The science of smoke
It’s an essential part of the barbecue experience. But why, exactly, does woodsmoke make food taste so good?
Live fire and smoke are 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 curl of the smoke. It is the flavour left behind by the smoke, or more specifically the flavour left behind by the right type of smoke, that 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?
First, let us be 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 it intuitively to keep the burn alive, with just the right amount of flame and sweet smoke rising.
To understand the flavours produced by woodsmoke 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 because of 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 are required to ignite the fuel), any water inside the wood starts to boil, then evaporates. 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 floating around in a vapour 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 this is where most of the flavour is contained.
Smoke gets in your tastebuds
The individual flavours produced by smoke are released at different times in the burning process, and various 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 fruity, flowery and bready aromas. Lignin breakdown creates phenolics (aromatic compounds), bringing pungency and spiciness. These flavours penetrate only the outer millimetres of the meat and are unstable, meaning that they would begin to disappear after a few weeks – if it was ever possible to resist a plate of barbecued food that long.
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 (a protein), visibly present as the smoke ring at a depth of 8mm–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: the Kansas City Barbecue Society, one of the governing bodies, has now removed it from its judging criteria, based on the knowledge that it is easily faked by dusting the meat with nitrate-based curing salt.
Hardwood or softwood?
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 softwoods do not.
Oak is one of the most popular hardwoods for barbecue, thanks to its steady rate of burning, which makes the heat easier to control; also, it produces a mellow smoke that does not overwhelm. Incidentally, the structure of oak makes it particularly good for wine barrels, because of the size of its ‘radial rays’ (perpendicular to the growth rings we 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. Fruitwoods (apple, cherry) produce popular flavours, too, because of 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 (300C–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 burn, 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, dirty smoke
The term ‘clean smoke’ basically means smoke that contains as little carbon or soot as possible. We have all seen dirty smoke at some point, even if only 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 – in other words, 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 getting as close as possible to complete combustion outside the lab.
The desirable level of combustion is indicated by thin blue smoke – but how do we achieve this? By ensuring that a 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 at the same time we 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 toas ‘seasoning’.
That word ‘intuition’ crops up a lot when 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 to 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 should have some excellent ’cue.
Can things still go wrong? Sure. The most important ingredient of all is practice.
This article was originally published in Pit issue 01