section_path: Chemistry/Extractives
section_name: Extractives
paragraph_text: Extractives
  
  Aside from the structural polymers, i.e. cellulose, hemicellulose and lignin (lignocellulose), wood contains a large variety of non-structural constituents, composed of low molecular weight organic compounds, called extractives. These compounds are present in the extracellular space and can be extracted from the wood using different neutral solvents, such as acetone. Analogous content is present in the so-called exudate produced by trees in response to mechanical damage or after being attacked by insects or fungi. Unlike the structural constituents, the composition of extractives varies over wide ranges and depends on many factors. The amount and composition of extractives differs between tree species, various parts of the same tree, and depends on genetic factors and growth conditions, such as climate and geography. For example, slower growing trees and higher parts of trees have higher content of extractives. Generally, the softwood is richer in extractives than the hardwood. Their concentration increases from the cambium to the pith. Barks and branches also contain extractives. Although extractives represent a small fraction of the wood content, usually less than 10%, they are extraordinarily diverse and thus characterize the chemistry of the wood species. Most extractives are secondary metabolites and some of them serve as precursors to other chemicals. Wood extractives display different activities, some of them are produced in response to wounds, and some of them participate in natural defense against insects and fungi.
  
  These compounds contribute to various physical and chemical properties of the wood, such as wood color, fragnance, durability, acoustic properties, hygroscopicity, adhesion, and drying. Considering these impacts, wood extractives also affect the properties of pulp and paper, and importantly cause many problems in paper industry. Some extractives are surface-active substances and unavoidably affect the surface properties of paper, such as water adsorption, friction and strength. Lipophilic extractives often give rise to sticky deposits during kraft pulping and may leave spots on paper. Extractives also account for paper smell, which is important when making food contact materials.
  
  Most wood extractives are lipophilic and only a little part is water-soluble. The lipophilic portion of extractives, which is collectively referred as wood resin, contains fats and fatty acids, sterols and steryl esters, terpenes, terpenoids, resin acids, and waxes. The heating of resin, i.e. distillation, vaporizes the volatile terpenes and leaves the solid component – rosin. The concentrated liquid of volatile compounds extracted during steam distillation is called essential oil. Distillation of oleoresin obtained from many pines provides rosin and turpentine.
  
  Most extractives can be categorized into three groups: aliphatic compounds, terpenes and phenolic compounds. The latter are more water-soluble and usually are absent in the resin.