Where there's smoke, there's fire. But where there's fire, there is not necessarily smoke. Smoke is where the flavor is. And the kind of woody smoke that draws praise to a grill chef requires water.
Live wood is up to 50% water by weight, most of it moving just under a tree's bark. It carries the goodies that build this year's growth, fight off disease and feed leaves, flowers and roots. If a tree were an animal, this thin layer would be its flesh and blood.
Freshly cut "green" wood is so wet it is virtually unburnable—so all future firewood travels along one of two paths to become suitable for the grill. Both paths reduce moisture so combustion can happen.
Chunk (or lump) charcoal is not just charred wood. It is the product of a highly heated chamber—a type of kiln—one that is very short of oxygen. The procedure that makes charcoal is called pyrolysis. A pyrolytic chamber can be a primitive mound of earth or a high-tech industrial monster (or a Kingsford plant, which performs the same process on pressed sawdust). When wood is put inside, the oxygen-starved heat boils away the water and accompanying elements—called volatile compounds—without allowing it to fully ignite. The result is a lump of nearly pure carbon and ash dried to 1% to 4% moisture.
Charcoal is far lighter than same-size firewood, and stable (it won't rot) so it is very practical to store, package and transport. Outside the developed world, a significant portion of rain forest logging can be blamed on subsistence farmers making, using and selling the stuff.
When lit, charcoal produces steady heat with no visible smoke or flame, releasing only trace levels of the original wood's volatile compounds and moisture. Charcoal was wood.
Under natural conditions a felled tree will dry to about 15% moisture after a year, depending on split size and climate. When ready for the grill it still contains four to 15 times the water content of charcoal. These days the process is often accelerated in a kiln, which has the added benefit of killing invasive pests like the emerald ash borer or Japanese beetle. This is called "seasoning" the wood. And it's appropriate—this 15% moisture level is vital to grillers. It tips the balance in favor of ignition but leaves just enough of the wood's nature to throw off our beloved seasoning smoke. Drier and it burns too fast; wetter and your
soggy smoke column will attract the fire department.
In a properly seasoned wood fire, smoke is produced at the border of lit and unlit, the area that's approaching the 482° Fahrenheit ignition point. Watch a stick burn and you can see the border clearly. The advancing combustion is the reaction of carbon and hydrogen with oxygen in the air. At 200° to 482° *, the water is being driven out by the advancing fire.
The smoke you see and smell is that water—and the minerals and organic elements that make up the character of the piece of wood—being released into the air. (Some of them sound so terrifying that they are the basis of occasional hate mails sent me by clean-air fanatics and cancerphobes, but a whiff of these chemicals from the grill is no more dangerous than breaths taken while jogging a city street.)
"The key here is WATER as much as fire. You need both to produce smoke that will transmit wood flavor to your food. There's practically none of it in charcoal."
Well, again, moisture is largely responsible for making those burgers taste like they were grilled rather than fried. But in this case the flavor is not from the wood—that departed in the kiln. When grilled over a dry charcoal fire (or a gas one, for that matter) on a standard bar surface, dinner's juices, seasonings and fats drip into the heat and ignite, throwing off their own smoke, which in turn rises and flavors your food—often quite well. Hold a dripping, seasoned patty over a burner in the kitchen and you'll get you a similar result—but I don't recommend it.
The moisture, the sap, the thousands of substances that make up a piece of wood are responsible for the flavor we shoot for on a wood-fired grill. They're why hickory smoke smells different from mesquite; why apple smoke is almost sweet while cedar is pungent. When wood's individuality is burned away to make charcoal all you have left to light is a near-pure reaction between carbon, hydrogen and oxygen. Not too tasty.
Bear no hate for charcoal. It's vital to grilling. Once established in the heart of a fire it's responsible for the steady heat needed to cook predictably, it makes "low and slow" possible (alongside smoldering hickory or oak) and in a professional setting chunk charcoal is a perfect supplement to deliver a high volume of plates on a limited supply of wood.
But charcoal only? Like a steak without salt.
Thanks to Vivien Lecoustre, PhD, or
"Dr. Fire," for checking my science.