What Is Smoking? Preserving and Flavouring With Compounds From Burning Wood

Wood heated without enough oxygen to burn cleanly breaks down into hundreds of compounds, and which ones dominate depends on the temperature. Cellulose and hemicellulose break down first and produce compounds contributing sweet and caramel notes. Lignin, which is the rigid component of wood, breaks down at higher temperatures and produces phenolic compounds that supply most of the characteristic smoky flavour and much of the preservative effect. Organic acids are produced and lower the surface acidity. Carbonyl compounds react with proteins on the food surface to produce colour, which is why smoked food develops its distinctive appearance. The mixture is complex enough that the flavour differs noticeably between woods, and the temperature of the fire matters more than most people assume, since a smouldering fire produces a different mixture from a hot one.

Several mechanisms operate together and none is sufficient alone:

• •Phenolic compounds deposited on the surface are directly antimicrobial and antioxidant
• •Formaldehyde and related compounds react with surface proteins, forming a barrier
• •Acids lower the surface pH, which inhibits growth
• •Drying, since smoking involves warm moving air that removes water and lowers the water available to microbes
• •Salt, since smoking is nearly always preceded by salting and the two work together
• •Antioxidant effects that slow fat becoming rancid, which is what limits the keeping of fatty foods

The two approaches are distinct operations and confusing them is a safety matter. Cold smoking holds the food below the temperature at which proteins cook, generally under thirty degrees, so the product remains raw and is flavoured and partly preserved without being cooked, which is how smoked salmon and many traditional hams are made. Because the food is never heated, cold smoking does not kill pathogens and depends entirely on the salting, the drying and the smoke compounds for safety, which is why the curing step is not optional and why the traditional procedures specify salt concentrations and times precisely. Hot smoking cooks the food while smoking it, generally between sixty and ninety degrees, producing a fully cooked product that keeps less well but is safe immediately. Warm smoking sits between and is used for some sausages.

Which wood is burned changes the result measurably and the traditional pairings reflect that rather than habit. Hardwoods are used almost exclusively, since softwoods contain resins that produce sooty acrid smoke and deposit unpleasant compounds. Within hardwoods, oak gives a strong general-purpose smoke and is the standard across much of Europe. Hickory and mesquite are stronger still and are associated with particular regional traditions. Fruit woods including apple and cherry are milder and sweeter and suit poultry and fish. Beech and alder are gentle and are traditional for fish in northern Europe. Peat, seaweed and even dung have been used where wood was scarce, and each leaves its own signature, which is why some traditional products taste of the fuel available in the place they came from rather than of a chosen flavouring.

Some of the compounds deposited are carcinogenic and the concern is real and quantifiable. Polycyclic aromatic hydrocarbons form when fat drips onto a heat source and when combustion is incomplete, and they deposit on the food surface, with heavily smoked traditional products carrying measurable amounts. Epidemiological studies in populations consuming large quantities of smoked and salted food have found elevated rates of certain cancers, with the evidence strongest where consumption is high and sustained. Modern commercial practice reduces exposure by generating smoke externally and filtering it, by controlling temperature, and by using liquid smoke, which is condensed and purified smoke with the heavier compounds removed and which is a reasonable technique rather than a shortcut. Occasional consumption of traditionally smoked food is not the situation the epidemiology concerns.

Wood breaking down without enough oxygen yields phenolic compounds that flavour and preserve, acids that lower surface pH, and carbonyls that react with proteins to give colour. Preservation depends on those plus drying and salt together. Cold smoking leaves the food raw and relies entirely on the cure for safety, while hot smoking cooks it. Incomplete combustion deposits carcinogens.