What You Need To Know About Wood, Smoke, And Combustion
"When a lovely flame dies, Smoke gets in your eyes." Song "Smoke Gets In Your Eyes" by Jerome Kern and Otto Harbach, performed by The Platters
Smoke is what differentiates barbecue from other types of cooking.
Originally all barbecue and grilling was done with logs of dried hardwood as the fuel source.
Heat cooked the meat, and smoke from the wood and the dripping juices landed on the meat imparting a distinctive seductive scent that is the essence of grilling and barbecue. But it is difficult to control the heat and flavor when cooking with logs, so today, only a few expert pitmasters cook with logs only.
Today, most barbecues use charcoal or gas to produce the heat, and they get flavor and aroma from the addition of measured amounts of wood in the form of chips, chunks, bisquettes, pellets, logs, or sawdust. Click here for a good buying guide to smokers.
Hardwoods, 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 evergreens (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 wood choices below.
Logs, charcoal, gas, pellet, and electric cookers each produce tastably different flavors because each fuel produces a unique combination of combustion byproducts.
Electric cookers use a hot coil for heat, so there is no combustion and thus no combustion gases. Even if you put wood chips on an electric coil, the flavor of the smoke is vastly different, and to my palate, inferior because the food lacks the complexity that combustion gases the other fuels produce. The secret to good flavor is the right combustion gases plus smoke from wood.
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. The one on right was cooked on a gas smoker. You can see and taste the difference.
What is combustion?
The simplified technical definition, as it applies to barbecue and grilling, is that combustion is a the sequence of chemical reactions between materials, oxygen and at least one other fuel, producing a change of the chemistry of the fuels, producing heat, light, smoke, and gases. Here's how it applies in the real world: Wood and pellets. 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, climate, and vintage.
Cellulose is a long chain molecule, a type of carbohydrate. Hemicelluloses are made of different carbohydrates and sugars. Lignin is another complex polymer from the cell walls that gives the wood strength. The minerals in wood include oxygen, hydrogen, nitrogen, potassium, carbon, sulfur, sodium, chlorine, and heavy metals. These minerals can significantly impact the aroma and smoke flavor.
Hardwoods have more minerals than softwoods, but, according to The Forest Encyclopedia, their smoke flavor is 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.
Wood. Wood combustion starts to take place in the 500 to 600°F range and requires significant amounts of oxygen. Let's call the combustion point 575°F on average for the sake of discussion. At that temp the wood has absorbed a lot of heat and the water has been driven off as vapor. It is ready to burn.
Charcoal. Charcoal is almost pure carbon made from wood that has been preburned in an oven with very little oxygen. When burned charcoal produces heat, carbon dioxide, carbon monoxide, water, and other gases. It burns hotter and cleaner than wood. For a detailed explanation of how it is made and how it works, read my article on the Zen of Charcoal.
Propane and liquid natural gas. When propane and oxygen are ignited, they produce heat, light, water vapor, carbon dioxide, carbon monoxide, and other gases.
What is flame
Primary combustion is the burning of oxygen and the other fuel, and secondary combustion is the burning of the gases emitted at high temps. It is the combustion of gases that produces flames. This consumption of gases also produces most of the heat.
In primary combustion, heat creates gases that rise from the fuel creating something like a gaseous bubble. The gas bubble is surrounded by air that contains about 20% oxygen. If there is enough energy the gas ignites and starts the process of combining its molecules with the oxygen. Where the gas and air meet, and if all the gas is able to combine with the oxygen, a blue flame is visible. If the gases don't burn completely, the flame glows yellow or orange. If unburned gases escape, the bubble cools and turns into smoke. Meanwhile, the energy from the combustion causes more gas to evaporate.
When there is too much oxygen, the flame is yellow and orange. A gas grill has a venturi, a valve that blends the gas and oxygen like a carburetor. When properly blended the flame is mostly blue.
What is smoke?
When burned thoroughly in a lab, wood produces about 8,600 BTUs per pound of heat, about half of the mass is converted to carbon dioxide and about half to water vapor. In the real world, wood is never burned thoroughly in a grill or smoker. If the wood does not get enough oxygen it can still undergo primary combustion, but not secondary. It will not burst into flame, it will smolder, and smoldering wood produces lots of smoke and a different flavor than burning wood. That's why ribs smoked by a fire made from logs tastes different than ribs smoked by a fire made from charcoal, gas, or electricity.
Smoke includes as many as 100 compounds in the form of microscopic solids including char, creosote, ash, as well as combustion gases that include carbon monoxide, carbon dioxide, nitrogen oxides, polymers, and liquids such as water vapor, and phenols. When they contact food they can stick to the surface and flavor it. Most of the flavor comes from the gases, not the smoke particles, according to the AmazingRibs.com science advisor, Dr. Greg Blonder, and the composition of the gases depends on composition of the wood, the temperature of combustion, and the amount of oxygen available.
Smoke from wood or charcoal for cooking can range from bluish, to white, to gray, to yellow, brown, and even black. 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, under a micron in size, about the size of the wavelength of light. Pure white smoke consists of larger particles, a few microns in size, and they scatter all wavelengths in all directions. Gray smoke contains particles large enough to actually absorb some of the light and colors. The most desirable smoke is blue as seen coming from the smoker on the left in the photo below by Grant Erwin in Seattle.
Blue smoke is the holy grail of low and slow pitmasters. 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, under a micron in size, about the size of the wavelength of light. 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. If it doesn't get enough and if the fuel is not emitting gases for secondary combustion, the fuel smolders and produces white smoke.
If you are cooking hot and fast, white smoke is a great way to get some smoke flavor on the food in a hurry. But white smoke usually has a lot of contaminants from an incomplete secondary combustion and prolonged exposure to white smoke can still make good food, but not as good a blue smoke.
Sterling Ball, a champion pitmaster who owns a guitar string business, describes the art of making blue smoke as similar to tuning a guitar. "You need control of your tools, the pit, fuel, oxygen, fire, heat, and practice." Here are some tips on how to get blue smoke for long cooks.
Keep your pit clean. Carbon buildup, soot, and sticky grease on the inside can create off flavors and drip on the food. Often that "flavorful" grease is creosote. Many competition pitmasters power wash after a cookoff.
Use dry wood. Some pitmasters will even put the wood on top of their smoker to drive off any remaining water. Some remove the bark from their wood.
Use large chunks or logs. Don't use chips or pellets. You need something to burn down to embers.
Build a small hot fire. You want to see flame. Hot fires burn off the impurities that are created 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. Don't let your embers sit in ash. Keep them on a grate above the bottom of the firebox. Knock ash off occasionally and if necessary, remove it.
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 (a.k.a. SNPP), Char-Broil Silver Smoker, Char-Broil American Gourmet, and especially the Char-Griller Smokin Pro. They are nothing but headaches. Serious Offset Pits (EOS) include Horizon, Jambo, Klose, Lang, Meadow Creek, Peoria, Pitmaker, and Yoder. They are superb cooking tools.
Allow the pit to warm up. Start the fire at least an hour before the food goes on. Adjust your airflow and get the temp, fire, and smoke stabilized. Warm the walls of the cooker. It is harder to get blue smoke in cold weather.
Start the fuel on the side. If you can't get the cooker to cooperate, or if you have temp yo-yoing up and down, start your fuel on the side and add only hot coals. If you are cooking with wood, start burning it on the side and add only glowing embers. If you are using charcoal, use briquets. I use a wheel barrow. Lump is not often completely carbonized and can create more smoke than briqs. Remember, properly burning charcoal doesn't produce much smoke.
Use good thermometers. Digitals are the best.
Practice. Do dry runs without food until you can anticipate when more fuel is needed, how to adjust the airflow, and how to react when the smoke starts going bad.
Cook indirect. If the meat drips on the fire, water and fat will burn and make dirty smoke. These drippings can create flavor, especially for short fast cooks, but for long low and slow cooks, they can create dark smoke. Use your senses. It's hard to see the color of the smoke at night, but the smell should be sweet, with meat and spice fragrances dominating. The smoke aromas should be faint and seductive, perhaps like vanilla, not like a bonfire smell.
Creosote is among the compounds in smoke and it is the Jekyll and Hyde of smoke cooking.
On the Dr. Jekyll side, creosote contributes positively to the flavor and color of smoked foods and acts as a preservative, among the reasons that smoking meat was used for preservation before refrigeration.
Blonder says "Creosote is always present in charcoal or wood smoke, and a few components of creosote (guaiacol, syringol, and phenols) are the largest contributors to smoke flavor. No creosote, and the meat might as well have been boiled."
On the Mr. Hyde side, "If the balance of the hundreds of chemicals in creosote shifts, it can taste bitter rather than smoky."
What is creosote and where does it come from, and why is it two faced?
Commercial creosote is produced by distilling tars from primarily beechwood or bituminous coal (not charcoal). Careful control of the combustion temperature, oxygen flow, and pressure produce a wide range of aromatic oils and tars. Creosote from coal tar is the black stuff used to preserve telephone poles and railroad ties. Coal tar creosote is classified as a possible carcinogen. Anybody who has a wood burning fireplace knows that creosote from logs can cling to chimneys, clog them, and even ignite, burning down the house.
These industrial chemicals give the creosote found in barbecue smoke a bad name. I am unable to find any research that implicates the small amounts of creosote in barbecue with health risks.
Many cooks believe that the best smokers and grills are coated with a thick black "seasoning" from use. Alas, much of the seasoning on the inside of a smoker is creosote. Some of it is carbon, and some of it is just plain grease. It is not likely to combust, but it can vaporize or drip on the food. Many of the best cooks powerwash the interior of their cookers regularly. For more, read my article on grill and smoker cleaning and maintenance.
According to Blonder, "When you smoke low and slow at temperatures like 225°F, many smokers require you to control the fire by damping the oxygen supply which moves it below the ideal combustion zone, creating black smoke, soot, and more creosote. The best smokers combust at a high temp to create the ideal flavor profile and direct a small fraction of the smoke across the meat.
"To control creosote, use less charcoal so you can burn it hot and more efficiently, but the smaller number of embers will provide less heat. You will have to replenish the coals more often, but the food will taste better and there will be less creosote. Avoid resinous woods such as pine and cedar.
"Humidity can affect creosote production and its flavor profile because moisture in smoke can scavenge some of the creosote compounds selectively and then it can condense on cool meat and cool smoker walls, especially in winter. All this means there is plenty of room for barbecue skill and experience."
Here are other ways to avoid creosote formation:(1) Avoid resinous woods such as pine and cedar; (2) Don't add a lot of cold meat to the smoker at once; (3) If you are using charcoal or logs, use a small hot fire that does not have to be stifled by closing dampers to keep the heat down; (4) Cook in warm weather; (5) Preheat the smoker and give the walls time to warm up; (6) Don't add a lot of cold charcoal, start it up first in a chimney; (7) If you are cooking with wood, preheat the logs by letting them sit on the firebox or in the cooking chamber; (8) Leave the chimney open at all times; (9) If the fire gets too hot, don't close the dampers, open the firebox door to let out some of the heat; (10) Check the cooking chamber temperature with a digital thermometer at least every 30 minutes; (11) Practice, practice, practice.
Smoke and food
In a smoker or grill, after combustion, the smoke rises and flows from the burn area into the cooking area. Some of it comes in contact with the food, but most goes right up the chimney and very little contacts the food.
Blonder explains why: "Around every object is a stagnant halo of air called the boundary layer. Depending on airflow, surface roughness, and so on, the stagnant layer of air around a piece of meat might be a millimeter or two in thickness. When smoke particles approach the meat's surface, they follow that boundary layer around the food. Very few ever touch down. We've all cursed a form of this piece of physics while driving: Gnats follow the airstream over the windshields, while larger insects leave green sticky splats at the point of impact."
To see the way smoke sticks to food, Blonder did some experiments. He suspended three cotton disks in a smoker at 230°F for 30 minutes. One disk was dry, one soaked in oil, one soaked in water. The results are pretty dramatic. Smoke adhered to the oiled surface more than the dry surface, and far more stuck to the wet surface. Remember, the atmosphere inside a smoker is as dense as a London fog, yet no visible smoke got stuck on the dry cotton pad.
Why does the wet pad gather so much more smoke? Blonder explains that smoke impacting the dry pad simply bounces off because there is nothing to hold them. But the oily and wet pads are tackier. But why does the wet pad attract more smoke than the oily pad? The answer is thermophoresis, according to the physicist.
Thermophoresis is a force that moves particles from a warm to a cold surface. He showed this by placing two smooth, dry, glazed tiles in the pellet smoker. The left tile is a control. It did not go in the smoker and is shown here just so we can see what it looks like before smoking. The middle tile was warmed to 225°F before being placed in the smoker, and the right hand tile was chilled to 29°F.
In the first experiment, the wet cotton pad was cooled by the evaporation of the moisture in the pad so it was well below the temp of the others. That's why it was smokier.
The same thing happens to meat when you put it in the smoker. If it is cold and wet it will hold more smoke. As it warms and dries out, less smoke is absorbed.
Smoke flavor is almost all on the surface of the food
As we see, smoke particles glom onto the surface of foods. There they may dissolve and penetrate a bit below the surface but not very far. Rarely more than 1/8". This is the same phenomenon with marinades. Meats, especially, are hard to penetrate. Don't believe me? Here's how to prove it to yourself. Get a nice thick piece of meat, at least 1" thick, like a cross section of pork loin or a brisket or pot roast. Smoke the heck out of it at 225°F until it is 190°F to 200°F internal temp. Let it cool a bit and taste it. Yummmmmm! Smoky! Now cut off the surface and edges in 1/8" thick slices. Make sure the center meat doesn't get to roll around on the cutting board in the juices from the surface. Now taste the center. No smoke!
Also, building enough smoke to create flavor takes time. On a thin skirt steak for fajitas, there will be much less smoke flavor than on a 1" thick ribeye steak cooked to medium rare (about 130°F). A thick steak will have much less smoke flavor than a chicken breast the same thickness because the chicken needs to be cooked longer, to 165°F. And a turkey breast will have more smoke flavor than all of them.
Enough is enough
One of the biggest mistakes we frequently make is using too much smoke. Too much smoke can make your meat bitter or taste like an ash tray. Smoke is like salt. You can always add more but you can't take it out. Do not try to cook with wood. It is too hard to control the temp and the amount of smoke. When you become an expert, you may be able to cook with wood only, but at the outset stick to charcoal, propane, or electricity. I cannot give you a precise amount because each cooker is different and the amount of wood to get the right flavor will depend on the volume of the cooking chamber, the airflow, leaks, how often you peak, the kind of wood you use, and of course, your preferences. You will need to experiment, but a good rule of thumb is start experimenting with about two ounces of wood, regardless of the cut or weight. For dense, thick cuts of meat such as pork butts for pulled pork or beef brisket, you can double or triple the amount of smoke. If the results are not smoky enough, you can add more wood on your next cook.
Putting wood to work
So here's how to take advantage of this info.
Don't soak the wood. It does no good. Water can't penetrate it. That's why they make boats from wood. Read my article on soaking wood and look at the photos for proof.
Size matters. For long cooks, chunks of wood from golf ball to baseball size work best. They burn more cooly, smolder and last longer, while chips and pellets tend to combust and disappear more quickly. 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.
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. But restrict oxygen to the wood. Too much air and it will burst into flame. Experiment with containers for the wood. Here you can see a foil packet with holes punched in it and a small aluminum loaf pan crimp it to restrict airflow. You want the wood to smolder, which is the point just before ignition when it produces smoke but hasn't yet burst into flame. The difference is oxygen. So if your wood bursts into flames, next time wrap it in foil and poke a few holes in the foil to limit the oxygen supply.
Use cold meat. As described above, smoke is attracted to cold meat. Do not let the meat come to room temp. Besides, it takes forever for meat to come to room temp.
Use a spice rub. Rough up the surface with a spice rub, a layer of spices and herbs to reduce the boundary layer.
Keep the meat moist. Use water pans in your smoker. They add water to the atmosphere, but more importantly the water condenses on the meat's surface. You can also do this manually by spritzing the meat with a spray bottle you can buy in a drug store. A mop or basting brush will just wash off your spice rub. You can use apple juice if you wish, but it adds very little to the flavor in comparison to the rub and sauce. You can use cranberry or pomegranate for color. But really, all you need is water. And don't worry, opening the cooker every 30 minutes to spritz will not slow the cooking process measurably. This is another myth Blonder has debunked. Click the link.
Cured (dried) hardwoods with low sap are the best for barbecue, especially fruit and nut woods. When dry they produce the best smoke. They all have slightly different flavors, and it is impossible to describe them.
To make things complicated, there are different kinds of each wood. For example, there's shagbark hickory, scrub hickory, pignut hickory, and red hickory. The climate the tree is grown in can impact flavor. Texas oak tastes different than Michigan oak. Furthermore, the amount of bark and can significantly impact flavor, far more than the type of tree you pick. How long it has dried and the percent water left in the wood impacts flavor and aroma.
The internet is full of guides attempting to describe the flavors of different woods. They remind me of the florid descriptions wine lovers use. I don't find these wood descriptions very useful. Think about it. Apple might taste one way on pork, but will taste entirely different on beef or turkey. It's like visualizing the color red. No problem. But when you mix it with yellow, it is vastly different, and very different from when you mix it with blue. That said, I can often tell the wood by the smell, but rarely can I guess it by the taste. To make matters worse, there is no guarantee that the wood in the bag is the wood on the label. Coffee, olive oil, and fish markets are regularly rocked by scandals of fraudulent labeling.
Here's the best I can do based on the woods I have used. And remember, I have judged food and wine around the world and won wine tasting championships, and I would love nothing more than to tell you that a particular wood has "nuances of spice with an undertone of mushrooms". I just can't do it.
- Mild (best for foods that are not heavily seasoned or sauced). Alder, apple, cherry, grape, maple, mulberry, orange, and peach.
- Strong (best for strong flavored foods with lots of spice and/or sauce). Hickory, mesquite, oak, pecan, walnut, and whiskey barrel.
I avoid mesquite. It can be harsh, bitter, and pungent. Hickory is the tried and true mate for pork, but some people find it too aggressive and occasionally it can taste bitter. Fruit woods tend to impart a sweetness.
Then there are the exotic woods that I haven't tried yet: Citrus, dried citrus peels, pistachio wood, corn cobs, nut shells, coconut shells, mango, and even mahogany.
The choice of wood is crucial when you are using wood for both heat and flavor, if it is your fuel. It is not very important when you are throwing a few chips and chunks into a pile of charcoal or onto a gas grill or even on a pellet smoker.
Bottom line: Stop obsessing over which wood to use. Pick one and stick with it for a whiole. The quality of the meat, the spice rub, the cooking temp, the meat temp, and the sauce impact the final taste profile far more than the name on a bag of wood. Once you have everything else under control, then you can try different woods.
Bad wood. Whatever you do, never use wood from conifers such as pine, fir, cyprus, spruce, redwood, or cedar. They contain too much sap and that sap contains turpines, and they can make the meat taste funny. Some have been known to make people sick. Yes, I know that cedar planks are popular for cooking salmon on, but I don't know anyone who burns cedar as a smoke wood. I have also heard that elm, eucalyptus, sassafras, sycamore, and liquid amber trees impart a bad flavor. Oleander smoke is poisonous, and I am told laurel is too.
Never use lumber scraps. Some lumber is treated with chemicals that are poisonous. Never use wood that has been painted. If you have branches fall from trees, make sure they are not moldy. By the way, that's why I don't use lump charcoal. You can see lumber scraps in there and it makes me wonder how careful they are to prevent treated lumber from getting in there.
Never use wood that is moldy.
Use air dried wood. Green woods have more sap, burn irregularly, and impart different flavors than dried wood. Air dried wood is slightly wetter than kiln dries, and the water provides steam that makes the droplets larger and stickier. Kiln dried tends to taste smokier, and when wood is the heat source, getting enough smoke is rarely a problem, so top pitmasters prefer air dried. How wood is dried doesn't matter if you are burning charcoal, gas, or pellets.
Bark or no bark? Some wood has more bark than others and that can impact the flavor. Some folks say you should remove the bark. I don't bother, but there may be something to this.
Make the wood smolder. You don't want the wood to catch on fire and go through rapid combustion. You want the wood to smolder. The way to do this is to starve it by limiting oxygen. If your wood is constantly breaking into flames, wrap it losely in foil and poke holes in the foil.
What does Meathead prefer? If I was on a dessert island I would want a bag of apple chunks and a bag of small apple chips or pellets. I would use the chunks for steady slow release smoke, and the chips or pellets for quick smoke.
Add wood early. Meat soaks up more wood flavor at the start of the cook, and the colder the meat the more smoke it absorbs.
Where to get it? There are a number of barbecue specialty stores opening around the country and there may be one near you. Most hardware stores carry only hickory or mesquite, but a few carry expanded barbecue supplies and a selection of woods. Watch the newspaper for ads from stores promoting a lot of grills. Then give them a call. Another option is to go to an orchard and ask if you can have some dead limbs. Also there are a number of places to buy wood on the net. Click here for contact info for online wood suppliers.
How much is enough? It is best to weigh the amount of wood you use so you can increase or decrease it as you wish in future cooks. The amount you need will vary depending on your preferences, how tight your cooker is, they type of fuel, the thickness of the meat, and if you use chunks, chips, or pellets. Pellets are especially good for measured amounts.
Here's where to start your experiments: On charcoal, use no more than 8 ounces by weight of wood for ribs. Use no more than 16 ounces for pulled pork and brisket, and no more than 4 ounces for turkey and chicken. Add it in doses. Put on about two ounces when you put on the meat and add another two ounces when you can no longer see smoke. On gas grills, double the amount.
Take notes! Measure the wood and write it down so you can add more or less the next time you cook. Use less than you think you need. Keep records of your experiments on a cooking log.
Click here to see the original unedited data from Blonder's experiments.
This page was revised 1/7/2013
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