Everything you Want to Know About Wood, Smoke, And Combustion

Posted on Posted in Smoked BBQ

 

Smoke is the essence of barbecue.

It is what differentiates barbecue from other types of cooking.

Originally all barbecue and grilling was done with logs of dried hardwood as the sole fuel source. Heat cooked the meat, and smoke from the wood and from dripping juices imparted a distinctive seductive scent that is the essence of barbecue. But it is difficult to control the heat and flavor when cooking with logs, so today, only a few expert pitmasters outfitted with special rigs cook with logs only.

Today, most grills and smokers use charcoal or gas to produce the heat, although a few use wood pellets or electricity. They get flavor and aroma from the addition of wood in the form of wood chips, chunks, bisquettes, pellets, logs, or sawdust. When heated, they make smoke. Smoke can also come from meat drippings, which are laden with fat, protein, spices, and even sugars from sauces. Adventurous cooks can also get smoke from dried herbs, tea, and even hay. But not all smoke is created equal.

Smoke is a combination of tiny particles that we can see in an aerosol mixed with water vapor and a complex cocktail of gases. The exact mixture is crucial and it can add elegant vanilla and brown spice notes or coarse bitter ash tray taint.

Cookers that get heat from logs, charcoal, gas, pellets, and electricity each produce noticeably different flavors because each fuel produces a unique combination of combustion byproducts.

Combustion, as it applies to barbecue and grilling, is a sequence of chemical reactions between oxygen and another fuel that are ignited producing a change in the chemistry of both, creating heat, light, and smoke with its blend of particles, water vapor, and gases.

Burning wood

Hardwoods, deciduous trees which include fruit and nut woods, have compact cell structures, and they are the best woods for cooking. Softwoods, like pine, fir, spruce, redwood, hemlock, and cypress, are all evergreen, coniferous trees, and they have more air, more pungent sap, and they burn fast. They are not recommended for cooking. We’ll talk more about different wood types below.

Fresh cut hardwood has a lot of water in it, up to 50% by weight, it produces a lot of steam and off flavors during combustion, and it takes up to 45% more energy than charcoal or gas to dry it out, so most wood for cooking is hardwood that has been air dried.

Dried hardwood is rarely totally dry, perhaps 5% water. Of the remaining 95%, about 40% is cellulose, about 40% hemicelluloses, 19% lignin, and 1% minerals. Actual numbers will vary depending on the wood species, subspecies, age, soil, and climate.

Cellulose and hemicelluloses are large molecules made of carbohydrates and sugars. Lignin is another complex compound that gives wood strength, and it is found mostly in cell walls.Wood also contains oxygen, nitrogen, carbon, and hydrogen among many other molecules. The minerals in wood include, potassium, sulfur, sodium, chlorine, and heavy metals. Although there are only trace amounts, these minerals can significantly impact the aroma and smoke flavor.

According to The Forest Encyclopedia, smoke flavoris influenced more by the climate and soil in which they are grown than the species of wood. This is very important to note, especially when you are caught up in the game of deciding which wood to use for flavor. This means that the differences between hickory grown in Arkansas and hickory grown in New York may be greater than the differences between hickory and pecan grown side by side. More on this below.

When burned thoroughly in a lab, wood produces about 8,600 BTUs of heat per pound, about half of the mass is converted to carbon dioxide and about half to water vapor. In the real world of a grill or smoker, wood is never burned thoroughly.

Wood has a lot of airspace in it and is a good insulator. That’s why you can set one end of a stick on fire and hold the other end. It is also why wood burns unevenly with different parts at different temperatures.

Dr. Blonder explains that during combustion wood goes through four stages. When it is really going good, all four stages can be happening at once.

Stage 1 – Dehydration (up to about 500°F). In this stage wood must be heated from an external source like a match, kindling, rolled up newspaper, or (horrors) lighter fluid. The wood drys out the rest of the way, water steams and evaporates, and some gases like carbon dioxide are given off, but there is no flame or heat produced.

Stage 2 – Gassification and pyrolysis (500 to 700°F). Here’s where combustion begins. The compounds in wood begin to change, and some come off as flamable gases, some form oily liquids and tars. The gases will burn if you give it an ignition source like a flame or spark, but they will not ignite on their own. Let’s call the combustion point 575°F on average for the sake of discussion.

Stage 3 – Burning bush (700-1,000°F). An apt term coined by Dr. Blonder to describe the phase most important to cooks. Flame appears and more gases emerge, among them nitric oxide (NO) which is essential for formation of the smoke ring in meat. In the sweet spot of about 650 to 750°F, the best aromatic compounds for cooking come off, among them guaiacol and syringol, which are primarily responsible for the aromas we call smoke. Some are ethereal and dissipate, and that’s why barbecue doesn’t taste the same after it has been reheated. As the temp rises above 750°F, acrid, bitter, and possibly hazardous compounds are formed.

During the burning bush stage, heat creates gases that rise from the fuel creating something like a gas bubble, but the wood itself is not consumed. The bubble is surrounded by air that is about 20% oxygen. If there is any source the gas bursts into flame as the gases and oxygen combine. If all the gases are able to combine with the oxygen, a blue flame is visible. This never happens with wood or charcoal so the flame glows yellow or orange. If unburned gases escape, the bubble cools and creates smoke. Charcoal and wood fires are inefficient and so their flames are orange, yellow, and red.

Stage 4 – Charcoal formation (above 1,000°F).Most of the organic compounds have burned off leaving behind pure carbon, or char, which burns with little smoke.

Burning propane and natural gas

A gas grill has a venturi, a valve that blends the gas and oxygen like a carburetor. When there is too little oxygen some of the fuel is unburned and sooty. The soot glows yellow and orange, like an ember. When properly blended all the fuel is completely burned and the flame is nearly blue. Click here to read more about gas grills and smokers and how they work.

When propane or natural gas combine with oxygen and they are ignited, they produce water vapor, carbon dioxide, carbon monoxide, and not

What is smoke?

Smoke includes as many as 100 compounds in the form of microscopic solids including char, creosote, ash, and phenols, as well as combustion gases that include carbon monoxide, carbon dioxide, nitric oxide, syringol, and liquids such as water vapor and syringol, an oil.

Trace amounts syringol are responsible for much of the smoke aroma we love and trace amounts of guiacol are responsible for much of the taste of smoke. The composition of smoke depends a great deal on composition of the wood, the temperature of combustion, humidity, and the amount of oxygen available.

If the wood does not get enough oxygen it can still undergo pyrolysis and gassification, but not burning bush. It will not burst into flame, it will smolder, and smoldering wood produces lots of smoke and a different flavor than burning wood. Why would it not get enough oxygen? If the intake vents and chimney are not open wide enough. Click here to learn more about controlling vents and chimneys. Getting enough air can be a problem in kamado type smokers that are so well insulated and retain so much heat that we often have to choke off airflow to keep it from running too hot.

Wood also plays a role in the color of the meat and the formation of the crust on the meat, also called the bark. Below are two slabs of ribs with the same spice rub but no sauce. The one on the left was cooked on a charcoal smoker and the one on right was cooked on a gas smoker. You can see and taste the difference. They were both excellent, but different. The one on the right had a bit of a bacon or ham undertone, typical of gas smokers.

Blue smoke for long cooks

Smoke from wood or charcoal for cooking can range from bluish, to white, to gray, to yellow, brown, and even black. The most desirable smoke is almost invisible with a pale blue tint. You can see it below.Blue smoke is the holy grail of low and slow pitmasters, especially for long cooks.

Dr. Blonder explains that the color depends on the particle size and how it scatters and reflects light to our eyes. Pale blue smoke particles are the smallest, less than a micron in size, about the size of the wavelength of light. Just because you can’t see it, doesn’t mean it’s not there. Pure white smoke consists of larger particles, a few microns in size, and they scatter all wavelengths in all directions. Gray and black smoke contains particles large enough to actually absorb some of the light and colors.

Black and gray smoke happen when the fire is starving for oxygen, and they can make bitter, sooty food tasting like an ash tray. Billowing white smoke is common when you just start the fire, and when the fuel needs lots of oxygen as it goes through stages 1 (dehydration) and 2 (gassification). If it doesn’t get enough and if the fuel is not emitting gases for stage 3 (burning bush combustion), the fuel smolders and produces white smoke. For short cooks of thin meats, you might just want the heavier, more intense white smoke, and we’ll discuss that below. Here are some tips on how to get blue smoke for long cooks.

Get your smoke from wood. Don’t worry if your wood bursts into flame. A lot of beginners fret over their burning wood and want it to smolder and belch smoke. You’ll use more wood if you let it burn, and you’ll have to struggle a bit to maintain temperature control, but you’ll get better flavor.

Charcoal belches flamboyant white smoke when it is igniting. You don’t want to add meat until the coals are fully aflame and at their peak in heat, when it has a white coat of ash. Then add wood at the start of the cook. Remember, charcoal is for heat, not flavor. If the temp runs up a bit at the start of the cook, it’s not a big problem. The meat is cold and it can take a little extra heat. Then, when the wood burns out, stabilize the temp.

Keep your cooker clean. Sticky grease on your cooking grates can create black smoke and drip on the food. Grease smoke is not good smoke. Click here for more on cleaning your cooking grates. The black stuff on the walls probably has a lot of condensed creosote in it. A thin layer of neutral carbon is harmless, but black sticky goo is not. Many competition pitmasters power wash after a cookoff. Click here for more on cooker cleaning and maintenance.

Control oxygen. Make sure coals have plenty of oxygen. If coals are choking for lack of oxygen, they burn incompletely and can coat your food with gray soot. If that happens, get the meat off, rinse it, adjust the fire and put it back on. Don’t let your embers sit in ash which can smother them. Keep them on a grate above the bottom of the firebox. Knock ash off occasionally and if necessary, remove it.

Use dry wood. Wet wood makes bad smoke. It also cools the fire. Read my article on soaking wood. Some pitmasters will even put the wood on top of their smoker to drive off any remaining water.

Size matters. For long cooks, chunks of wood from golf ball to baseball size work best. For short cooks, like a steak, chicken, or fish, small chips and especially pellets work best because they produce more smoke in a short burst. Some remove the bark from their wood.

Build a small hot fire. You want to see flame. Fires burning in the 650 to 750°F range in the hot spots burn off the impurities that can becreated in an incomplete secondary combustion. That means that you need a lot of oxygen so you want your exhaust vent open all the way. The hot air rising through the chimney will draw in air through the intake vent. You will probably want it open wide or close to it. Low smoldering wood creates dirty smoke.

This is why high quality offset smokers, the ones that look like a big barrel on its side with a small barrel attached, are so popular with experienced pitmasters. But there is a big difference between the cheap offsets at the hardware stores and the serious pits made for competition teams and caterers. Cheapo Offset Smokers (COS) include Brinkmann Pitmaster, Brinkmann Smoke’N Pit Professional, Char-Broil Silver Smoker, Char-Broil American Gourmet, and especially the Char-Griller Smokin Pro. They are nothing but headaches. The doors don’t fit properly so you can’t control oxygen intake, the walls are thin so they don’t retain heat, and they rust. Expensive Offset Pits (EOS) include Horizon, Jambo, Klose, Lang, Meadow Creek, Peoria, Pitmaker, and Yoder to name a few. They are superb cooking tools. The picture below is inside the firebox of Darren Warth’s Jambo. His team, Iowa’s Smokey D’s, one of the winningest cooks on the circuit. Notice that most of it is glowing embers and a log almost burnt to embers, and there on top is a small block of hardwood for flavor.