acquire one, the patina becomes virtually nonstick.
Over the next few months, I stir-fried almost every day in the carbon steel wok, and even acquired a smaller carbon steel vessel as an accompaniment. A slick, dark mahogany patina emerged on both woks, yet a deeper problem had become apparent from the outset.
No matter how fiercely I preheated the woks, the moment I tossed in my ingredients, the heat would quickly dissipate. For stir-frying, the absence of high heat is crippling. Without high and dry heat, the items steam instead of sear, precluding the delicately charred, elusive taste of a good stir-fry. Regretfully, I put away the carbon steel woks and went back to the drawing table.
Second Attempt: The Cast Iron Wok
For my second material, I chose a cast iron wok. Fourteen inches in diameter, it weighed a hefty ten pounds. Though it took both of my arms to lift the wok onto the burner, I was enchanted by its power. At last, I could sear strips of meat and vegetables in minutes without collecting any steam at the base of the wok.
The all-important sizzle sound lasted the entirety of the cooking time rather than petering out prematurely. Still, the relationship between the cook and her wok was less than ideal. Unable to lift it with one arm, I found myself resorting to a scooping method to remove items from the heat. Worse, the cast iron never took well to added starch, an integral part of certain stir-frys. Like all uncomfortable relationships, soon the negative aspects outweighed the positive.
Third Attempt: The Nonstick Aluminum Wok
After the cast iron wok, I took a hiatus from stir-frying to think things through. I needed a light material that could still sustain heat. I wanted a patina that could sear and char quickly with a surface that wouldn't stick to a starch-based slurry. Finally, I caved in and bought an anodized-aluminum nonstick wok just like the one my mother owns.
For years my mother insisted that her single wok was better than all of mine put together, but I didn't want to believe her. While aluminum is an excellent conductor of heat, the idea of nonstick cookware has never appealed to me. Nonstick vessels don't grow and mature with the cook. The surface comes ready from the factory and can only get worse over time. Still, I should have remembered that my mother is usually right about most things.
My mother is a scientist. It's really too bad she hadn't explained things to me in scientific terms, because there's a very good reason why the aluminum wok works so well. In sum, there are two properties to consider when choosing the ideal wok material: heat capacity and conductivity.
Heat capacity is defined as the amount of energy needed to change the temperature of a given material. Heat capacity works both ways, meaning that if a lot of heat is required to make the vessel hot, a lot of heat must be given off before it cools down again. Conductivity means exactly what it sounds like--the efficiency of the material to redistribute heat evenly. By examining the heat capacity and conductivity measurements for common cooking materials, I was finally able to understand my struggles with past woks:
|Volumetric Heat Capacity (in joules per cubic centimeter per °C)||Thermal Conductivity (watts per thousandth of a °C)|
Why Was Aluminum So Successful?
If we examine the heat capacity figures alone, the aluminum is the poorest performer. However, what aluminum lacks in heat capacity, it more than makes up for in terms of conductivity. We've all noticed this fact when cooking with aluminum--it takes very little time for the pan to heat up evenly after we add food to the surface. Aluminum is bested only by copper but even so, aluminum is three times faster at heat conduction than iron, and six times faster than steel. Carbon steel loses on both fronts, being both a material that cools down too quickly as well as a poor conductor of heat. (Of course, the clear winner here is copper, which is why people have put up with its toxicity by coating it with tin and selling it for exorbitant prices.)
But what about the matter of heat capacity, cast iron lovers say? Isn't the heat retention so superb because the cast iron is so heavy and thick? Certainly. Heat capacity depends on pan thickness. While aluminum performs well due to its conductivity, the sheer thickness of a cast iron skillet compensates for its poorer conductivity.
The ratio of thickness to heat capacity is linear--if the cast iron skillet is twice as thick as another material, then it will take twice as much heat to bring the cast iron up to the same temperature, and twice as long for the heat to dissipate. This explains exactly why we cast iron lovers are so passionate in our devotion to the material. Once searing hot, a thick cast iron skillet will stay that way for a very long time.
Think of cast iron and aluminum as two separate models. If cast iron is a huge grain silo, then aluminum is a smaller silo with faster workers. Both models can distribute grain efficiently--the cast iron does so by retaining a very high grain level (i.e. temperature), while the aluminum simply refills the silo as soon as it's been depleted.
So, Does the Perfect Wok Exist?
Ideally, it would be a very thick copper vessel lined with a thin layer of stainless steel, not tin, which would melt after hitting 450°F. Barring such an extravagantly expensive wok, a very thick aluminum wok would be superb for its high heat capacity, high conductivity, light weight, and nonstick surface that won't cling to starch slurries.
I wonder why cookware manufacturers haven't gotten in on this action. What manufacturers call "heavy gauge aluminum" is usually no more than 1/4 inch thick. Why not make an aluminum wok that's one inch thick, or even two inches at that? The idea of a grossly thick aluminum wok seems feasible given the lightness of the material to begin with. If I were Jeffrey Steingarten, I'd call up Calphalon or All-Clad and cajole them into making an extra-thick aluminum wok, just for my purposes.
Even so, the anodized-aluminum wok I currently own has been my steady stir-frying companion. It's not as fiercely scorching as the cast iron yet its maneuverable weight and superior conductivity make it a pleasure to use.
Someday, there'll be a burner in my kitchen, about ten times as big as any other, that will match the heat of a restaurant flame. For now, my anodized aluminum is the workhorse wok, turning out dish after reliably good dish. After many years and many woks, I am one content stir-fryer.