What they are and how much
Pour alcohol or acetone over sawdust and the liquid turns coloured and smells of resin. Everything that dissolved is extractives. Usually it is a few percent of the wood mass.
Extractives are compounds that are not part of the cell wall framework and can be removed by neutral solvents without destroying it. They fall into three large groups: terpenes and terpenoids (including resin acids), fats, waxes and sterols, and phenolic compounds. In wood analysis they are removed first so they do not distort lignin and carbohydrate results.
Terpenes and resin
Conifer resin smells of pine because of terpenes. Distilling oleoresin gives turpentine, and what remains is rosin, used on violin bows and in adhesives and paints.
Terpenes are built from C5 isoprene units. Volatile monoterpenes (α- and β-pinene, 3-carene) make up turpentine; non-volatile diterpenoid resin acids (abietic and pimaric types) make up rosin. Resin sits in the resin canals and parenchyma of conifers. In kraft pulping of softwood, terpenes are distilled off as sulfate turpentine, while resin and fatty acids rise as soap, which is acidified with sulfuric acid to give crude tall oil.
Fats, waxes and sterols
In the living cells of sapwood the tree stores fats, much as we store energy. Birch outer bark is rich in betulin, which is largely what gives it its white colour.
Fats (mainly triglycerides of unsaturated fatty acids, above all linoleic and oleic) and sterols (β-sitosterol) concentrate in the parenchyma cells of sapwood. During wood storage, triglycerides hydrolyse to free fatty acids. Hardwoods contain practically no resin acids: their lipophilic extractives consist mainly of fats, waxes and sterols. Birch outer bark is rich in the triterpene betulin.
Phenolic compounds
The heartwood of many species is soaked with phenolic substances. They are toxic to fungi, so heartwood rots more slowly. Oak passes such compounds into wine aged in barrels, and larch yields dihydroquercetin.
Phenolic extractives include stilbenes (pinosylvins in pine heartwood), lignans (especially abundant in spruce knots, mainly hydroxymatairesinol), flavonoids (dihydroquercetin, also called taxifolin, in larch heartwood) and tannins (hydrolysable tannins of oak, condensed tannins of bark). They form as heartwood develops and protect it from biological attack.
Why they matter
Extractives decide the colour, smell and durability of wood. In papermaking, resin can stick to the machinery. And pulping by-products, turpentine and tall oil, are valuable raw materials in their own right.
Extractives affect the colour and odour of wood, the natural durability of heartwood, and gluing and finishing. In pulp and paper making, lipophilic extractives cause pitch problems (deposits on equipment and in paper). Sulfate turpentine, tall oil and rosin, as well as betulin and dihydroquercetin, are used as chemical raw materials.
Key facts
- pine, Sjöström
- 3.5%
- birch, Sjöström
- 3.2%
- groups
- terpenes, fats, phenolics
- turpentine
- C10 monoterpenes
- rosin
- C20 resin acids
- pulping by-product
- tall oil
Sources for this chapter
- Sjöström E. Wood Chemistry: Fundamentals and Applications. 2nd ed. San Diego: Academic Press, 1993.
- Fengel D., Wegener G. Wood: Chemistry, Ultrastructure, Reactions. Berlin; New York: De Gruyter, 1984.
- Ek M., Gellerstedt G., Henriksson G. (eds.) Wood Chemistry and Wood Biotechnology. Pulp and Paper Chemistry and Technology, vol. 1. Berlin: De Gruyter, 2009.
- Азаров В.И., Буров А.В., Оболенская А.В. Химия древесины и синтетических полимеров: учебник. 2-е изд. СПб.: Лань, 2010.