Smoke and Food
Smoke has many component molecules, and its interaction with food has been extensively studied. Common smoked foods include fish (especially salmon), ham and bacon, sausages, cheeses, chipotle (smoked jalapeño) peppers and many others — including, of course, smoked malt used in the brewing of smoked beers and in the production of whiskey.
Smoke contains a variety of compounds that retard spoilage, including formaldehyde, acetic acid (the acid in vinegar) and various phenolic compounds. Some of these phenolics also have anti-oxidant properties. The anti-oxidant properties of smoke were a big reason it was used on fish before refrigeration. The anti-oxidants kept the fats and oils on the surface of the fish from going rancid. (Rancidification is the decomposition of fats or oils.) This — usually in combination with salting or drying — allowed fish to be stored for weeks. The pH of smoke is also very low, usually under 2.5, which makes it hostile to microorganisms.
These days, food is smoked primarily for the flavor contribution of the smoke. When wood is burned, smoke can smell of caramel, bread, fruits, flowers, vanilla, clove and many other things. It also, of course, has a smoky pungency to it. The character of the smoke depends largely on what kind of wood is being burned and under what conditions. Many of the phenolic compounds in smoke are quite reactive and thus smoke flavors can diminish in foods, sometimes in a matter of weeks.
For all the wonderful properties of smoke, it does have the potential to cause health problems. Burning wood gives off polycyclic aromatic hydrocarbons (or PAHs). Many of these compounds have been found to be carcinogenic. Commercially produced smoked malt is made under conditions to minimize PAH content and in the sidebar on smoking malt, I’ll explain how to reduce your risk. PAHs are found in many foods — most especially foods grilled over wood — and you can never completely eliminate your exposure to them.
What’s in Wood
Wood is composed of three major materials, cellulose, hemicellulose and lignin. Cellulose is a linear chain of glucose molecules, similar to starch, but bonded together in such a way as to be indigestible by animals. (Termites can digest the cellulose in wood due to bacteria in their guts that can degrade cellulose.) A typical chain of cellulose contains 7,000 to 15,000 sugar subunits.
Hemicellulose is similar to cellulose, but is composed of many different types of sugars beside glucose. These include xylose, and lesser amounts of mannose, galactose and arabinose. Hemicellulose usually contains around 200 sugar subunits in a branching configuration.
When cellulose or hemicellulose is burned, it gives off many compounds, including furans (which smell sweet, bready and floral), lactones (which smell like coconut or peach) acetaldehyde (green apple), acetic acid (vinegar) and diacetyl (buttery).
Lignin is a class of molecule composed of many rings of carbon atoms. The more lignin in wood, the hotter it burns. The phenolics in smoke come from the decomposition of lignin. These include guaiacol (which smells smoky or spicy), vanillin (the main aromatic compound in vanilla), phenol (pungent), isoeugenol (sweet or clove-like) and syringol (spicy, sausage-like).
One of the minor components of wood is protein, so smoke can also contain Maillard products, those “browning products” formed by the interaction of sugar and amino acids under heat. If you are an advanced homebrewer, you probably recognized many of the compounds mentioned above. If not, though, don’t worry, the point is that smoke contains a large number of molecules that can confer flavors or aromas to smoked malt.
Malt
If you’re going to brew a smoked beer at home, you first need to find a source of smoked malt. There are two commonly-available commercial smoked malts available to North American homebrewers , Weyermann rauchmalz (smoked malt) and peat-smoked malt. Peated malt is available from maltsters Hugh Baird, Thomas Fawcett and Simpsons. In addition, you can smoke your own malt — see the sidebar on page 34.
Weyermann rauchmalz is a pale, base malt (1.7–2.8 °L), smoked over beechwood. Beech is native to Europe and North America and is used frequently as firewood because it burns steadily and cleanly. (Beech is also the wood that Anheuser-Busch uses to age Budweiser on. In their conditioning tanks, it serves as a place for the yeast to settle.) Rauchmalz has a fairly mild smoke character and can be used for up to 100% of the grist.
Peat-smoked malts are also light in color — generally 2–3 °L — but have a much more intense smoke character. Peat is partially-decomposed organic matter (mostly mosses and lichens) that forms in acidic wetlands called bogs. In many countries, it is dried and burned as fuel and its smoke has a very distinctive aroma. For awhile in North American homebrewing, it was practically de rigueur to add peat-smoked malt to Scottish-style ales. These days, the pendulum has swung the other way and this practice is avoided as it never was a traditional commercial practice.
Some maltsters make peat-smoked malt with different levels of smoke character. In general, peat-smoked malt should be used in very small quantities, perhaps up to 5% maximum for the lightly-peated versions. The primary customer for peat-smoked malts is distilleries.