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Science of Smoke
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The Science of Smoke

Smoke is an essential part of the barbecue flavour profile, but how, exactly, does smoke flavour food? Helen Graves looks at the chemistry.

Live fire and smoke is what sets barbecue apart from other forms of cooking. Enthusiasts become hooked on the crackle of wood, the eddy of red and gold sparks, the twist and curl of smoke. It is the flavour left behind by that smoke, or more specifically the flavour left behind by the right type of smoke, which makes the results of barbecue cooking so appealing. But what exactly is that flavour? How does the infusion of smoke produce the changes in colour and flavour that we associate with this style of cooking? Which wood produces the best smoke for barbecuing?

Firstly let us make it clear that we are talking about smoke from burning wood, not charcoal. This is the domain of the southern American pit master, stoking the fire, tending intuitively to keep the burn alive with just the right amount of flame and sweet smoke rising.

To understand the flavours produced by wood smoke we must first understand the basic structure of wood, which is composed predominantly of three organic compounds: cellulose, hemicellulose and lignin. Green plants all contain cellulose, but a plant becomes woody when it contains lignin, which can be thought of as filler between cell walls, adding strength and rigidity. Harold McGee, in his book On Food and Cooking, illustrates nicely by asking: ‘Ever had a woody stem on asparagus? That’s lignin starting to form.’

Wood is the most desirable fuel for the traditional barbecue cook due to the complex flavour it produces, but it is also the hardest to control. So, what happens when wood burns? Well first, it dries out. When heat and oxygen are present (both required to ignite the fuel), any water inside the wood starts to boil, then evaporate. When this moisture has gone, it is the turn of volatile gases to be released. The temperature continues to rise and eventually these gases combust, forming flames.

The smoke that is produced during this burning process contains all three states of matter: solid particles of soot and droplets of liquid, both floating around in a vapour made of air and chemicals. It is the solid soot particles (black) and microscopic droplets of liquid (blue) that make smoke visible as a haze, but the vapour of air and chemicals responsible for suspending everything is invisible, and that is where most of the flavour is contained.

The Flavours of Smoke

The individual flavours produced by smoke are released at different times during the burning process, and different types of wood bring distinct flavour profiles because they have differing proportions of cellulose, hemicellulose and lignin. Cellulose and hemicellulose are chains of glucose (sugar) molecules, so when burnt they produce sweet flavours such as peach, coconut and green apple. Many of the sugars break apart into the same molecules as those found in caramel, producing fruit, flowery and bready aromas. Lignin breakdown creates phenolics (aromatic compounds) bringing pungency and spiciness. These flavours only ever penetrate the outer millimetres of the meat and are unstable, meaning that after a few weeks they would begin to disappear if a plate of barbecue food ever would, or could, last that long.

Wood Components and Smoke Flavours (reproduced from On Food and Cooking by Harold McGee, 2004, Hodder and Stoughton, with permission from the author).

Wood Component % of Dry Weight Combustion Temperature Combustion By-Products and Their Aromas
Cellulose 40-45% 540-610F

280-320C

Furans: sweet, bready, floral
Lactones: coconut, peach
Acetaldehyde: green apple
Acetic acid: vinegar
Diacetyl: buttery
Hemicellulose
(cell wall filler, from mixed sugars) 20-35%
390-480F

200-250C

Lignin (cell wall strengthener, from phenolic compounds)
20-40%
750F

400C

Guaiacol: smoky, spicy
Vanillin: vanilla
Phenol: pungent, smoky
Isoeugenol: sweet, cloves
Syringol: spicy, sausage-like

The Smoke Ring

When wood burns, a very small amount of nitric oxide gas can be produced and, when it hits the surface of the meat, it diffuses into the meat where it reacts with the myoglobin to produce a stable pink pigment, visually present as the smoke ring at a depth of 8-10mm. The same compound provides the characteristic pink colour in cured meats, such as bacon, where it is formed by the reaction of curing salts with myoglobin.

Many barbecue chefs desire a deep, obvious smoke ring as a kind of trophy, and if that is something you covet then the key lies in keeping the surface moist, so a mop sauce can help, as well as, obviously, adding flavour. In competition however it is largely pointless chasing a smoke ring, since the Kansas City Barbecue Society have removed it from their judging criteria, based on the knowledge that it is easily faked by dusting nitrate based curing salt onto the meat.

The Ideal Wood for Barbecue

Hardwoods are best for low and slow cooking because they burn slower with less intensity than softwood varieties. Just to confuse matters, the terms hardwood and softwood have nothing to do with lignin content (the stuff that makes wood hard). The distinction is actually to do with seeds, and the fact that hardwoods release their seeds with a covering (e.g. a fruit, or an acorn) and soft woods do not.

Oak is one of the most popular hardwoods for barbecue due to its steady rate of burning, meaning it is easier to control the heat. Plus, it produces a mellow smoke which does not overwhelm. Incidentally, the structure of oak also makes it particularly good for wine barrels, due to the size of its ‘radial rays’ (perpendicular to the growth rings we have all counted as children), which give strength when shaped into barrels. Burning the inside of a barrel before adding whisky or wine allows the oak to release its vanilla and butter flavours into the liquid. Another popular hardwood, mesquite, brings different flavours and is much more powerful – strong and smoky with a hint of sweetness. Fruit woods too (apple, cherry) produce popular flavours due to the moderate balance of their organic compounds.

Combustion temperature is also important for flavour, with the ideal range at the lower end of the scale (300-400C), where the most flavour compounds are released. Past this point, flavour molecules are broken down too far, becoming harsh or losing pungency entirely. The reason we are able to use high lignin woods yet harness their flavours is in the skill of the pit master; their combustion must be slowed by controlling airflow and beginning with the right moisture content. If wood is too wet, or has been cut from a tree that was very recently alive (green wood), then it will be very hard to burn, and when it does, the amount of evaporation taking place due to the high water content cools the combustible gases, making it harder to burn them successfully. This produces what is known as ‘dirty smoke’.

Clean Smoke and Dirty Smoke

The term ‘clean smoke’ basically means smoke containing as little carbon or soot as possible. We have all seen dirty smoke at some point, even if just on TV – it is that billowing, black and quite obviously nasty stuff coming from a ship’s funnel or factory chimney. It is dense with soot, a thick belch into the atmosphere. You will not see this coming from a barbecue for long, but there will be a period of thick white smoke, which is also undesirable.

Dirty smoke is a result of incomplete combustion. All barbecue fires represent incomplete combustion to some degree, because if complete combustion were to occur, only water and carbon dioxide would be produced – i.e. we would have no smoke, something that can only happen under lab conditions. The goal, however, is to produce the cleanest smoke we can, which essentially means we need to get as close as possible to complete combustion outside of the lab.

The desirable level of combustion is indicated by thin, blue smoke – but how do we achieve this? By ensuring sufficient oxygen supply reaches the wood and that it is not too wet, or too dry. We do not want that green wood from recently alive trees mentioned earlier, but we also do not want it to be parched – the ideal moisture content is somewhere around the 20 per cent mark, according to Aaron Franklin and Jordan Mackay in their book, Franklin Barbecue: A Meat-smoking Manifesto. Leaving wood out to dry to the correct moisture content can be quite the test of intuition, apparently, and is a process referred to as ‘seasoning’.

That word ‘intuition’ crops up a lot when we are talking about barbecue cooking, where there are no gas mark temperature settings or instructions on the back of a packet. What barbecue cooks are trying to do is harness the power of a natural process to the best of their ability, by controlling as many elements of the fire as possible. Combine this with good quality meat, proper seasoning, time and patience and you have potentially got yourself some excellent ’cue. Can things still go wrong? Sure. The most important ingredient of all is practice.

Illustration by Jamie Jones


Helen is an award-winning freelance food writer and author of Food Stories, one of the UK’s first and most influential food blogs. Helen is the proud owner of four BBQs, and loves carbs, cats, crabs and kebabs. She has a strong physical reaction to bad pulled pork.

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