Wood/Bark Extractives

Objective: obtain a better feel for the types of extractives present in woody plants and their function.

Important Concepts
Secondary metabolites
Major classes of extractives
Role of extractives in mature cell wall
Extractives and wood processing
Extractives are the compounds present in trees that can be solubilized (extracted) by organic solvents. They are found in higher concentrations in the bark of most woods and are generally considered to be biosynthesized in order to slow or prevent pathogen invasion. Their production is under strict genetic control, and some individual compounds are limited to individual species. Compounds such as these are broadly classified as secondary metabolites. This terminology dates from when these compounds were originally characterized. The compounds appeared to have no intrinsic role in physiological processes, and were thus considered secondary. In fact, until relatively recently, they were considered waste products of plant metabolism.
Extractive contents are quite variable within individual species and are also site specific (ie., the extractive content of two genetic clones of an individual species will vary depending on the site the trees are placed). Values also depend on method of extraction, time of extraction vs. date of felling, amount of heartwood, etc. Extractives in woody plants tends to be under-emphasized in classes devoted to Wood Chemistry basically because there is no real simple way to describe all the the materials that can be formed. This is unfortunate because extractives are responsible for many useful and practical aspects of wood, and they can also be a severe detriment to wood and fiber processing. For example, western red cedar cannot be used for pulping as some compounds in it corrode steel. Staining for that nice look on finished lumber essentially involves reactions with extractives. The deleterious grey stain that can occur during kiln drying of oak is also due to the extractives. The flavor of wine and spirits are enhanced by oak extractives. It really isn't important that an individual knows what each particular species has as its predominant extractives, that is reserved for natural products chemists. What is important is to know that they exist and could contribute to problems or be advantageous in certain situations that you may encounter. This set of pages and pdf files is to help you wade through the seemingly enormous array of compounds that are present in low concentrations in woody plants, and hopefully provide you with a better appreciation for them.
The reason most extractives are produced by trees is to provide protection from predators that wish to consume the cell wall structural components. The plant has decided to expend energy into making them rather than using the same energy to make more cellulose, hemicellulose and lignin. Making more structural stuff would allow the tree to grow faster and taller; but instead of doing this it invests energy into "wood preservation." Predators can be insects that eat the leaves, or termites which chew the wood to let bacteria in their stomachs digest the polysaccharides. Fungi obviously rot wood as anyone can attest to by taking a walk in the woods or looking at some unprotected timber. The rate of fungal degradation is very much species dependent as different tree species have different extractives which differ in their ability to prevent degradation. Some of these compounds (by chance as far as the tree is concerned) are also effective for treatment of human maladies. Thus the efforts at isolating taxol from the Pacific Yew tree and using it to treat cancer. Humans, by trial and error, have discovered which plants have materials which are useful for conditions such as coughs (choke cherry and black cherry bark) and sore throats (witch hazel). Gleaning information from medicine men or great grandma is useful in searching for potentially important chemicals and is termed ethnobotany.
 
For additional information: "Natural Products of Woody Plants I and II," Springer Series in Wood Science (TS 930/N38), published in 1989.
General Points Concerning Extractives
1. Typically 2-10% of wood. Higher is specific regions such as bark or heartwood.
2. Amount and type of extractives are species and site specific.
3. Detrimental to pulping processes. Consume chemicals and cause pitch problems and pulp yellowing.
4. Cause problems in wood finishing. Extractive movement can cause problems with staining and/or gluing.
Classification of Wood Extractives
1. Volatile Oils---mainly softwoods
i. Terpenes are widespread in nature and are based on 5-carbon building block.
ii. Monoterpenes are fragrant (10 carbon molecules)
iii. Turpentine, tropolones.
2. Wood Resins---mainly softwoods
i. Acidic diterpenes (20 carbon molecules).
ii. Resins of softwoods.
iii. Basis of tall oil.
3. Fats and Waxes---minor less than 0.5 %, suberin.
4. Tannins---hardwoods and softwoods
i. Compounds which tan leather. Old leather and dye industries used hemlock, oak and chestnut.
ii. Hydrolyzable tannins built upon glucose and gallic acid. Found in the Fagaceae (oaks and chestnuts).
iii. Condensed tannins are flavonoid-based and found in most woody plants.
5. Lignans---hardwood and softwoods.
i. Optically active lignin dimers. Free radical coupling process is controlled.
6. Carbohydrates---typically food reserves.
Bark
1. Extractive content can range from 20-40%.
2. Bark can comprise 15% of tree mass making debarking imperative for pulping.
3. Excess bark can be a problem as it is considered a hazardous waste by the EPA. Typically burned for fuel value. Some research into medicinals or use in adhesives has been done. Certain barks cannot be used as mulch due to toxicity or potential pathogen spread.
4. Bark Components
  Extractives  Polysaccharides Lignin Others (ash)*
Pine  25 35 35 5
 Hardwoods 17 45 30 8
*Bark has about 5 times as much inorganic matter as wood. Mostly calcium salts.
 
5. pH of Bark. Softwoods range from 3.1 to 3.8 due to the resin acids, whereas the wood ranges from 3.3 to 6.0. Hardwood bark pH ranges from 4.9-5.5, with the wood coming in at 3.5-5.5.
Heartwood
Heartwood formation in trees is the final biochemical event associated with woody plant growth and development. While the signalling and biochemical processes of heartwood formation are still poorly understood, the net result of heartwood formation is the deposition of relatively large quantities of secondary metabolites, the types and amounts of which are species and site dependent. While previous theories suggested that this production of metabolites was due to the elimination of excess carbohydrate, current thought is that these compounds provide a deterent to pathogen attack in regions of the tree where a biochemical response is not possible (passive resistance).
Natural restriction to wood decay in many species is due to the secondary metabolites broadly classified as tannins. Durable hardwoods such as the oaks and chestnuts are thought to obtain much of their resistance through the deposition of ellagitannins, compounds in which gallic acid (3,4,5-trihydroxy benzoic acid) is attached via an ester bond to a D-glucose core, and also biaryl-coupled to an adjacent galloyl group. Black locust (Robinia pseudoacacia) obtains its durability from flavonoids and condensed tannins, and cedar durability is due to a wide range of toxic terpenes. Resistance to fungal invasion in many but not all cases is due to the ability of the tannins to precipitate protein , and/or removing metal cofactors through their strong affinity for metal ions. The blu-black color observed with a nail in contact with oak wood is the result of a tannin-iron chelate (or complex).
Generalized Biosynthetic Pathway.
Aromatic Biosynthetic Pathway.
Lignans.
Terpenes and Resin Acids.
Extractive Contents of Several Woods
 Softwood % Hardwood %
 Abies balsamea (Balsam fir) 4.0 Acer rubrum (Red maple) 6.2
 Larix occidentalis (Western larch) 6.0 Carya illinoensis (Pecan) 7.4
 Picea mariana (Black spruce) 4.1   Carya tomentosa (Mockernut hickory)  5.0
 Pinus banksiana (Jack pine) 7.0   Fagus grandifolia (American Beech)  3.4
 Pinus echinata (Shortleaf pine) 5.3   Fraxinus americana (White ash) 5.8
 Pinus elliotti (Slash pine) 5.0   Liquidambar styraciflua (Sweetgum)  3.0
 Pinus monticola (Western white pine)  5.1  Liriodendron tulipifera (Yellow-poplar)  2.4
 Pinus palustris (Longleaf pine)  4.8  Nyssa sylvatica (Black tupelo)  2.7
 Pinus ponderosa (Ponderosa pine)  5.9  Platanus occidentalis (Sycamore)  4.4
 Pinus resinosa (Red pine)  5.2  Populus deltoides (Eastern cottonwood)  2.0
 Pinus strobus (White pine)  7.0  Quercus alba (White oak) 5.4
 Pinus taeda (Loblolly pine)  3.5  Quercus coccinea (Scarlet oak) 6.6
 Pseudotsuga menziesii (Douglas fir)  4.3  Quercus falcata (Southern red oak) 9.6
Sequioa sempervirens (Redwood)
old growth
new growth

 

10.8

0.9

 Quercus prinus (Chestnut oak)  6.6
 Taxodium distichum (Baldcypress) 5.5  Quercus rubra (Northern red oak) 4.4
 Thuja occidentalis (Northern white cedar)  6.4 Quercus virginiana (Live oak) 13.2
 Thuja plicata (Western red cedar)  15.0  Sassafras albidum (Sassafras) 2.4
 Tsuga canadensis (Eastern hemlock)  3.3  Ulmus americana (American elm) 2.1 
Percent values above are based on dry weight. Extractive contents are variable within individual species and are site specific. Values also depend on the method of extraction, time of extraction vs. date of felling, amount of heartwood, etc.
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