Wood smoke Hardwoods are made up mostly of three materials: cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are the basic structural material of the wood cells; lignin acts as a kind of cell-bonding glue. Some softwoods â€” especially pines and firs â€” hold significant quantities of resin, which produces a harsh-tasting soot when burned. Because of this, these woods are generally not used for smoking. Cellulose and hemicellulose are aggregate sugar molecules; when burnt, they effectively caramelize, producing sweet, flowery, and fruity aromas. Lignin, a highly complex arrangement of intelocked phenolic molecules, also produces a number of distinctive aromatic elements when burnt, including smoky, spicy, and pungent compounds like guaiacol, phenol, and syringol, and sweeter scents like the vanilla-scented vanillin and clove-like isoeugenol. Guaiacol is the phenolic compound most responsible for the "smokey" taste, while syringol is the primary contributor to smokey aroma. (Hui 512) Wood also contains small quantities of proteins, which contribute roasted flavors. Many of the odor compounds in wood smoke, especially the phenolic compounds, are unstable, dissipating after a few weeks or months. A number of wood smoke compounds act as preservatives. Phenol and other phenolic compounds in wood smoke are both antioxidants, which slow rancidification of animal fats, and antimicrobials, which slow bacterial growth. Other antimicrobials in wood smoke include formaldehyde, acetic acid, and other organic acids, which give wood smoke a low pH â€” about 2.5. Some of these compounds are toxic to people as well, and may have health effects in the quantities found in cooking applications. The compounds best demonstrated to have long-term health consequences are the polycyclic aromatic hydrocarbons, or PAHs, many of which are known or suspected carcinogens. Hotter wood fires make more PAHs; hot-burning mesquite produces twice as much as cooler-burning hickory. Since different species of tree have different ratios of components, various types of wood do impart a different flavor to food. Another important factor is the temperature at which the wood burns. High-temperature fires see the flavor molecules broken down further into unpleasant or flavorless compounds. The optimal conditions for smoke flavor are low, smoldering temperatures between 300 and 400 Â°C (570â€“750 Â°F). This is the temperature of the burning wood itself, not of the smoking environment, which sees much lower temperatures. Woods that are high in lignin content tend to burn hot; to keep them smoldering requires restricted oxygen supplies or a high moisture content. When smoking using wood chips or chunks, the combustion temperature is often lowered by soaking the pieces in water before placing them on a fire.