Botanically, wood used as a smoke source falls into two broad categories, namely hardwoods and softwoods, which in some cases do not correlate with the actual physical hardness of wood. Proteins represent another potentially interesting component in wood. During the aging of harvested wood the extractives composition can change significantly, resulting in the loss or degradation of unsaturated compounds, fats, and fatty acids. The term hemicellulose can be considered to be generic since originally it was thought that all wood components were derived from cellulose. Distinctive differences in the odors of various wood species are due to the volatile or essential oil composition of each specific wood. Terpenes in wood include both acyclic and cyclic compounds. Various polyhydric alcohols are also present in wood. The fatty acid glyceride of glycerol is the most common such compound. Organic nitrogen-containing compounds can also be represented in wood by alkaloids. Not all woods contain alkaloids.