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Executive Summary
Introduction
Methodology
The UK Forest Resource
Secondary Metabolites From Trees
Non-Timber Markets For Trees
Extraction Technologies For Tree Metabolites

Cellulose extraction
Hemicellulose
Lignin
Minor Metabolites
Supercritical Extraction
Reactive Extraction
Enzymatic Processing
Extraction of Fermentation Broths
Modification of Materials Using scCO2
Overview of Opportunities
References

Adding Value To Tree Metabolites
Further Research
Modelling Tools
Extraction Technologies For Tree Metabolites
This section provides a review of current and future technologies for the extraction of the value-added components from wood. Except for cellulose, virtually all the substances occurring in wood are currently employed to generate heat and electricity for wood processing (e.g. pulping). With the appropriate processes nearly all the tree components can be isolated from wood, broken down into useable fragments or used directly, adding many times more value than burning the wood immediately or allowing it to rot. In past these extraction processes (e.g. organic solvents) often produced low yields, were relatively non-selective (complex downstream processing would be required to remove contaminants), destructive to labile tree metabolites, environmentally damaging and hazardous to operators. Consequently the isolation of the value-added products was often uneconomic or of borderline viability.
For example, most wood processing plants are energy-intensive facilities and hence consume wood chips, bark and other trimmings to generate energy and thereby reduce manufacturing costs. Removal of potential value-added metabolites using traditional aqueous technologies, meant that the residue left after extraction was saturated with water and therefore unsuitable for combustion (Hergert 1989). Here we review a number of new technologies that promise to assist in the removal of added-value materials at low-enough cost and at sufficient yield.
Cellulose extraction
Figure 10. Scheme for extraction and modification of cellulose
Cellulose is generally extracted by one of two methods, sulfite or by prehydrolysis Kraft pulping (sulphate method). The Kraft pulping method is the most popular and is responsible for around 80% of world cellulose production. Kraft pulping involves chemical treatment of wood with caustic soda and disodium sulfite at high temperature, high pH and high pressure. Lignin in the wood is converted to water-soluble lignosulfonate (the molecular weight is also reduced through hydrolysis) that comes out of the wood, and is often referred to as Kraft black liquor. Some hemicellulose and other secondary metabolites are also removed during this process. The cellulose yield is of the order of 30 - 35% and is around 95% pure (hemicellulose being the major contaminant). However, the final cellulose needs bleaching (if white cellulose is the desired product), which requires extra chemical treatment steps. Typical chemicals that have been employed as bleaches are chlorine, hypochlorite, chlorine dioxide, oxygen and hydrogen peroxide. The method is extremely odorous and is of social concern for residents in close proximity to the plant. Significant research effort has been directed at improving the yields of the Kraft process.
The by-products of Kraft-pulping (the Kraft black liquor, KBL) are also potentially valuable chemicals. The range of chemicals that can be isolated from KBL are summarised in Table 11 below. In the majority of processes, these chemicals are not recovered as they are burnt to generate steam and electricity for the pulping process.
More recently a combined separation method has entered the developmental stage in a collaboration between Eastman Chemical Company, the National Renewable Energy Laboratory and a major producer of chemical-grade cellulose. This process involves a mixture of organic solvents and water to separate cellulose from hemicellulose and lignin. Essentially, at the end of the process the organic phase contains the lignin, the aqueous phase contains hemicellulose and the insoluble cellulose precipitates out as a solid for collection (though it still requires some bleaching). Evaporation of the organic phase produces lignin and the hemicellulose can be isolated as a solid from the aqueous phase (although there are some purification problems that need to be overcome).
Table 11. Materials extracted from spent pulp liquor
Kraft Black Liqour component Approx content %a Separation method Example end product uses
Lignin 46 Precipitation with acid or carbon dioxide Dispersing agents, additives, polymers, vanillin production
Hydroxy acidsb 30 Tricky to separate, vacuum distillation a possibility Starting materials, polymers (lactic acid), sequestering agents
Formic acidb 8 Distillation Textile dying, leather tanning , coagulating latex rubber
Acetic acidb 5 Distillation Widely used organic acid
Turpentine 1 Distillation (mostly α-pinene) Varnish solvent, chemical feedstock
Resin and/or fatty acids 5 Skimmed from black liquor then vacuum distillation for separation and purification Coatings, polymers, adhesives, soaps and lubricants
a % of dry solid weight b Carbohydrate degradation products
Hemicellulose
Hemicellulose is generally extracted from wood by the use of sodium or potassium hydroxide (in some cases with the addition of sodium borate). However, the alkaline conditions have the problem that they deacetylate the hemicelluloses almost completely (reducing in most cases the end-value of the hemicellulose). The hemicellulose fraction is precipitated through neutralisation with acid. Xylan can be extracted from holocellulose (a mixture of cellulose and hemicellulose) through dissolution in dimthylsulfoxide (DMSO); but this method does not lead to chemical modification of the xylan (however, DMSO is not a "Green" solvent).
Lignin
There are two general methods that can be used to isolate lignin from a wood source. The first involves hydrolysis (either chemical or enzymatic) of the cellulose and hemicellulose fractions. This is not an ideal method if other fractions in wood are also to be extracted for commercial use (which in most cases they would be). The second method involves isolation as lignosulfonates, which is similar to the lignin products produced during pulping as described above (i.e. the lignosulfonates become soluble in aqueous solution). However, this methodology results in a change in the structure of lignin. Lignin can also be dissolved as alkali lignin by treatment with sodium hydroxide (or better still with sodium sulfide) at elevated temperatures (170°C). It can then be converted in turn into alkali-soluble derivatives by solution of hydrochloric acid and thioglycolic acid at 100°C.
Delignification of wood
Wood extractives play an important role in the delignification processes (Sjostrom 1993). Increased knowledge of wood resin chemistry will help to develop effective methods for the deresination of wood pulp. The standard method for analysis of wood resin involves fractionation of the ether or acetone extracts. Supercritical toluene, dioxane, tetrahydrofuran and acetone have all been found to be effective in the solubilization of lignin and cellulose. Extraction with super-critical CO2 could potentially be used to remove resinous materials from wood chips prior to pulping or other commercial wood conversion processes. There are numerous advantages to removal of valuable turpentine and tall oil from wood prior to pulping, as these extractives can hinder the pulping process and lower the final product quality. Resinous extractives can consume pulping chemicals, decrease penetration of pulping liquors, increase organic loads on recovery furnaces and reduce fibre-to-fibre bonding, causing a reduction in paper strength. In addition, these extractives contribute to significant air and water pollution problems associated with the pulp and paper industry.
High-pressure extractions and delignification of red spruce has been reported using binary acetic acid-water, acetic acid-carbon dioxide fluids and ternary acetic acid-water-carbon dioxide fluids. (Demirbas 2001)
Minor metabolites - extractives and extraction technologies
Trees contain a large and diverse number of hydrophobic and hydrophilic components that are soluble in neutral organic solvents or water; these are generally called extractives. The extractives can be considered as non-structural wood constituents, almost exclusively composed of extracellular, low molecular weight compounds. The composition of the extractive varies markedly in the different parts of the tree (e.g. leaves, roots, barks etc.) and also between species of trees.
The active components in the extractives can broadly be split into three classes of compounds:
  1. Terpenoids and steroids
  2. Fats and waxes
  3. Phenolic constituents
A number of the metabolites are of commercial and scientific interest (Table 9). Traditionally, such compounds have been isolated by solvent extraction using acetone, alcohol or water. Separation of the various classes of compounds is facilitated by separation through column chromatography, using increasingly polar solvents. Further separation of the individual metabolites or closely related families of metabolites (such as individual steroids from the terpenoid fraction) is achieved through further, specialised chromatographic techniques (such as a polyamide column material in the case of flavanoids). Qualitative and quantitative assessment analysis of the metabolites is normally carried out by gas-liquid (and high-pressure liquid) chromatographic methods in combination with mass spectrometry.
Such methods use large quantities of solvents, especially those classed as volatile organic compounds (VOC's) and harmful chlorinated solvents, such as dichloromethane. The volume of solvent used compared to the amount of extractive isolated is extremely high. Alternative extraction and separation methods are required if the commercial potential of these valuable extractives is to be exploited. One such technique, supercritical fluid extraction, shows a great deal of promise in this area and some recent examples are detailed in the next section.
Supercritical extraction and reactive extraction
Supercritical fluid extraction (SFE) has potential as an efficacious means of recovering secondary metabolites from the wood, such as waxy lipids, terpenes and phenolics. Supercritical fluids (dense gases that are maintained above their critical temperature, the temperature at which they cannot be liquefied by pressure) are less viscous and diffuse more quickly than liquids and can therefore extract plant products more effectively and faster than conventional organic solvents. SFE using carbon dioxide, at temperatures above 31°C and pressures beyond 73 atmospheres, has been the subject of increasing interest during the last twenty years (Kiran & Balkan 1994). It has been used on an industrial scale for coffee decaffeination for many years and smaller plants are also in operation using supercritical carbon dioxide (scCO2) as a solvent in the UK e.g. Botanix Ltd. and Thomas Swan & Co. The current users of SFE include the beverage industry (decaffeination, extracts for brewing and cocoa de-fatting), the food industry (spices, natural colours (paprika and turmeric), vegetable oils and defatting of cereal and nuts) and the cosmetics industry (ginger extract for use in toothpaste, black pepper extract for use in mouthwash and paprika extract for use in lipsticks) (McHugh & Kruknois 1994). It can be classified as a 'natural' and 'organic' solvent, which has accelerated its uptake in these areas. In the field of plant product extraction, several groups of non-polar compounds have been extracted including medicinal compounds, lipids, carotenes and alkaloids. However, technophobia and a reluctance to change still dominate in many potential areas of application. The equipment necessary, even though increasingly available on pilot-scale plant commercially, is still expensive and leads to high capital installation costs.
Supercritical CO2 has the following properties and advantages as an extraction solvent:
  1. It is a chemically pure, non-polar, stable solvent (available in >99.9% pure form, £70/$110 per 25kg.).
  2. It is colourless, odourless and tasteless.
  3. It is easily removed, allowing simple product isolation by evaporation to 100% dryness.
  4. It is highly selective.
  5. It is a 'tunable' solvent. Its density can be varied (by adjusting the temperature and pressure) to control product solubility.
  6. It is non-toxic and non-flammable.
  7. It is environmentally friendly (no liquid waste / solvent effluent is generated).
  8. Further processing is possible (e.g. impregnation).
  9. Low temperature (typically 30-100 °C) extraction conditions result in minimal degradation of volatile compounds.
  10. Higher product yields are possible compared with steam distillation.
  11. The spent material is undamaged unlike steam distillation or VOC solvent extraction so it can be used in composts or other by-product uses.
  12. It is capable of fractionating products without additional distillation steps that may alter the chemical composition of the products. Botanicals can be fractionated to produce a natural colour fraction, an aroma fraction, an anti-oxidant fraction and/or a flavour fraction (Jarvis & Morgan 1997).
  13. SFE often offers additional advantages over VOC extraction of a smaller solvent volume requirement, smaller space requirement, shorter extraction time and, after recovery of initial installation costs, a cheaper method of extraction (Modey, Mulholland, & Raynor 1996).
Supercritical 2 extraction
Pine wood and bark from the southern and ponderosa pine has previously been extracted using scCO2 with and without ethanol as a co-solvent (Sihvonen et al. 1999). The extractive yields ranged from 20-60% relative to the total diethyl ether extractive content. The yields as expected were dependent on temperature, pressure, particle size and fluid to wood ratio. Fatty acids were more soluble in scCO2 than diethyl ether. Scanning electron microscopy revealed that the SFE process did not appear to significantly alter the wood surface structure.
Despite its poor solvent power, scCO2 can dissolve many biologically important molecules, especially in the presence of small amounts of polarity modifiers. Any solvent residues in the product destined for human consumption would incur further costs for removal. However, the use of polar modifiers will probably be necessary to maximise and selectively extract biologically important molecules. As a minimum ethanol would be used in this regard, as it is non-toxic and available from renewable resources. The amount and identity of polar modifier and scCO2 density can be varied to optimise the extraction process to balance the yield of product with the cost of extraction. Further variables to consider include pre-treatment of the material being extracted e.g. size of wood pieces or pulp, dry or green wood. Generally SFE is not considered suitable for wet raw materials. Water is only 0.3% soluble in scCO2, but could play an important role in the extraction (Hawthorne, Miller, & Kreiger 1989). If excess water remained in the extraction vessel, highly water-soluble solutes would prefer to partition into the aqueous phase and therefore the SFE recovery would be low. There are several options available to circumvent this problem. The material could be dried; but this may affects the quality of the extract depending on which method is used. The two best methods with regard to natural product extraction are freeze-drying and air drying at ambient temperature (Hawthorne, Miller, & Kreiger 1989). Another process involves the addition of a modifier which removes the water such as 2, 2-dimethoxypropane (Hawthorne, Miller, & Kreiger 1989).P
It is worth noting that scCO2 extraction technology does not add to the 'greenhouse effect' as on a large/pilot plant scale, any CO2 used will be recycled. In addition, CO2 generated in pressurising and recycling the gas will be used to replenish the system. New membranes are under development, which would reduce the need for pressure cycling in such a system (Ritter & Campbell 1991). A cellulose acetate reverse osmosis membrane was applied to perform the separation of nutmeg essential oil and scCO2. The membrane presented good CO2 permeability and resisted well to the severe pressure conditions applied.
Water is another viable alternative to VOC in extraction processes. Steam distillation is widely used in extracting most essential oils, fragrances and flavour compounds. As a traditional and well-established technique it offers low capital running costs and designs are available to suit all volumes. However, unpredictable degradation of some classes of compounds can occur due to the higher temperature involved compared with extraction by scCO2. In addition, during sequential extraction of further products from the residue, using VOCs, there can be difficulties due to high moisture level.
All extraction processes require technically skilled operators; however, there are companies available e.g. the high-pressure equipment company, Swagelok, who are able to provide the necessary training.
Sub-critical water extraction
A technique related to SFE, continuous subcritical water extraction (CSWE) is also showing promise in the field of essential oil isolation (Lang & Wai 2001). Generally, the quality of oil from this technique is better than with scCO2 SFE because of the high content of oxygenated (aroma producing) compounds and lower content of terpenes.
Commercial equipment is currently unavailable for this technique. It requires a high-pressure pump, a pre-heater, an oven, temperature controller, extraction vessel, cooling system and high pressure and temperature resistant tubing.
N.B. the critical point of water is 221 bar and 374 °C. Temperatures used for CSWE are usually between 100 and 374 °C. However, pressures must be kept below the critical point but be sufficient to maintain the liquid state.
Table 12. Comparison between scCO2SFE and CSWE for the isolation of essential oils from plants
Aspect ScCO2SFE CSWE
Drying stage Yes No
Co-extraction of cuticular waxes Yes No
Installation cost High Medium
Extraction conditions Mild Medium
Pre-concentration effect Yes No
Environmentally clean character Yes Yes
Supercritical acetone
SFE of wood samples using acetone as the solvent, a high-pressure process, has previously been compared with low-pressure pyrolysis. It was found that both systems afford similar oil products in similar yields (Sjostrom 1993).
Reactive extraction
Reactive extraction can also be performed. This process results in the degradation of the wood fibres to glucose and other saccharide building blocks. This has been performed to some extent with wood using supercritical fluids such as water, acetone and ammonia (Spricigo et al. 2001). However, these supercritical fluids are corrosive, difficult to handle and require much higher temperatures and pressures. Therefore, they pose a greater risk to personnel.
Enzymatic processing
Experiments have been performed in scCO2 (at 50 °C and at pressures of up to 160 atm) on the hydrolysis of cellulose by the enzyme, cellulase (Luque de Castro, Jim�nez-Carmona, & Fern�ndez-P�rez 1999). Quantitative conversion to glucose was achieved and at rates significantly higher than those under atmospheric conditions.
ScCO2 pre-treatment of lignocellulose for enzymatic hydrolysis of cellulose has been investigated (Ehara, Saka, & Kawamoto 2002). Hard and softwoods with varying moisture contents were treated with scCO2 at varying temperatures, pressures and lengths of time. By increasing the water content of the lignocellulose before scCO2 treatment and enzymatic hydrolysis, yields of sugar were dramatically increased.
Extraction of Fermentation Broths
ScCO2 counter-current column extraction is currently being investigated as a new process for the extraction of bioactive compounds from fermentation broths. This process offers an inexpensive method to extract and simultaneously fractionate compounds of interest without leaving organic residues in the product (Sasaki et al. 2000).
Modification of materials using scCO2
Supercritical CO2 has been used to prepare wood-polymer composite materials, impregnate dyes into cellulose fibres and also spray-coat glass beads with cellulose based polymers (Demirbas 1994).
In situ chemical derivatization under supercritical conditions can be used to increase the extraction efficiency of polar organics, as it promotes the conversion of these groups (hydroxyl and carboxyl) into other less polar functions (ether, ester and silyl derivatives) which are more readily soluble in scCO2 B(Park, Ryu, & Kim 2001). It has recently been shown that the introduction of polar carbonyl groups, which afford a favourable dipolar interaction with the solvent and the introduction of flexible ether linkages, which reduce intermolecular solute-solute interactions, can be used to prepare designer scCO2-soluble materials (Kim & Hong 2001). The derivatized compounds would also be more amenable to conventional chromatographic column conditions.
The small molecules that are used in derivatization reactions are soluble in supercritical carbon dioxide and because of the enhanced permeability of this solvent it can be used to modify porous materials such as expanded cellulose efficiently. In addition to this, many cellulose derivatives such as ethyl cellulose and cellulose acetate are soluble in carbon dioxide and ethanol binary fluids (Fabre, Condoret, & Marty 1999).
Overview of opportunities
With the appropriate processes, nearly all the tree components can be isolated from wood, then broken down into useable fragments or used directly to add many times more value than burning the wood immediately or allowing it to rot. In the past, these extraction processes (e.g. using volatile organic solvents or steam distillation) often produced low yields, were relatively non-selective (complex downstream processing would be required to remove contaminants), destructive to labile tree metabolites, environmentally damaging and hazardous to operators. As a consequence, the isolation of the value-added products was often uneconomic or of borderline viability. For example, most wood processing plants are energy-intensive facilities and hence consume wood chips, bark and other trimmings to generate energy thereby reduce manufacturing costs. Removal of potential value-added metabolites using traditional technologies such as steam distillation left processing residues that were saturated with water and therefore unsuitable for combustion and energy generation.
New extraction technologies, such as supercritical fluid extraction, will enable operators to recover and modify tree metabolites (e.g. waxes, terpenes and phenolics) from wood more efficiently, with greater selectivity, and with far fewer potentially environmentally damaging consequences than conventional solvent technologies. These new technologies offer many additional advantages over previous ones (e.g. steam distillation) so that the plant material is now relatively undamaged and is available for further processing or energy generation. They have already been adopted by many food, beverage and cosmetic industries to extract the desired chemicals or remove the undesirable constituents of many natural products. However, technophobia and a reluctance to change still dominate in many potential areas of application. The equipment necessary to run pilot scale plants to test commercial viability, is still expensive and leads to high capital installation costs, though once installed it is often cheaper to run. Further R&D is required to support the evaluation and adoption of the new extraction techniques in wood processing.
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References
Demirbas,A. (1994) Chemicals from forest products by efficient extraction methods. Fuel Science Technology International 12, pp. 417-431
Demirbas,A. (2001) Supercritical fluid extraction and chemicals from biomass with supercritical fluids. Energy Conservation Management 42, pp. 279-294
Ehara,K., Saka, S., & Kawamoto, H. (2002) Characterization of the lignin-derived products from wood as treated in supercritical water. Journal of Wood Science 48, pp. 320-325
Fabre,C.E., Condoret, J. S., & Marty, A. (1999) Extractive fermentation of aroma with supercritical CO2. Biotechnology & Bioengineering 64, pp. 392-400
Hawthorne,S.B., Miller, D. J., & Kreiger, M. S. (1989) Coupled SFE-GC - a rapid and simple technique for extracting, identifying, and quantitating organic analytes from solids and sorbent resins. Journal of Chromatographic Science 27, pp. 347-354
Hergert,H.L. (1989) Lignans. Natural products of woody plants. (ed Rowe,J.W.), pp. 349-511. Springer-Verlag, Berlin
Jarvis,A.P. & Morgan, D. (1997) Isolation of plant products by supercritical-fluid extraction. Phytochemical Analysis 7, pp. 1-15
Kim,K.H. & Hong, J. (2001) Supercritical CO2 pretreatment of lignocellulose enhances enzymatic cellulose hydrolysis. Bioresources Technology 77, pp. 139-144
Kiran,E. & Balkan, H. J. (1994) High-pressure extraction and delignification of red spruce with binary and ternary mixtures of acetic-acid, water, and supercritical carbon-dioxide. Journal of Supercritical Fluids 7, pp. 75-86
Lang,Q. & Wai, C. M. (2001) Supercritical fluid extraction in herbal and natural product studies - a practical review. Talanta 53, pp. 771-782
Luque de Castro,M.D., Jim�nez-Carmona, M. M., & Fern�ndez-P�rez, V. (1999) Towards more rational techniques for the isolation of valuable essential oils from plants. Trends in Analytical Chemistry 18, pp. 708-716
McHugh,M.A. & Kruknois, V. J. (1994) Supercritical fluid extraction: pinciples and practices. Butterworth-Heinemann, Boston
Modey,W.K., Mulholland, D. A., & Raynor, M. W. (1996) Analytical supercritical fluid extraction of natural products. Phytochemical Analysis 7, pp. 1-15
Park,C.Y., Ryu, Y. W., & Kim, C. (2001) Kinetics and rate of enzymatic hydrolysis of cellulose in supercritical carbon dioxide. Korean Journal of Chemical Engineering 77, pp. 475-478
Ritter,D.C. & Campbell, A. G. (1991) Supercritical carbon-dioxide extraction of southern pine and ponderosa. Wood Fibre Science 23, pp. 98-113
Sasaki,M., Fang, Z., Fukushima, Y., Adschiri, T., & Arai, K. (2000) Dissolution and hydrolysis of cellulose in subcritical and supercritical water. Industrial Engineering & Chemistry Research 39, pp. 2883-2890
Sihvonen,M., Jarvenpaa, E., Hietaniemi, V., & Huopalahti, R. (1999) Advances in supercritical carbon dioxide technologies. Trends in Food Science Technology 10, pp. 217-222
Sjostrom,E. (1993) Chemistry: Fundamentals and applications. Academic Press Inc, New York
Spricigo,C.B., Bolzan, A., Machado, R. A. F., Carlson, L. H. C., & Petrus, J. C. C. (2001) Separation of nutmeg essential oil and dense CO2 with a cellulose acetate reverse osmosis membrane. Journal of Membrane Science 188, pp. 173-179