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Bark andExtractives
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Bark is the outermost layers of branches, stems and roots of woody plants. Plants with bark include trees, woody vines, and shrubs.*Amount: Branches > Roots > Stem
Bark: Introduction
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Outer bark Cork (PHELLEM) Cork cambium (PHELLOGEN) Secondary cortex (PHELLODERM)
Structure
Periderm
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Cork cells (phellem) Cells pushed from the cork cambium to the outside. Generally have
layering of suberin in their walls, and thereby become cork cells (phellem).
Suberin, often composed of suberic acid and phellonic acid, is a fatty and waxy substance, which makes the cells mostly impervious to water and unable to exchange gases and nutrients, hence these cells soon die and entrap air.
Phelloderm Cells pushed from the cork cambium to the inside are called phelloderm Can remain alive because they have unthickened and unspecialized cell
walls and, hence, can exchange gases and obtain nutrients.
Lenticels Permit the exchange of gases between the outer atmosphere and the
internal tissue of the stem. Occur in most woody trees.
Outer Bark
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Sieve elements Companion cells Phloem parenchyma Phloem fiber
Inner Bark
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Sieve elements Sieve elements are the conducting elements of the phloem. They
have thick primary walls. Contain sieve plate. The sieve elements are arranged one above the other and form
vertical sieve tubes.
Inner Bark (cont’d)
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Companion cells The thin-walled, elongated, specialized parenchyma cells, which are
associated with the sieve elements, are called companion cells. They assist the sieve tubes in the conduction of food materials.
Phloem parenchyma The parenchyma cells associated with the phloem are called phloem
parenchyma. These are living cells. They store starch and fats. in some plants they also contain resins and tannins.
Phloem fibres The fibres of sclerenchyma associated with phloem are called phloem fibres
or bast fibres. They are narrow, vertically elongated cells with very thick walls and a small
lumen. Among the four kinds of phloem elements, phloem fibres are the only dead
tissue and are the strengthening and supporting cells.
Inner Bark (cont’d)
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The lignin content of bark is much higher than that of wood, and the polysaccharide or sugar content is correspondingly lower.
The extractive-free cellulose portion of bark is only 20-35%, compared to 40-45% for wood.
The ash content of bark is usually higher than that of wood. Extractive content of bark is higher compared to wood,
commonly amounting to 15-26% of unextracted bark weight compared to 2-9% for wood.
Bark extractives include various starches, resins, and waxes. A major portion of the extractable chemicals, one-fourth to one-
half by weight, is tannic acid. Bark extract is usually more highly acidic than extract from wood
of the same species.
Bark: Chemical Composition
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1. Transport: through sieve elements2. Growth3. Biomechanical support: provide rigidity and flexibility is to
upright tree stem4. Defense: against both biotic and abiotic stress5. Storage: store carbohydrates, fats, oils, latex, and resins
Bark: Functions
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Bark volume determination processDobie and Wright’s formula (Dobie and Wright 1975) for calculating bark volume
Here, DOB = diameter measured outside the bark DIB = diameter measured inside the bark.
We used DOB and DIB measurements that were averages of the midpoint and small and large end diameters.
Huber’s equation for log volume calculation Volume = L x A Smalian’s equation for log volume calculation Volume = {( A1+ A2) /2 } x L
Here, A1 = cross sectional areas of the large end A2 = cross sectional areas of the large end , L = Length.
We still have to determine total log volume by one of the stated methods.
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Bark Wood relationship
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Bark: Different Uses
Backpack made of birch bark Bark of pine was used as emergency food in Finland during famine
Made from the bark of birch bark
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Trees contain some compounds known as extractives in very limited amounts but still affect the wood properties greatly and serve a variety of functions including:
Protection (from insects, animals, and rot). Attractants (flowers, fruits) Food storage Membranes Enzymes
Extractives add significant properties to wood:• Color• Odor• Density
Extractives: Introduction
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For protection Terpenes Resin Acids Flavonoids Lignans Stilbenes Tannins Phenolics and others
For other functions Fats and fatty acids Proteins Flavonoids Sterols
Types
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Softwoods:- Resin acids: 40-45% of extractives- Fatty acids: 40-60%- Monoterpenes (turpentine)- Phenolics
Hardwoods:- No resin acids or monoterpenes- Fatty acids: 60-90%- Phenolics
Content
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Most extractives are located in resin canals, gum ducts and/or parenchyma cells
High extractives content in heartwood of pine Extractives level decreases higher up in the tree The general composition of the extractives varies over the stem cross
section of wood 20-40 % extractives in bark
Distribution
Vertical resin canal Horizontal resin canal
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The fragrance of softwood trees are typically from a class of chemicals named terpenes, hydrocarbon derivatives of isoprene.
Over 4000 have been identified in trees. Terpene: Hydrocarbons Terpenoid: Contains oxygen containing functional group.
Terpenes/Terpenoids
Prefix # C-atoms # Isoprene units Occurence
Mono 10 2 Softwood oleoresin
Sesqui 15 3 Hardwood resin
Di 20 4 Resin acids
Tri 30 6 In many plants
Poly >40 >8 Leaves
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Terpenes/Terpenoids (cont’d)
Monoterpenes 2 * 5C
Isoprene1 * 5C
Sesquiterpenes 3 * 5C
Diterpenes 4 * 5C Triterpenes
6 * 5C
Resin Acids
• Mainly carboxylic derivates of neutral tricyclic diterpenoids
• Classified in pimarane and abietane acids
• One lipophilic and one hydrophilic end
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Found in the heartwood of most tress. Found in large amounts in bark and roots. Green tea can contain up to 30% flavonoids.
o Protectiono Colorationo Higher flavonoid content in diet reduces the chance of:
Heart Disease Stroke Cancer: all types
Flavonoids
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Lignans: Lignans are phytoestrogens. Widely distributed in softwoods and
hardwoods (typically amount: < 1% of heartwood but can be upto 30%).
Greatly responsible for coloration of heartwood. Many lignans have fungicidal, insecticidal, and antioxidant
properties.
Tannins: Protection of plants against insects/animals
Bad Taste (bitter taste) Toxic to bacteria Uses of condensed tannins
Leather tanning: 10,000+ year old industry Adhesives
Lignans and Tannins
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Variety of aromatic compounds Aromatic odor
Balsam odor Foliage, bark, fruits
Sometimes in heartwood Typically in small amounts (1%<)
Phenolics
OHOCH3
CHO
Vanillin
OCH3
OH
CH2 CH CH2
Eugenol(Oil of Clove)
OCH3
OHCH3O
CH2 CH CH2
Elemicn
OO CH2
CH3O
CH2 CH CH2
Myristicin(Extracted fron Nutmeg)
OCH3
OCH3
CHO
Veratraldehyde(Smells like vanilla)
OH
COOH
Salicylic Acid(asprin - close)
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Similar structures in all plants. What varies is the fatty acids attached to the glycerol and the percentage of tri, di, and mono-glycerides.
There are high levels of fats in seeds Corn and soybeans contain 10-20% oils. Canola and sunflower contain up almost 50% oils. Nuts 50-70% oils.
Fats are found in living cells – parenchyma. Functions
Serves as a storage material. The amount of fat in the tree is seasonally dependent.
Fats and Oils
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Waxes Water resistant protective coatings of leaves, fruits, & trees.
Minor wood component 0.08-0.09% Much higher percentage in foliage, fruits and some barks.
Physical Properties Soluble in organic solvents. Water insoluble Melting/Boiling point: like candles
b Carotene found in dark green and orange yellow vegetables. In the human body, it is converted to vitamin A. Has been shown to be have many health related benefits.
Waxes and b Carotene
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Highly colored compounds Anthraquinone
Found in Teak Used as pulping catalyst
Naphthaquinones Used as UV absorbers Used in polymer products
Benzoquinones
Coumarin Chemical which has the aroma of fresh mowed hay (or vanilla)
Proteins & Amino acids Plant contain small amounts (~0.5%) of proteins. Chlorophyll (structure)
Others
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Pitch Consists of about 50/50 terpenes and resin acids. When a softwood tree is wounded, it releases pitch to “seal” the
wound Terpenes serve as the antiseptic and then evaporate. Resin acids serve as the bandage.
Rosin The term rosin refers to a mixture of resin acids produced
through the distillation of pitch. Gum rosin: distillation of gum resins (wood). Tall oil rosin: pine oil rosin, produced during kraft pulping. Wood rosin: Distillation of old stumps.
Others (cont’d)
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ExtractionExtraction TechniqueSteam Distillation ®
ProductsTerpenesPhenolsHydrocarbonsLignans
Ether Extraction ® Fats/WaxesResin AcidsSterols
Alcohol Extraction® FlavonoidsPhlobaphenesTanninsStilbenes
Water Extraction ® CarbohydratesProtein/AlkaloidsInorganics
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